U.S. Department of the Interior

U.S. Geological Survey

Guidelines and Standard Procedures for Continuous

Water-Quality Monitors: Station Operation,

Record Computation, and Data Reporting

Techniques and Methods 1–D3
Front Cover. Upper left—South Fork Peachtree Creek at Johnson Road near Atlanta, Georgia, site 02336240 (photograph 

 by Craig Oberst, USGS)

Center—Lake Mead near Sentinel Island, Nevada, site 360314114450500 (photograph by Ryan Rowland, USGS)

Lower right—Pungo River at channel light 18, North Carolina, site 0208455560 (photograph by Sean D. Egen, USGS)

Back Cover. Lake Mead near Sentinel Island, Nevada, site 360314114450500 (photograph by Ryan Rowland, USGS)
Guidelines and Standard Procedures for Continuous 

Water-Quality Monitors: Station Operation, 

Record Computation, and Data Reporting

By Richard J. Wagner, Robert W. Boulger, Jr., Carolyn J. Oblinger, and Brett A. Smith

Techniques and Methods 1–D3

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

P. Lynn Scarlett, Acting Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B.A., 2006, Guidelines and standard procedures for continuous water-quality monitors—Station operation, record computation, and data reporting: U.S. Geological Survey Techniques and Methods 1–D3, 51 p. + 8 attachments; accessed April 10, 2006, at http://pubs.water.usgs.gov/tm1d3
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................1

Acknowledgments................................................................................................................................2

Water-Quality Monitoring Station Operation ............................................................................................2

Site Selection.........................................................................................................................................2

Monitor Selection .................................................................................................................................5

Monitor Configurations and Sensors ................................................................................................5

Types of Sensors..........................................................................................................................5

Temperature.........................................................................................................................7

Specific Conductance........................................................................................................7

Salinity .........................................................................................................................8

Dissolved Oxygen ...............................................................................................................8

Percentage of Dissolved Oxygen Saturation ........................................................8

pH ..................................................................................................................................9

Turbidity ................................................................................................................................9

Instrument Acceptance Criteria..............................................................................................10

Placement of Sensors in the Aquatic Environment ......................................................................10

Stream Cross-Section Surveys ...............................................................................................10

Lake or Estuary Vertical Profiles .............................................................................................11

Use and Calibration of Field Meters ................................................................................................11

Temperature................................................................................................................................11

Specific Conductance...............................................................................................................11

Dissolved Oxygen.......................................................................................................................11

pH .........................................................................................................................................12

Turbidity .......................................................................................................................................12

Monitor Operation and Maintenance..............................................................................................12

Standard Protocol......................................................................................................................13

Maintenance...............................................................................................................................13

Sensor Inspection and Calibration Check .............................................................................14

Steady-State Conditions..................................................................................................14

Rapidly Changing Conditions ..........................................................................................14

Alternative Methods for Servicing a Monitor..............................................................16

Monitor Calibration Criteria .....................................................................................................16

Field Cleaning of Sensors.........................................................................................................17

Field Calibration of Sensors .....................................................................................................17

Temperature Sensors.......................................................................................................18

Specific Conductance Sensors......................................................................................18

Dissolved Oxygen Sensors..............................................................................................18

pH Sensors.........................................................................................................................19

Turbidity Sensors ..............................................................................................................20

Troubleshooting Procedures....................................................................................................20

Requirements for Field Notes and Instrument Logs ............................................................22

Record Computation....................................................................................................................................22
iv

Data-Processing Procedures ...........................................................................................................23

Initial Data Evaluation ...............................................................................................................23

Application of Data Corrections..............................................................................................24

Corrections for Fouling ....................................................................................................31

Corrections for Calibration Drift .....................................................................................33

Other Corrections .............................................................................................................35

Evaluation of Cross-Section Data Corrections ..................................................35

Computation of Salinity.............................................................................................................36

Computation of Percentage of Dissolved Oxygen Saturation............................................37

Final Data Evaluation.................................................................................................................37

Maximum Allowable Limits for Reporting Continuous Data......................................37

Publication Criteria...........................................................................................................38

Preparation of the Review Package................................................................................................39

Record Checking........................................................................................................................39

Station Description....................................................................................................................39

Station Analysis..........................................................................................................................40

Documentation...........................................................................................................................40

Level of Review Required.........................................................................................................41

Contents of the Review Package ............................................................................................41

Data Reporting..............................................................................................................................................45

Data Reports........................................................................................................................................45

Data-Qualification Statements .........................................................................................................47

Reporting Parameter Codes and Units............................................................................................47

Archiving of Records...................................................................................................................................48

Summary........................................................................................................................................................49

References....................................................................................................................................................49

Attachments..................................................................................................................................................53

Figures

1. Photograph of Light 5 on the Pamlico River near Bath, North Carolina, 

and schematic of in-situ water-quality monitoring station....................................................3

2. Photograph of Ramapo River at Pompton Lakes, New Jersey, and schematic 

of flow-through water-quality monitoring station ..................................................................4

3. Photograph of Delaware River and Araitan Canal feeder at Raven Rock, 

New Jersey, and schematic of internal-logging water-quality monitoring 

sensor and recording system.....................................................................................................7

4. Example of rapidly changing recorded values of specific conductance 

at U.S. Geological Survey streamgaging station 02300554, Little Manatee River 

at Shell Point near Ruskin, Florida ...........................................................................................15

5. Specific conductance record at Duwamish River at Tukwila, Washington, 

September 5–10, 1998, was truncated at 330 microsiemens per centimeter, 

which was the default maximum setting for the data logger .............................................23

6. Turbidity values at the Little Arkansas River at Highway 50 near Halstead, 

Kansas, October 2003.................................................................................................................24
v

7. Turbidity values at the North Fork Ninnescah River above Cheney Reservoir, 

Kansas, October 2004.................................................................................................................25

8. Example of fouling of specific conductance sensor.............................................................26

9. Example of a constant data correction of –1.4 degrees Celsius applied in 

September 2005 to correct for programming error that produced an incorrect 

offset at South Fork Tolt River near Carnation, Washington ..............................................27

10. Linearity of specific conductance sensor response, Palouse River at Hooper, 

Washington, September 10, 2002, showing best-fit regression .........................................29

11. Graphic representation of a three-point data correction for nonlinear 

calibration drift............................................................................................................................30

12. Data corrections for fouling of a specific conductance sensor at Palouse River at 

Hooper, Washington, May–June 2002.....................................................................................32

13. Two-point variable data correction for calibration drift of a specific conductance 

sensor at Palouse River at Hooper, Washington, August–September 2002.....................34

14. Two-point variable data correction for calibration drift of a dissolved oxygen 

sensor at Little Arkansas River near Sedgwick, Kansas, March–April 2004....................34

15. Example of a review graph of measured and computed values of specific 

conductance at Palouse River at Hooper, Washington, August 2002................................41

16. Example of a discharge hydrograph at U.S. Geological Survey streamgaging 

station 13351000, Palouse River at Hooper, Washington ....................................................42

17. Example of a review graph of computed values of related water-quality field 

parameters at U.S. Geological Survey streamgaging station 12121500, 

Puyallup River at Puyallup, Washington, August 2001 ........................................................42

Tables

1. Factors for consideration in the placement and installation of continuous 

water-quality monitoring systems..............................................................................................2

2. Hydraulic conditions of the ideal gage site .............................................................................5

3. Principal advantages and disadvantages of continuous water-quality monitoring 

systems...........................................................................................................................................6

4. Standard protocol for the operation and maintenance of a continuous 

water-quality monitor.................................................................................................................13

5. General maintenance functions at a water-quality monitoring station.............................13

6. Modified standard protocol for the operation and maintenance of a continuous 

water-quality monitor at a site with rapidly changing conditions......................................15

7. Calibration criteria for continuous water-quality monitors ................................................16

8. Example of the effects of temperature on pH calibration standard solutions..................19

9. Troubleshooting problems with water-quality monitors ......................................................21

10. Criteria for water-quality data corrections ............................................................................25

11. Example of a constant data correction...................................................................................27

12. Example of a one-point variable data correction .................................................................28

13. Example of a two-point variable data correction based on percentage error ................29

14. Example of a three-point variable data correction for nonlinear calibration drift...........31

15. Rating table for conversion of specific conductance, in microsiemens per 

centimeter, to salinity, in practical salinity units ..................................................................36
vi

16. Rating table for conversion of specific conductance, in microsiemens per 

centimeter, to salinity, in parts per thousand ........................................................................37

17. Maximum allowable limits for continuous water-quality monitoring sensors ................37

18. Accuracy ratings of continuous water-quality records ......................................................38

19. Example of an ADAPS station analysis report.......................................................................43

20. Example of an ADAPS end-of-year summary report............................................................44

21. Example of an ADAPS daily-values table ...............................................................................46

22. Example of a manuscript station description.........................................................................47

23. Parameter codes and reporting units for measured field parameters 

at water-quality monitoring sites .............................................................................................48

Attachments

1. U.S. Geological Survey continuous water-quality monitor field form................................55

2. ADAPS primary data tables: (a) example of a standard format ADAPS primary 

data table, and (b) example of a historical format ADAPS primary data table ................59

3. Output from Excel spreadsheets for data-correction calculations for temperature, 

specific conductance, dissolved oxygen, and pH, and a correlation test 

for sensor linearity .....................................................................................................................61

4. Evaluation and application of a cross-section data correction..........................................67

5. Example of accuracy rating using “Expanded shifts/corrections” output: 

(a) “Expanded shifts/corrections” daily output, and (b) “Expanded 

shifts/corrections” hourly output for rating transition days, and (c) final rating 

of accuracy based on “Expanded shifts/corrections” output ...........................................69

6. Example of a station description for the Yampa River near Maybell, Colorado...............73

7. Example of a station analysis for specific conductance on the Yampa River 

near Maybell, Colorado .............................................................................................................81

8. Calibration criteria, data-correction criteria, maximum allowable limits, 

and ratings of accuracy.............................................................................................................83

Conversion Factors, Definitions, Datums, 

and Abbreviations and Acronyms

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)
vii

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Vertical coordinates: Unless otherwise specified, vertical coordinate information is referenced 

to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinates: Unless otherwise specified, horizontal (latitude and longitude) 

coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Abbreviations and Acronyms

AC alternating current

ADAPS Automated Data Processing System

CD-ROM compact disk - read only memory

DC direct current

DECODES Device Conversion and Delivery System

DO dissolved oxygen

FNU formazin nephelometric unit

HIF Hydrologic Instrumentation Facility

ISO International Standards Organization

LED light-emitting diode

mg/L milligram per liter

NAD27 North American Datum of 1927

NAWQA National Water-Quality Assessment Program

NFQA National Field Quality Assurance Program

NGVD29 National Geodetic Vertical Datum of 1929

NFSS National Field Supply Service

NIST National Institute of Standards and Technology

NTU nephelometric turbidity unit

NWIS National Water Information System

PSS Practical Salinity Scale 1978

psu practical salinity unit

S salinity

SDVB styrene divinylbenzene

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

± plus or minus

> greater than

≥ greater than or equal to

Abstract

The U.S. Geological Survey uses continuous waterquality monitors to assess the quality of the Nation’s surface 

water. A common monitoring-system configuration for 

water-quality data collection is the four-parameter monitoring 

system, which collects temperature, specific conductance, 

dissolved oxygen, and pH data. Such systems also can be 

configured to measure other properties, such as turbidity or 

fluorescence. Data from sensors can be used in conjunction 

with chemical analyses of samples to estimate chemical 

loads. The sensors that are used to measure water-quality field 

parameters require careful field observation, cleaning, and 

calibration procedures, as well as thorough procedures for the 

computation and publication of final records. 

This report provides guidelines for site- and monitorselection considerations; sensor inspection and calibration 

methods; field procedures; data evaluation, correction, and 

computation; and record-review and data-reporting processes, 

which supersede the guidelines presented previously in 

U.S. Geological Survey Water-Resources Investigations 

Report 00– 4252. These procedures have evolved over the past 

three decades, and the process continues to evolve with newer 

technologies. 

Introduction

Water quality can change frequently over time, necessitating frequent, repeated measurements to adequately 

characterize variations in quality. When the time interval 

between repeated measurements is sufficiently small, the 

resulting water-quality record can be considered continuous. 

A device that measures water quality in this way is called a 

continuous water-quality monitor. These monitors have sensors and recording systems to measure physical and chemical 

water-quality field parameters at discrete time intervals at 

point locations. Operation of a water-quality monitoring 

station provides a nearly continuous record of water quality 

that can be processed and published or distributed directly by 

telemetry to the Internet. The water-quality record provides a 

nearly complete record of changes in water quality that also 

can serve as the basis for computation of constituent loads at a 

site. Data from the sensors also can be used to estimate other 

constituents if a significant correlation can be established, 

often by regression analyses. 

Continuous monitoring of water-quality field parameters, 

such as temperature, specific conductance, pH, dissolved 

oxygen (DO), and turbidity, takes place in a wide variety of 

aquatic environments, ranging from clear, pristine, freshwater 

streams to biologically productive estuaries. Procedures for 

continuous monitoring in pristine, freshwater streams differ 

from those needed in coastal environments. Continuous monitoring in coastal environments can be challenging because 

of rapid biofouling from microscopic and macroscopic 

organisms, corrosion of electronic components from salt and 

high humidity, and wide ranges in values of field parameters 

associated with changing weather and tidal conditions.

Temperature and conductivity are true physical properties 

of water bodies, whereas DO and pH are concentrations, and 

turbidity is an expression of the optical properties of water 

(ASTM International, 2003). For the purposes of this report, 

all of these properties or constituents and the sensor values 

recorded by the monitors are referred to as field parameters. 

Sensors also are available to measure other field parameters, 

such as oxidation-reduction potential, water level, depth, 

ammonia, nitrate, chloride, and fluorescence. In addition to the 

measured field parameters, some monitors include algorithms 

to report calculated parameters, such as specific conductance, 

salinity, total dissolved solids, and percentage of DO saturation. Emerging sensor technology broadens the variety of 

measurable chemical constituents and reduces the limits of 

detection. Because it has become possible to make near realtime water-quality monitoring data available on the Internet, 

continual progress is being made to improve applications and 

refine quality-control procedures.

Purpose and Scope

This report provides basic guidelines and procedures for 

use by U.S. Geological Survey (USGS) personnel in site and 

Guidelines and Standard Procedures for Continuous 

Water-Quality Monitors: Station Operation, 

Record Computation, and Data Reporting

By Richard J. Wagner, Robert W. Boulger, Jr., Carolyn J. Oblinger, and Brett A. Smith 
water-quality monitor selection, field procedures, calibration 

of continuous water-quality monitors, record computation 

and review, and data reporting. Two techniques for servicing 

continuous monitors are discussed in this report: (1) a method 

for servicing monitors in a well-mixed, stable, or slowly 

changing aquatic environment; and (2) a method for servicing 

monitors in a dynamic, rapidly changing aquatic environment 

as is commonly found in estuaries. A discussion of alternative 

methods for servicing monitors also is included. These basic 

guidelines are minimal requirements that may need to be 

modified to meet local environmental conditions. Knowledge 

of the operation of the monitoring equipment and first-hand 

knowledge of the watershed form the core of the dataevaluation process. Record-computation procedures presented 

in this report provide a uniform set of minimum requirements 

for computing records. Examples of the application of scientific judgment in the evaluation of data records are discussed 

and are, by necessity, site specific. Other specific examples 

also are included to demonstrate the range of environmental 

conditions that affect the evaluation process. 

Acknowledgments

The authors appreciate the support, technical contributions, and advice of USGS colleagues in the preparation of this 

document. The authors wish to thank Janice Ward, Yvonne 

Stoker, Jacob Gibs, Andrew Ziegler, Heather Heckathorn, 

Marlene Sanderson, and Stephen Schmitt for their diligent 

technical reviews of the manuscript. Many others contributed 

to the content of this report. Special thanks to Trudy Bennett 

for her assistance in review of the record computation sections; to Kevin Grimsley for his assistance with the salinity 

computation section; and to Jerad Bales, Wendi Young, and 

Kathy Sarver for their assistance with the program for calculating percentage of DO saturation. The authors are grateful 

for the encouragement and support from the USGS Office of 

Water Quality, particularly from Steve Sorenson, Tim Miller, 

Kathy Fitzgerald, and Franceska Wilde.

Water-Quality Monitoring Station 

Operation

Physical and chemical measurements made in streams at 

frequent, discrete intervals provide a nearly continuous record 

of water quality in an aquatic environment. Major considerations in the design of a continuous water-quality monitoring 

station include selection of the monitor configuration, types of 

monitors and sensors, site selection, locations of the sensors 

in the aquatic environment, the use and calibration of field 

meters, and the actual operation of continuous water-quality 

monitors. Sensor and site selection are guided by the purpose 

of monitoring and the data objectives. The main objective in 

the placement of the sensors is the selection of a stable, secure 

location that is representative of the aquatic environment.

Site Selection

The main factors to consider in selecting a water-quality 

monitoring site are the purpose of monitoring and the dataquality objectives. All other factors used in the site-selection 

process must be balanced against these two key factors. 

Defining the purpose of monitoring includes making decisions 

about the field parameters to be measured, the period and 

duration of monitoring, and the frequency of data collection. 

Stream characteristics, site characteristics, and data-quality 

objectives determine whether a data sonde will be placed in 

situ (fig. 1) or whether a flow-through receptacle with a pumping sampler (fig. 2) will be a better choice. More site-specific 

considerations in monitor placement include site-design 

requirements, monitor-installation type, physical constraints of 

the site, and servicing requirements (table 1).

Table 1. Factors for consideration in the placement and 

installation of continuous water-quality monitoring systems.

Site characteristics

Potential for water-quality measurements at the site to be representative of the location being monitored.

Degree of cross-section variation and vertical stratification.

A channel configuration that may pose unique constraints.

Range of stream stage (from low flow to flood) that can be expected.

Water velocity.

Presence of turbulence that will affect water-quality measurements.

Conditions that may enhance the rate of fouling, such as excessive 

fine sediments, algae, or invertebrates.

Range of values for water-quality field parameters.

Need for protection from high-water debris damage.

Need for protection from vandalism.

Monitor installation

Type of state or local permits required before installation can 

begin.

Safety hazards relevant to monitor construction and installation.

Optimal type and design of installation.

Consideration of unique difficulties or costs of installation.

Logistics (maintenance requirements)

Accessibility of site, including parking or boat access.

Safe and adequate space in which to perform maintenance.

Presence of conditions that increase the frequency of servicing 

intervals needed to meet data-quality objectives.

For stream sites, proximity to an adequate location for making 

cross-section measurements.

Accessibility and safety of the site during extreme events (for 

example, floods or high winds).

Availability of electrical power or telephone service.

Need for real-time reporting.

 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Figure 1. Light 5 on the Pamlico River near Bath, 

North Carolina, and schematic of in-situ water-quality 

monitoring station.

Once the purposes of monitoring and data-quality 

objectives are defined, balancing the numerous considerations 

for placement of a continuous water-quality monitoring system 

still can be difficult. Obtaining measurements representative of 

the water body usually is an important data-quality objective. 

The optimum site consideration for achieving this objective 

is placing the pump intake, sensor package, or sonde in a 

location that best represents the water body being measured. 

Thus, an optimal site is one that permits sensors to be located 

at a point that best represents the section of interest for the 

aquatic environment being monitored. 

For streams, cross-section surveys of field parameters 

must be made to determine the most representative location for 

monitor placement. A site must not be selected without first 

determining that the data-quality objective for cross-section 

variability will be met. Sufficient measurements must be made 

at the cross section to determine the degree of mixing at the 

prospective site under different flow conditions and to verify 

that cross-section variability at the site does not exceed that 

needed to meet data-quality objectives. Additional crosssection measurements must be made after equipment installation to ensure that the monitor installation is representative 

of the stream during all seasons and hydrographic flow 

conditions. 

Water-quality characteristics in lakes, bays, estuaries, or 

coastal waters also may be variable, making it difficult to find 

a single location that is representative of the entire water body. 

Sufficient measurement surveys of field parameters must be 

made to provide adequate confidence that the magnitude and 

spatial distribution of variability are understood. Verticalprofile surveys should be made in lakes, deep rivers, or 

estuaries. If substantial horizontal or vertical variability 

is determined, consideration should be given to choosing 

another site with less vertical or horizontal variability, or 

using a different approach to meet the data-quality objectives 

(see Placement of Sensors in the Aquatic Environment). For 

example, estuaries, lakes, or large rivers may be chemically 

or thermally stratified. Sensors or pump intakes at multiple 

depths may be a solution for providing adequate data in 

stratified bodies of water. Multiple sensors or multiple pump 

intakes for a flow-through monitoring system may be needed 

to meet the monitoring and data-quality objectives for measuring field parameters in deep lakes, estuarine sites, or other 

vertically stratified sites.

The best location for a monitoring site is often one that 

is best for measuring surface-water discharge. Although 

hydraulic factors in site location must be considered, it is more 

important to consider factors that affect water-quality conditions. The same hydraulic factors that must be considered 

when selecting a specific site for measuring discharge in a 

channel also should be considered in selecting a water-quality 

monitoring location. Both purposes require a representative 

site that approaches uniform conditions across the entire 

width of the stream. Rantz and others (1982) identified nine 

5

Figure 1. Light 5 on the Pamlico River near Bath, North Carolina, and

schematic of in-situ water-quality monitoring station. (Photograph by

Sean Egen, U.S. Geological Survey) 

Satellite

telemetry

Solar

panel

Data

logger

Sonde

Water-Quality Monitoring Station Operation 

Sean Egen, U.S. Geological Survey
Data logger

Figure 2. Ramapo River at Pompton Lakes, New Jersey, and schematic of

flow-through water-quality monitoring station. (Photograph by Michael J. 

Deluca, U.S. Geological Survey)

location in the channel center may be more 

representative of areas farther upstream in 

the drainage basin. Large streams and rivers 

usually are monitored from the downstream 

side of bridge abutments, assuming that 

safety hazards and other difficulties can be 

reduced or overcome. 

The measurement point in the vertical 

dimension of larger flow systems also needs 

to be appropriate for the primary purpose 

of the monitoring installation. The vertical 

measurement point can be chosen for low-, 

medium-, or high-flow conditions; if bed 

movement or sensor location during low flow 

is a problem, consideration should be given 

to moving the sensors along the bridge to 

the optimal location. For a medium to small 

stream with alternating pools and riffles, 

the best flow and mixing occurs in the riffle 

portion of the stream; however, if flooding 

changes the locations of shoals upstream 

from the monitoring site, the measurement 

point may no longer represent the overall 

water-quality characteristics of the water 

body. Streams subject to substantial bed 

movement can result in the sensors being 

located out of water following a major 

streamflow event, or at a point no longer 

representative of the flow. A site may be 

ideal for monitoring high flow but not 

satisfactory during low flows. Site selection 

often is a choice of meeting as many of the 

applicable criteria as possible.

Assessment of a site also is dependent 

on fouling potential, ease of access, susceptibility to vandalism, and susceptibility of 

instruments or telemetry to interference from 

high-tension power lines or radio towers. The 

configuration and placement of water-quality 

monitoring sensors in cold regions require 

additional considerations in order to obtain 

data during periods of ice formation. White 

(1999) discusses environmental factors in the 

site selection of an automated water-quality 

station in British Columbia, Canada, but also 

generalizes morphological stream factors 

Figure 2. Ramapo River at Pompton Lakes, New Jersey, and schematic of flowthrough water-quality monitoring station.

hydraulic conditions for an ideal gage site, and these also must 

be considered in site selection for water-quality sites (table 2).

Some aquatic environments may present unique 

challenges for optimal site location. Lateral mixing in large 

rivers often is not complete for tens of miles downstream 

from a tributary or outfall. Turbulent streamflow may aid in 

mixing, but turbulence can create problems in monitoring 

field parameters, such as DO or turbidity. A location near 

the streambank may be more representative of local runoff 

or affected by point-source discharges upstream, whereas a 

 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data Michael J. Deluca, U.S. Geological Survey

and the importance of selecting a site that has minimal chance 

of damage or destruction from natural forces and vandalism. 

White (1999) emphasizes that a site should (1) meet minimum 

stream-depth requirements for instruments, (2) be safe and 

accessible under all conditions, and (3) be located to avoid the 

danger of vandalism. Also emphasized is exposure to direct 

sunlight if optical sensors are deployed. White (1999) adds 

that sites should be selected based on program objectives and 

field reconnaissance under several flow conditions. 
Monitor Selection

The selection of a water-quality monitor involves four 

major interrelated elements—(1) the purpose of the data 

collection, (2) the type of installation, (3) the type of sensor 

deployed at the installation, and (4) the specific sensors 

needed to satisfy the accuracy and precision requirements of 

the data-quality objectives. 

Sensors are available as individual instruments or as a 

single combined instrument that has several different sensors 

in various combinations. For clarity in this report, a sensor is 

the fixed or detachable part of the instrument that measures 

a particular field parameter. A group of sensors configured 

together commonly is referred to as a sonde. A sonde typically 

has a single recording unit or electronic data logger to record 

the output of multiple sensors. The term monitor refers to the 

combination of sensor(s) and the recording unit or data logger. 

The most widely used water-quality sensors in monitoring 

installations are temperature, conductivity, DO, pH, and 

turbidity. These sensors are the focus of this report.

Monitor Configurations and Sensors

In general, three types of configurations are used for 

water-quality monitors. Each configuration has advantages 

and disadvantages in relation to site location and data-quality 

objectives. The flow-through monitoring system has a pump 

that delivers water from the measuring point to the sensor(s) 

or sonde housed in a shelter (fig. 2). Typical pumps for a 

flow-through monitoring system require 110-volt alternating 

current (AC) and pump about 10 gallons of water per minute. 

Access to power is a requirement for flow-through monitoring systems, but advantages and disadvantages of all of the 

monitoring configurations must be evaluated based on the 

monitoring objectives (table 3).

The second configuration is one in which only the 

sensors are placed directly at the measuring point (in situ) 

in the aquatic environment, and communication cables are 

run to the data logger and power system located in a waterresistant shelter (fig. 1). The primary advantages of the in-situ 

configuration are that no power is needed to pump water, 

small shelters can be used, and systems can be installed at 

remote locations where AC power is not available (table 3). 

Direct current (DC), 12-volt batteries easily meet the power 

requirements of the sensors and recording equipment, and 

solar panels may suffice in some areas.

The third type of water-quality monitoring system is an 

internal-logging, combined sensor and recording sonde that 

is entirely immersed and requires no external power, thus 

reducing its exposure to vandalism (fig. 3). Power is supplied 

by conventional batteries located in a sealed compartment, 

and sensor data are stored within the sonde on nonvolatile, 

flash-memory, recording devices. The primary advantages of 

the internal-logging configuration are that AC power or large 

batteries and shelters are not needed. 

Types of Sensors

Sensors are available for continuous measurement of 

many field parameters and chemical constituents, but five of 

the most commonly used sensors are temperature, specific 

conductance, DO, pH, and turbidity. General concepts 

and calibration procedures are described in this report, but 

manufacturers’ instructions and recommendations should be 

read carefully and followed. Although the concepts of monitor 

operation and record computation also apply to other types 

of sensors, insufficient information is available to specify 

calibration criteria, data-correction criteria, and maximum 

allowable limits for sensors measuring other field parameters 

(see Application of Data Corrections). If a sensor is used 

for which these criteria have not been specified, sufficient 

quality-assurance data must be collected to define and apply 

quality-control limits. This information should be documented 

in a quality-assurance plan. 

Table 2. Hydraulic conditions of the ideal gage site (modified from Rantz and others, 1982).

1. The general course of the stream is straight for about 300 feet upstream and downstream from the gage site.

2. The total flow is confined to one channel at all stages, and no flow bypasses the site as subsurface flow.

3. The streambed is not subject to scour and fill, and is free of aquatic growth.

4. Banks are permanent, high enough to contain flood waters, and free of brush.

5. Unchanging natural controls are present in the form of a bedrock outcrop or other stable riffle for low flow and a channel 

constriction for high flow, or a falls or cascade that is not submerged at all stages.

6. A pool is present upstream from the control at extremely low stages to ensure a recording stage at extremely low flow, and 

to avoid high velocities at the streamward end of gaging-site intakes during periods of high flow.

7. The gaging site is far enough upstream from a confluence with another stream or from tidal effect to avoid any variable 

influence on stage at the gage site from the other stream or tide. 

8. A satisfactory reach for measuring discharge at all stages is available within reasonable proximity of the gage site. (It is 

not necessary that low and high flows be measured at the same stream cross section.)

9. The site is readily accessible for ease of installation and operation of the gaging station.

10. The site is not susceptible to manmade disturbances, nearby tributaries, or point-source discharges.

Water-Quality Monitoring Station Operation 
Table 3. Principal advantages and disadvantages of continuous water-quality monitoring systems.

[AC, alternating current]

Advantages Disadvantages

Flow-through monitoring system

Unit can be coupled with chlorinators to reduce membrane fouling. 110-volt AC power source is needed.

Expensive sensor systems can be secured in vandal-proof shelters. Large shelters are required, incurring higher installation costs.

Calibration can be performed in the shelter. Pumps in streams can clog from algal fouling or high sediment loads.

Sample water from more than one measuring point can be pumped to 

a single set of sensors.

In shallow bank or poorly mixed installations, properly locating 

intakes or sensors in the cross section is difficult.

With satellite telemetry, data can be transmitted to an office location. Electrical shock protection is required.

System can be monitored remotely for problems and needed service. Pumps may be damaged by sediment or corrosive waters.

Freeze protection can be provided to the sensors. Pump maintenance may be necessary.

Pumping may cause changes in water quality.

In-situ monitoring system

Remote locations are possible. Sensors are susceptible to vandalism.

Small shelters can be used. Sensors are more prone to fouling than in a flow-through system.

No power is needed to pump water, and electrical hazards are 

reduced.

Servicing sensors during flooding can be difficult.

With satellite telemetry, data can be transmitted to an office location. In shallow bank or poorly mixed installations, properly locating 

intakes or sensors in the cross section is difficult.

System can be monitored remotely for problems and needed service. Sensors are susceptible to debris or high flow.

Pump maintenance is not necessary. Shifting channels may require adjustments to sensor placement.

Susceptible to freezing.

Internal-logging monitoring system

Location options are flexible. Sensors are susceptible to vandalism.

No electrical hazards. Sensors are more prone to fouling than in a flow-through system.

Exposure to vandalism may be reduced. Servicing sensors during flooding can be difficult.

Pump maintenance is not necessary. In shallow bank or poorly mixed installations, properly locating 

intakes or sensors in the cross section is difficult.

Data are available only during site visits.

Sensors are susceptible to debris or high flow.

Shifting channels may require adjustments to sensor placement.

Status of the equipment can only be checked while servicing.

Data cannot be viewed without a site visit, and loss of data is unknown until a site visit.

Susceptible to freezing.

 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Temperature

Temperature affects the density of water, the solubility 

of constituents (such as oxygen in water), pH, specific 

conductance, the rate of chemical reactions, and biological 

activity in water (Radtke and others, 2004). Continuous 

water-quality sensors usually measure temperature with 

a thermistor, which is a semiconductor having resistance 

that changes with temperature. Thermistors are reliable, 

accurate, and durable temperature sensors that require little 

maintenance and are relatively inexpensive. The preferred 

water-temperature scale for most scientific work is the 

Celsius scale. Modern thermistors can measure temperature 

to plus or minus (+) 0.1 degree Celsius (°C), but the user 

must verify the accuracy claimed by the manufacturer for 

the range of application. 

Specific Conductance

Electrical conductivity is a measure of the capacity 

of water to conduct an electrical current and is a function 

of the types and quantities of dissolved substances in 

water (Radtke and others, 2005). As concentrations of 

dissolved ions increase, conductivity of the water increases. 

Specific conductance is the conductivity expressed in units 

of microsiemens per centimeter. The USGS measures 

and reports specific conductance in microsiemens per 

centimeter at 25 °C (μS/cm at 25 °C). Specific conductance 

measurements are a good surrogate for total dissolved 

solids and total ion concentrations, but there is no universal 

linear relation between total dissolved solids and specific 

conductance. Rather, the relation between specific conductance and constituent concentration must be determined for 

each site. A continuous record of specific conductance can 

be used in conjunction with chemical analyses and continuous discharge records to estimate constituent loads (Clifton 

and Gilliom, 1989; Hill and Gilliom, 1993; Christensen and 

others, 2000). 

Specific conductance sensors generally are of two 

types—contact sensors with electrodes and sensors without 

electrodes. Continuous specific conductance sensors 

generally have electrodes that require the user to choose a 

cell constant for the expected range of specific conductance. 

Multiparameter monitoring systems should contain 

automatic temperature compensation circuits to compensate 

specific conductance to 25 °C. This should be verified by 

checking the manufacturer’s instruction manual. All modern 

sensors are designed to measure specific conductance in 

the range of 0–2,000 μS/cm or higher. In general, specific 

conductance sensors are reliable, accurate, and durable 

but are susceptible to fouling from aquatic organisms and 

sediment.

Figure 3. Delaware River and Araitan Canal feeder at Raven 

Rock, New Jersey, and schematic of internal-logging water-quality 

monitoring sensor and recording system.

Figure 3. Delaware River and Araitan Canal feeder at Raven Rock,

New Jersey, and schematic of internal-logging water-quality

monitoring sensor and recording system. (Photograph by Bonnie Gray, 

U.S. Geological Survey)

Sensors

Water-Quality Monitoring Station Operation 

Bonnie Gray, U.S. Geological Survey
Salinity

Although salinity is not measured directly, some sondes 

include the capability of calculating and recording salinity 

based on conductivity measurements. Conductivity has long 

been a tool for estimating the amount of chloride, a principal 

component of salinity, in water (Albert, 1964). Salinity is most 

commonly reported using the Practical Salinity Scale 1978 

(Lewis, 1980), a scale developed relative to a standard 

potassium-chloride solution and based on conductivity, temperature, and barometric pressure measurements (American 

Public Health Association, 1998). Before development of the 

Practical Salinity Scale (PSS), salinity was reported in parts 

per thousand. Salinity expressed in the PSS is a dimensionless 

value, although by convention, it is reported as practical salinity units. Salinity in practical salinity units is nearly equivalent 

to salinity in parts per thousand. Because salinity is not 

directly measured but is derived from conductivity, the USGS 

recommends that salinity data stored in the National Water 

Information System (NWIS) be calculated from processed 

specific conductance records (see Computation of Salinity). 

If specific conductance values have been compensated to 

25 °C and water depths are sufficiently shallow that pressure 

corrections are not necessary, salinity can be calculated using 

the equations described by Schemel (2001).

Dissolved Oxygen

Sources of DO in surface waters are primarily atmospheric reaeration and photosynthetic activity of aquatic 

plants (Lewis, 2005). DO is an important factor in chemical 

reactions in water and in the survival of aquatic organisms. 

In surface waters, DO concentrations typically range from 2 

to 10 milligrams per liter (mg/L). DO saturation decreases as 

water temperature increases, and DO saturation increases with 

increased atmospheric pressure. Occasions of super saturation 

(greater than 100-percent DO saturation) often are related to 

excess photosynthetic production of oxygen by aquatic plants 

as a result of nutrient (nitrogen and phosphorus) enrichment, 

sunlight, and warm water temperatures, which often occur in 

lentic environments or in streams during low-flow conditions. 

Occasions of saturated oxygen commonly are related to 

cascading flow conditions, both natural and artificial. DO may 

be depleted by inorganic oxidation reactions or by biological 

and chemical processes that consume dissolved, suspended, or 

precipitated organic matter (Hem, 1989). 

The DO solubility in saline environments is dependent on 

salinity as well as temperature and barometric pressure. DO 

in waters that have specific conductance values greater than 

2,000 μS/cm should be corrected for salinity. Most modern 

sensors automatically compensate for the effects of salinity 

or have manual compensation techniques, but this should be 

verified by checking the manufacturer’s instruction manual.

Several new technologies are available for measuring 

DO concentrations, but the technology most commonly used 

for continuous water-quality sensors is the amperometric 

method, which measures DO with a temperature-compensated 

polarographic membrane-type sensor. Although polarographic 

membrane-type sensors generally provide accurate results, 

they are sensitive to temperature and water velocity and are 

prone to fouling from algal growth and sedimentation. The 

measuring process consumes DO; therefore, water flow past 

the sensor is critical. If the water velocity at the point of 

measurement is less than 1 foot per second (ft/s), an automatic 

or manual stirring mechanism is required. Alternatively, a 

different technology may be used. DO sensors also can be 

affected by high water velocity. A complete discussion of DO 

calibration, measurement, and limitations can be found in 

Lewis (2005). 

Because the permeability of the membrane and solubility 

of oxygen in water change as functions of temperature, it is 

critical that the DO sensors be temperature compensated. The 

Teflon® membranes of DO sensors are susceptible to fouling; 

the membrane and retaining ring are susceptible to loss of 

elasticity; and the cathode-anode measuring electrodes are 

susceptible to chemical alteration. Fouling of the membrane 

includes coating from oily or other organic substances, siltation, attachment of aquatic organisms (for example, barnacles 

in estuarine locations), growth of algae, or deposition of other 

materials. Chemical alteration of the DO electrodes can be 

caused by a strong oxidizing or reducing chemical agent, 

such as a chemical spill, by metal-rich drainage water, or 

by organic-rich waters, such as a wetland. A more common 

chemical alteration is sulfide poisoning of the anode in 

oceanographic or ground-water environments. Poisoning is 

corroborated by chronically low DO readings even after sensor 

membrane replacement. Sulfide poisoning is evident as a gray 

or black color on the silver anode. Refurbishing a poisoned 

anode or replacement of a damaged sensor membrane is 

simple but requires calibration and may eliminate the ability 

to distinguish calibration drift from fouling. Manufacturer’s 

instructions must be followed in refurbishing or repairing a 

fouled electrode.

The newest technology for measuring DO is the luminescent sensor that is based on dynamic fluorescence quenching. 

This method employs measurement of light-emission characteristics of a luminescent-based reaction at the sensor-water 

interface (Lewis, 2005). The sensor has a light-emitting diode 

(LED) to illuminate a specially designed oxygen-sensitive 

substrate that, when excited, emits a luminescent light with 

a lifetime that is directly proportional to the ambient oxygen 

concentration. The response time of this sensor technology is 

fast; there are few known interferences to an unfouled sensor; 

there is no dependence on flow; and the sensors are claimed to 

have long-term stability (Alliance for Coastal Technologies, 

2004).

Percentage of Dissolved Oxygen Saturation

A common method for expressing the oxygenation 

of a water body is the percentage of DO saturation relative 

to 100 percent. DO solubility is based on ambient water 

temperature, atmospheric pressure, and salinity. Some DO 

 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
sensors include the capability of recording the percentage of 

DO saturation based on measurement or stored information 

on water temperature, atmospheric pressure, and salinity. DO 

saturation is calculated by using the equations described by 

Weiss (1970) and the program described in USGS Technical 

Memorandum 99.01 (U.S. Geological Survey, 1998) and available online at http://water.usgs.gov/software/dotables.html. 

The USGS recommends that DO percentage of saturation 

be calculated from fully processed and corrected continuous 

records of DO, temperature, and specific conductance.

pH

The pH of an aqueous solution is controlled by interrelated chemical reactions that produce or consume hydrogen 

ions (Hem, 1989). The pH of a solution is a measure of the 

effective hydrogen-ion concentration (Radtke and others, 

2003). More specifically, pH is a measure that represents 

the negative base-10 logarithm of hydrogen-ion activity of a 

solution, in moles per liter. Solutions having a pH below 7 are 

described as acidic, and solutions with a pH greater than 7 are 

described as basic or alkaline. Dissolved gases, such as carbon 

dioxide, hydrogen sulfide, and ammonia, appreciably affect 

pH. Degasification (for example, loss of carbon dioxide) or 

precipitation of a solid phase (for example, calcium carbonate) 

and other chemical, physical, and biological reactions may 

cause the pH of a water sample to change appreciably soon 

after sample collection (Radtke and others, 2003).

The electrometric pH-measurement method, using a 

hydrogen-ion electrode, commonly is used in continuous 

water-quality pH sensors. Sensors used in submersible 

monitors typically are combination electrodes in which a 

proton (H+)-selective glass-bulb reservoir is filled with an 

approximate pH-7 buffer. A silver wire coated with silver 

chloride is immersed in the internal reference electrode buffer 

reservoir. Protons on both sides of the glass pH electrode 

(media and buffer reservoir) selectively interact with the glass, 

setting up an external potential gradient across the outer glass 

membrane. Because the hydrogen-ion concentration in the 

internal buffer solution is constant, this external potential 

difference across the outer glass membrane, which is determined relative to the internal silver/silver-chloride reference 

electrode, is proportional to the pH of the medium. A correctly 

calibrated pH sensor can accurately measure pH to +0.2 pH 

unit; however, the sensor can be scratched, broken, or fouled 

easily. If streamflow rates are high, the accuracy of the pH 

measurement can be affected by streaming-potential effects 

(Radtke and others, 2003). The pH sensors are particularly 

prone to sensitivity loss (Busenberg and Plummer, 1987), 

which may result from a partially clogged reference electrode 

junction or a change in the concentration of the filling solution. The treatment for suspected sensitivity loss from sensor 

drift is sensor reconditioning or replacement. It is possible to 

distinguish between pH sensor drift and electronic drift by 

determining the sensor slope; however, because the correction 

for each is the same, it is not necessary. Detailed instructions 

for the calibration and measurement of pH are described by 

Radtke and others (2003) and by the instrument manufacturer.

Turbidity

Turbidity is defined as an expression of the optical 

properties of a sample that cause light rays to be scattered and 

absorbed, rather than transmitted in straight lines through a 

sample (ASTM International, 2003). ASTM further describes 

turbidity as the presence of suspended and dissolved matter, 

such as clay, silt, finely divided organic matter, plankton, 

other microscopic organisms, organic acids, and dyes. Implicit 

in this definition is the fact that color, either of dissolved 

materials or of particles suspended in the water, also can affect 

turbidity. 

Turbidity sensors operate differently from those for 

temperature, specific conductance, DO, and pH, which 

convert electrical potentials into the measurement of the 

constituent of interest. Submersible turbidity sensors typically 

direct a light beam from a light-emitting diode into the water 

sample and measure the light that scatters or is absorbed by 

the suspended particles in the water. The sensor response is 

related to the wavelength of the incident light and the size, 

shape, and composition of the particulate matter in the water. 

The effect of temperature on turbidity sensors is minimal, 

and the software for modern sensors provides temperature 

compensation. Calibration and measurement of turbidity by 

using a submersible sensor are discussed by Anderson (2004). 

Sensors that are maintained and calibrated routinely should be 

relatively error free. 

Numerous methods and instruments can be used to 

measure turbidity. Because different measurement technologies result in different sensor responses to the same turbidity 

calibrant, a set of turbidity parameter codes, method codes, 

and reporting units has been developed to differentiate 

between various instruments and methodologies (Anderson, 

2004). Data from each instrument type should be stored in 

NWIS using parameter codes and measurement reporting 

units that are specific to the technology and the instrument 

(see Data Reporting). Turbidity meters should be calibrated 

directly rather than by comparison with another meter.

Most commercially available sensors report data in 

nephelometric turbidity units (NTU), with a sensor range of 

0–1,000 and an accuracy of +5 percent or 2 NTU, whichever 

is greater. Some sensors can report values reliably up to about 

1,500 NTU. The USGS, however, defines NTU specific to the 

measurement of light scatter from a white tungsten lamp with 

color temperature 2,200− 3,000 degrees Kelvin and a light 

detector at 90 (+30) degrees from incident light (Anderson, 

2004). Instruments with such nephelometric designs qualify 

for approved measurement of turbidity in drinking water by 

the U.S. Environmental Protection Agency (USEPA) Method 

180.1 (U.S. Environmental Protection Agency, 1993). Another 

USEPA-approved method (GLI Method 2) for measuring the 

turbidity of finished drinking water is a dual-beam and dualdetector technology that compensates for color and reduces 

Water-Quality Monitoring Station Operation 
erratic readings (U.S. Environmental Protection Agency, 

1999). Field turbidity meters that are based on USEPA Method 

180.1 technology will not produce the same sensor response 

to a 100-NTU calibrant as a submersible turbidity sensor 

that is based on International Standards Organization (ISO) 

7027 (International Organization for Standardization, 1999) 

technology. Turbidity sensors for most submersible continuous 

water-quality sondes are based on nephelometric near-infrared 

wavelength technology that is compliant with ISO 7027, 

and data should be reported in formazin nephelometric units 

(FNU) as described by Anderson (2004). 

Instrument Acceptance Criteria

Independent testing to ensure accuracy and reliability is 

an important part of any quality-assurance program for hydrologic field instrumentation. One of the primary responsibilities 

of the USGS Hydrologic Instrumentation Facility (HIF) is 

the testing, evaluation, and documentation of instrument 

performance. USGS Water Science Centers are encouraged to 

purchase or rent instruments through the HIF when possible. 

Otherwise, programs or projects are required to perform the 

necessary steps of independent testing to ensure accuracy 

and reliability as stated by the instrument manufacturer (U.S. 

Geological Survey, 1995). Water Science Center personnel are 

encouraged to work with the HIF to evaluate new instrumentation and actively participate by entering, reviewing, and 

overseeing data in the Web-based instrument quality-assurance 

database, managed by the HIF (http://1stop.usgs.gov/qa/), and 

by taking corrective actions when necessary.

Placement of Sensors in the Aquatic 

Environment 

Placement of a water-quality monitoring sensor is 

dependent on the purpose of monitoring and the data-quality 

objectives. The data-quality objectives for the measurement 

of loads or flux in a stream or river generally require placement of a water-quality monitoring sensor at a location that 

is representative of conditions in the stream cross section. 

Some environments, such as lakes, estuaries, or poorly mixed 

streams, preclude sensor placement at one representative point, 

and alternative monitoring strategies must be considered. 

For example, multiple sensors can be located in vertically or 

horizontally stratified aquatic environments. Another option is 

the use of a flow-through monitor configuration (see Monitor 

Configurations and Sensors) with intakes located at multiple 

depths or horizontal locations. Alternatively, if poor mixing 

occurs only during particular seasons or flow conditions, 

sensors can be placed at the optimal location, and the rating 

of the accuracy of the record (see Publication Criteria) can 

be downgraded during periods of poor mixing. In all cases, it 

is necessary to characterize the vertical and horizontal mixing 

at the monitor site with measurement surveys of vertical and 

horizontal cross-section variability (see Stream Cross-Section 

Surveys).

Stream Cross-Section Surveys

The data objectives for most continuous water-quality 

monitoring stream sites require that the sensors be placed at 

locations that are representative of average measurements 

in the stream cross sections. Before a monitoring site is 

installed, surveys of the cross-section variability of the field 

measurement(s) to be monitored are required to determine the 

most representative measurement point in the stream cross 

section and to determine if a cross-section correction is necessary. Data from cross-section surveys can be used to correct 

single-point measurements in poorly mixed streams to better 

represent the cross-section average. By choosing a monitoring 

site with well-mixed streamflow, however, cross-section 

corrections may not be needed. 

A sufficient number of cross-section surveys representing 

different flow conditions is required to determine if discharge 

or seasonal changes significantly affect mixing in the cross 

section for the field measurement(s) to be monitored. A 

minimum of two cross-section surveys per year is required 

to verify or revise results from previous surveys. Temporal 

changes in cross-section variability in some streams may 

require more frequent surveys. Documentation of vertical 

mixing is required at least once per year at a minimum of two 

depths for all cross sections. 

The most efficient means of obtaining cross-section 

surveys is with a calibrated multisensor sonde that can 

measure the same field parameters that are being recorded by 

the monitor. At locations with high stream velocities, it may be 

necessary to attach the sonde to a sounding weight. Discrete 

samples should not be composited for measurement of crosssection averages. The standard USGS procedure for selecting 

measurement points for making a cross-section survey and 

calculating a cross-section mean value is to divide the stream 

cross section into increments using either the discharge- or 

area-weighted method (Webb and others, 1999). Generally, 

measurements are needed in the centroid of a minimum of 

four equal-discharge increments to provide a dischargeweighted mean. These multiple measurements also establish 

the horizontal cross-section variability of a measured field 

parameter. Alternatively, middepth measurements can be made 

at the midpoint of equal-width increments to determine an 

area-weighted mean value. Generally, a minimum of 10 and a 

maximum of 20 equal-width increments across a large stream 

or river are needed to establish the area-weighted mean value 

and horizontal cross-sectional variability of a field parameter. 

Examples of both area- and discharge-weighted cross-section 

average calculations are given in Wilde and Radtke (2005).

Multiple vertical measurements may be needed depending on the degree of vertical mixing. If physical or chemical 

vertical stratification is observed, the number of vertical 

measurements may need to be increased from middepth to 

two measurements (0.2 and 0.8 of the depth) or more. Alternatively, measurements can be made at points relative to changes 

in field parameters, such as temperature or salinity gradients, 

if these are documented. If the vertical stratification is sharply 

10 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
defined, the measurements across the transition zone must be 

more closely spaced to represent the position and degree of 

stratification adequately. 

Lake or Estuary Vertical Profiles

For lakes and other still waters or estuaries with wide 

cross sections, sensors likely cannot be placed at one location 

that adequately represents a cross section. Study objectives for 

continuous water-quality monitoring at lake or estuary sites, 

however, may require determination of whether measurements 

made at a single vertical location are representative of conditions in the vertical profile (Wilde and Radke, 2005). Thus, 

profiles of vertical variability of the field measurement(s) to 

be monitored are required before a monitoring site is installed 

to determine the most representative measurement point in the 

water column. For many of these types of sites, measurements 

will need to be monitored at multiple depths to meet study 

objectives.

The number of vertical measurements needed is 

dependent on the degree of vertical mixing. Measurements 

can be made at evenly spaced intervals (such as every 1 foot 

from surface to bottom) or at points relative to changes in 

field parameters, such as temperature or salinity gradients. If 

the vertical stratification is sharply defined, the measurements 

across the transition zone must be more closely spaced to 

represent the position and degree of stratification adequately. 

Use and Calibration of Field Meters 

The three major uses for a field meter during servicing of 

a continuous water-quality monitor are (1) as a general check 

of reasonableness of monitor readings, (2) as an independent 

check of environmental changes during the service interval, 

and (3) to make cross-section surveys or vertical profiles in 

order to verify the representativeness of the location of the 

sonde in the aquatic environment. The field meter should not 

be used directly to calibrate the water-quality monitor nor in 

the computation of monitor records. With the exception of 

temperature, it is important not to give too much credence to 

meter-to-meter comparisons. Independent field measurements 

must be made before, during, and after servicing the monitor 

to document environmental changes during the service interval. Measurements are made at the monitoring site by locating 

calibrated field instruments as close to the sensor as possible 

and at 5-minute intervals, or more frequently if necessary. 

Before site visits, all support field meters should be 

checked for operation and accuracy. Minimum calibration 

frequency for each type of meter is detailed in Anderson 

(2004) and Wilde and Radtke (2005). All calibrations must be 

recorded in instrument logbooks, along with all calibrations, 

measurements, results from USGS National Field Quality 

Assurance (NFQA) Program samples, and information about 

sensor replacements, instrument upgrades, or other periodic 

calibrations. 

Temperature

Proper certification and documentation for liquid-in-glass 

thermometers and thermistor thermometers are detailed in 

Radtke and others (2004). Thermometers must be calibrated 

or checked against a calibration thermometer, which is either 

certified by the National Institute of Standards and Technology 

(NIST) or certified by the manufacturer as NIST traceable 

(Radtke and others, 2004). Liquid-in-glass thermometers 

and thermistors must be accurate within +0.2 °C. For both 

thermistors and liquid-in-glass thermometers, an annual 

five-point calibration is required over the temperature range 

of 0 to 40 °C using a temperature-controlled water bath and 

an NIST-certified or NIST-traceable thermometer to ensure 

accurate temperature measurement. In addition, two-point 

calibration checks over the maximum and minimum expected 

annual temperature range must be made three or more times 

per year for thermistors and two or more times per year for 

liquid-in-glass thermometers. Calibrated thermometers and 

thermistors must be marked with the date of calibration.

Specific Conductance

Proper calibration and documentation for specific 

conductance meters are detailed in Radtke and others (2005). 

Calibration and adjustments for multiparameter sensor systems 

are found in manufacturers’ servicing manuals. Calibration 

standard solutions of known quality that bracket the expected 

full range of anticipated values are used to calibrate the 

specific conductance meter to the appropriate units for 

particular field conditions. Calibration is performed at the 

field site with calibration standard solutions that have been 

allowed to equilibrate to the temperature of the water being 

monitored. The USGS reports specific conductance compensated to 25 °C. Most meters have automatic temperature 

compensation circuits that permit readings in microsiemens 

per centimeter at 25 °C, but this should be verified by checking the manufacturer’s instruction manual. The accuracy of 

the meter should be within 5 percent for specific conductance 

values less than or equal to 100 μS/cm, or within 3 percent for 

specific conductance values greater than 100 μS/cm. Specific 

conductance standards are available from the USGS National 

Field Supply Service (NFSS). Calibration standard solutions 

must be discarded after use as described by Wilde (chapters 

variously dated).

Dissolved Oxygen

Proper calibration and documentation for DO meters 

are detailed in Lewis (2005). Calibration and adjustments for 

multiparameter sensor systems are provided in manufacturers’ 

servicing manuals. The most commonly used DO sensors 

measure the partial pressure of DO by the flow of oxygen 

through a porous membrane and oxygen consumption at a 

cathode. The calibrated accuracy of DO meters should be 

Water-Quality Monitoring Station Operation 11
within the lesser of 5 percent or +0.3 mg/L. Meters must be 

calibrated to 100-percent DO saturation and checked with a 

zero DO solution to provide an indication of sensor-response 

linearity. Calibration of a DO meter at 100-percent oxygen 

saturation is made by adjusting the meter reading for air saturated with water vapor to a value obtained from a DO solubility table (http://water.usgs.gov/software/dotables.html; Lewis, 

2005) generated from the equations of Weiss (1970). The DO 

solubility is based on the water temperature, the uncorrected 

barometric pressure, and salinity. A reliable pocket altimeter 

can be used to measure uncorrected (true) barometric pressure 

to the nearest 1 millimeter (mm) of mercury; a specific conductance meter can be used to measure salinity. The accuracy 

of a DO meter at 0.0 mg/L is verified by measuring the DO 

of a fresh solution of sodium sulfite, prepared as described by 

Lewis (2005). The zero-DO measurement also serves to ensure 

the integrity of the electrolyte solution, the membrane, and 

the retaining ring. Calibration and operation procedures differ 

among instrument types and makes, and the manufacturer’s 

instructions must be followed closely. 

pH

A detailed description of the 10-step calibration process 

for pH meters, including a wide range of available equipment, 

is provided by Radtke and others (2003). Calibration and 

adjustments for multiparameter sensor systems are available 

in manufacturers’ servicing manuals. Accuracy of field pH 

meters should be at least +0.1 pH unit. Two standard buffer 

solutions bracketing the expected range of environmental 

values are used to calibrate a pH electrode, and a third is used 

as a check for calibration range and linearity of electrode 

response. The pH-7 buffer is used to establish the null point, 

and a pH-4 or pH-10 buffer is used to establish the slope of 

the calibration line at the temperature of the solution. The 

slope of a pH electrode is temperature sensitive, but the pH 

slope for modern sensors usually is adjusted to the observed 

temperatures through automatic temperature compensation 

by use of the theoretical Nernst equation (Radtke and others, 

2003). It is important that the temperatures of the buffers be as 

close as possible to the samples being measured. Immersing 

the pH buffer bottles in the aquatic environment for about 

15 minutes allows the buffer temperature to equilibrate to the 

aquatic environment. Standard buffers of pH 4, 7, and 10 are 

readily available from the NFSS. Proper calibration of pH 

sensors does not ensure accurate pH measurements for low 

specific conductance waters. Consult the USGS National Field 

Manual for the recommended procedure when the specific 

conductance of the water sample is less than 100 μS/cm 

(Busenberg and Plummer, 1987; Radtke and others, 2003).

Turbidity

Proper calibration and documentation for turbidity meters 

are described by Anderson (2004). The three types of turbidity 

calibrants are (1) reference turbidity solutions, (2) calibration 

turbidity solutions, and (3) calibration verification solutions 

and solids. Reference turbidity solution is a calibrant that is 

synthesized reproducibly from traceable raw materials by a 

skilled analyst. The reference standard is fresh user-prepared 

formazin, prepared as described by Anderson (2004) or 

American Public Health Association (1998). All other 

calibrants are traced back to this reference solution. Calibration of a turbidity instrument by using reference turbidity 

solutions should be done only in the laboratory. 

Meters are calibrated using calibration turbidity solutions, 

which must be traceable and equivalent to the reference 

turbidity calibrants. Acceptable calibration turbidity solutions 

include commercially prepared formazin, stabilized formazin 

(such as StablCalTM), and styrene divinylbenzene (SDVB) 

polymer standards (such as Amco AEPA-1TM Polymer). 

Calibration turbidity solutions for various ranges are available 

commercially. Formazin-based calibrants can be diluted by 

using a dilution formula; however, errors may be introduced 

during the dilution process, thus reducing the accuracy of 

the standard solution. Formazin-based calibrants also are 

temperature dependent, and accurate readings may be difficult 

to obtain during field conditions. Anderson (2004) suggests 

that the effect of thermal fluctuations can be minimized by 

calibrating the instrument at room temperature in an office 

laboratory using a reference or calibration turbidity solution. 

Instrument calibration can then be checked at the field site by 

using a calibration verification calibrant. 

Calibration verification calibrants may include, but are 

not limited to, calibration turbidity solutions; however, calibration verification calibrants that are sealed or solid materials 

must not be used to adjust instrument readings (Anderson, 

2004). Before placing the sensor in a calibration verification 

calibrant, the sensor must be cleaned, rinsed three times with 

turbidity-free water, and carefully dried. Turbidity-free water 

is prepared as described by Anderson (2004).

Monitor Operation and Maintenance

The operational goal for monitoring water quality is to 

obtain the most accurate and most complete record possible. 

The general operational categories include maintenance of 

the monitoring station and equipment, periodic verification of 

sensor calibration, troubleshooting of sensors and recording 

equipment, and thorough record keeping. 

A standard protocol is common for the operation of 

continuous water-quality monitors in nearly all aquatic 

environments and site characteristics; only the cleaning and 

calibration steps (steps 3–5 in table 4) differ for determining 

error as a result of fouling and calibration drift. The standard 

protocol supports two methods for cleaning and calibration: 

(1) a method for monitors in well-mixed, stable, or slowly 

changing aquatic environments; and (2) a method for monitors 

in stratified or dynamic, rapidly changing conditions, such as 

those typically found in estuarine environments or in warm, 

12  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
sluggish, biologically active environments. The second method 

is used when changes in the monitored field measurements 

are greater than the calibration criteria (see Monitor Calibration Criteria), when measured field parameters are rapidly 

changing, or when measurements are fluctuating (see Rapidly 

Changing Conditions). Rapid change is defined as change 

that exceeds the calibration criteria (see Monitor Calibration 

Criteria) within 5 minutes.

Standard Protocol

The standard protocol is a series of procedures that 

must be performed routinely at all continuous water-quality 

stations (table 4) as fully described in this report. Site characteristics, such as stratification or dynamic, rapidly changing 

environmental conditions, may make it necessary to modify 

the standard protocol (see Rapidly Changing Conditions). 

Alternative methods that combine aspects of these techniques 

for cleaning and calibrating monitors also are possible. Environmental conditions or data-quality objectives may dictate the 

most appropriate method. For example, under conditions of 

rapidly rising streamflow or extreme low-flow conditions that 

lead to rapidly changing field parameters, measurements that 

are more accurate and perhaps safer working conditions may 

be obtained by using a modified protocol for rapidly changing 

environmental conditions. Experience and knowledge of site 

conditions will aid in the choice of cleaning and calibration 

methods.

Maintenance

Maintenance frequency generally is governed by the 

fouling rate of the sensors, and this rate varies by sensor type, 

hydrologic and environmental conditions, and season. The 

performance of temperature and specific conductance sensors 

tends to be less affected by fouling than the DO, pH, and 

turbidity sensors. The use of wiper or shutter mechanisms on 

modern turbidity instruments has substantially decreased fouling in some aquatic environments. For sites with data-quality 

objectives that require a high degree of accuracy, maintenance 

can be weekly or more often (table 5). Monitoring sites with 

nutrient-enriched waters and moderate to high temperatures 

may require maintenance as frequently as every third day. In 

cases of severe environmental fouling or in remote locations, 

the use of an observer to provide more frequent maintenance 

to the water-quality monitor should be considered. 

In addition to fouling problems, monitoring disruptions 

as a result of recording equipment malfunction, sedimentation, 

electrical disruption, debris, ice, pump failure, or vandalism 

also may require additional site visits. Satellite telemetry can 

be used to verify proper equipment operation on a daily basis 

and can aid the field hydrographer in recognizing and correcting problems quickly. Satellite telemetry is recommended for 

sites where lost record will critically affect project objectives.

Specific maintenance requirements depend on the 

site configuration and equipment. A useful discussion of 

the maintenance requirements for the flow-through and 

USGS minimonitor installations is available in Gordon and 

Katzenbach (1983), but nearly all operational requirements 

are fulfilled by the completion of the USGS water-quality 

continuous monitor field-inspection form (Attachment 1). 

Manufacturer’s instructions must be followed for other types 

of equipment or sensors.

Table 4. Standard protocol for the operation and maintenance 

of a continuous water-quality monitor. 

1. Conduct site inspection

 a. Record monitor readings, time, and monitor conditions

 b. With an independent field meter, observe and 

record readings and time near the sensor(s)

2. Remove sonde from the monitoring location

3. Clean sensors

4. Return sonde to the monitoring location

 a. Record monitor readings and time

 b. Using an independent field meter, observe and 

record readings near the sensor(s)

5. Remove sonde, rinse thoroughly, and check calibration

 a. Record calibration-check values

 b. Recalibrate if necessary

6. Return sonde to monitoring location

 a. Record monitor readings and time

 b. Using an independent field meter, observe and 

record readings near the sensor(s)

Table 5. General maintenance functions at a water-quality 

monitoring station.

Daily maintenance functions (for sites equipped with telemetry)

Daily review of sensor function and data download

Battery (or power) check

Deletion of spurious data, if necessary

Maintenance functions during field visits

Calibration of the field meter(s)

Inspection of the site for signs of physical disruption

Inspection and cleaning of sensor(s) for fouling, corrosion, or 

damage

Inspection and cleaning of deployment tube

Battery (or power) check

Time check

Routine sensor cleaning and servicing

Calibration check (and recalibration, if necessary)

Downloading of data

Water-Quality Monitoring Station Operation 13
Sensor Inspection and Calibration Check

The purposes of sensor inspection are to verify that a 

sensor is working properly, to provide an ending point for the 

interval of water-quality record since the last maintenance 

visit, and to provide a beginning point for the next interval 

of water-quality record. This is accomplished by recording 

the initial sensor readings in the environment, cleaning the 

sensors, recording the cleaned-sensor readings in the environment, performing a calibration check of sensors by using 

appropriate calibration standards, and recalibrating the sensors 

if the readings are outside the ranges of acceptable differences 

(see Monitor Calibration Criteria). A final environmental 

sensor reading is required after the calibration check or after 

recalibration. The difference between the initial sensor reading 

and the cleaned-sensor reading is the sensor error caused by 

fouling; the difference between the cleaned-sensor readings 

in calibration standard solutions of known quality represents 

sensor error caused by calibration drift. If the calibrated sensor 

cannot be recalibrated or does not agree with the calibrated 

field meter, the faulty sensor must be repaired after verifying 

that the readings of the field meter are not in error (see 

Troubleshooting Procedures). The alternative is to replace the 

monitoring sonde or sensor with a calibrated backup unit and 

repair the malfunctioning monitor in the laboratory or return 

it to the manufacturer for repair. All information related to the 

sensor inspection must be recorded on a field form (Attachment 1), which then is the basis for data corrections made 

during the record-processing stage. Complete and thorough 

documentation of the sensor inspection is required. 

Steady-State Conditions

The standard protocol (table 4) is used for servicing 

sensors in stable or slowly changing (in one direction) 

environmental conditions. Slowly changing is defined as 

changes in field measurements during maintenance that are 

less than the calibration criteria (see Monitor Calibration 

Criteria). Readings to determine error caused by fouling and 

calibration drift are made while the monitor is deployed. The 

initial sensor readings (before cleaning) of the monitoring 

equipment are compared to readings from a calibrated field 

meter ideally located at the same measuring point in the 

aquatic environment. The sonde is then removed for servicing 

while the field meter remains in place. The initial sensor 

reading becomes the ending point of the data record since the 

last servicing, and the field meter reading provides a sense of 

the reasonableness of the monitor readings and an indication 

of potential electronic calibration drift and fouling errors. 

Field meter readings should be recorded every 5 minutes, or 

more frequently if necessary.

Upon removal from the water, the monitoring sensors are 

inspected for signs of chemical precipitates, stains, siltation, or 

biological growths that may cause fouling. These observations 

are recorded in the field notes before cleaning, and then the 

individual sensors are cleaned according to the manufacturer’s 

specifications. The cleaned sonde or sensor is then returned 

to the water, and the final (cleaned) sensor readings and field 

meter readings and times are recorded in the field notes after 

the sensor has equilibrated. If the conditions are steady state, 

the field meter readings should not change substantially 

during the time that the monitoring sensors are cleaned. The 

observed difference between the initial sensor reading and 

the cleaned-sensor reading is a result of fouling (chemical 

precipitates, stains, siltation, or biological growths). After all 

cleaned-sensor readings are recorded, the monitoring sensors 

are again removed from the water, calibration is checked in 

calibration standard solutions, and the readings are recorded 

(and recalibrated if necessary); the difference between the 

cleaned-sensor readings in calibration standard solutions and 

the expected reading in these solutions is the result of sensorcalibration drift error. The sonde is recalibrated if necessary 

and replaced in the aquatic environment, and a set of initial 

readings is taken as the start of the new record. 

Except for temperature sensors, the field meter readings 

are not used directly in record computation; the field meter is 

used only as a tool to assess cross-section variability and environmental changes that may occur while the monitor is being 

serviced. If the environmental conditions are slowly changing 

while the monitor is being serviced and do not fluctuate, 

the fouling and calibration drift error can be computed with 

consideration being given to these environmental changes (see 

Data-Processing Procedures). 

Rapidly Changing Conditions

The standard protocol with minor modifications (table 6) 

is used when environmental conditions change rapidly or when 

measurements are fluctuating (increasing and decreasing; 

fig. 4). Rapid change is relative to the length of time needed 

to service the monitor and generally is defined as change that 

exceeds the calibration criteria within 5 minutes (see Monitor 

14  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Table 6. Modified standard protocol for the operation and 

maintenance of a continuous water-quality monitor at a site 

with rapidly changing conditions.

1. Conduct site inspection

 a. Record monitor readings, time, and monitor 

conditions

 b. With an independent field meter, observe and 

record readings and time near the sensor(s)

2. Remove sonde from the monitoring location

3. Place the sonde and a field meter in an insulated 

5-gallon bucket filled with ambient water

 a. Record monitor readings, time, and monitor 

conditions

 b. With an independent field meter, observe and 

record readings and time near the sensor(s)

4. Clean sensors

5. Return sonde to the insulated 5-gallon bucket

 a. Record monitor readings and time

 b. Using an independent field meter, observe and 

record readings and time near the sensor(s)

6. Remove sonde, rinse thoroughly, and check calibration

 a. Record calibration-check values

 b. Recalibrate if necessary

7. Return sonde to monitoring location

 a. Record monitor readings and time

 b. Using an independent field meter, observe and 

record readings near the sensor(s)

Figure 4. Example of rapidly 

changing recorded values 

of specific conductance 

at U.S. Geological Survey 

streamgaging station 02300554, 

Little Manatee River at Shell 

Point near Ruskin, Florida.

Water-Quality Monitoring Station Operation 15

50,000

40,000

30,000

20,000

10,000

0

1 8 15

APRIL 2004

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

22 30

Figure 4Example of rapidly changing recorded values of specific conductance at USGeological Survey streamgaging station

Calibration Criteria). Rapidly changing conditions typically 

occur in lakes and estuaries; a steep salinity or DO vertical 

gradient can result in large changes in salinity or DO over very 

short vertical distances. Internal water movement may further 

destabilize the gradient, and boat movement in choppy waters 

can make it impossible to replace the monitor or hold the field 

meter in one location. This effectively prevents comparison of 

pre-cleaned and cleaned-sensor readings to determine fouling 

per the standard protocol under steady-state conditions. 

Hazardous working conditions, especially in estuaries or large 

lakes, may limit the time that can be spent at the site and may 

require removal of the sonde and replacement after servicing. 

These situations require the use of a modified standard 

protocol for rapidly changing conditions (table 6). Such conditions also can occur in small streams or rivers, when rapidly 

increasing streamflow or extremely low streamflow conditions 

lead to rapidly changing field parameters that prevent comparison of pre-cleaned and cleaned-sensor readings in the stream. 

Experience and knowledge of site conditions will aid in the 

choice of servicing methods.

The modified standard protocol for rapidly changing 

conditions (table 6) generally follows the standard protocol 

(table 4) except that all measurements are made in ambient 

water collected in an ice cooler or insulated 5-gallon bucket 

that provides a stable environment for readings. First, sonde 

readings and time are recorded; the sonde is carefully removed 

from the water with minimal disturbance to any biological 

growth or sedimentation, inspected for signs of chemical precipitates, stains, siltation, or biological growths that may cause 

fouling, and placed in the bucket. The observations of fouling 

are recorded in the field notes. A calibrated field meter is 
placed in the bucket next to the sonde, and pre-cleaned sensor 

readings and times are recorded. Field meter readings should 

be recorded every 5 minutes, or more frequently if necessary. 

The sonde is removed from the bucket and cleaned according 

to the manufacturer’s specifications. The cleaned sonde is then 

returned to the bucket. After the sensors have equilibrated, the 

cleaned-sensor readings, field meter readings, and times are 

recorded in the field notes. The observed difference between 

the pre-cleaned sensor reading and the cleaned-sensor reading 

is a result of fouling. After all cleaned-sensor readings are 

recorded, the sensor calibration is checked in calibration 

standards, and the readings are recorded. The difference 

between the cleaned-sensor readings in calibration standard 

solutions and the expected reading in those solutions is caused 

by sensor calibration drift error. The sonde is recalibrated if 

necessary and replaced in the aquatic environment, and a set of 

initial readings is taken to represent the start of the new record. 

If turbidity is being measured, it may be more appropriate 

to use tap or distilled water rather than ambient water in the 

insulated bucket because of the possibility of turbid particles 

settling out in still water.

Because of the inherently variable nature of turbidity, 

measurements of turbidity in the stream before and after 

cleaning the sensor may not provide an accurate measure of 

fouling. A more accurate determination of fouling of turbidity 

sensors can be made by making pre-cleaning measurements 

in a bucket of clean tap or distilled water. Fouling is then 

determined by comparing measurements of turbidity in the 

bucket after cleaning the sensor, taking into consideration any 

changes in turbidity in the bucket, with measurements by a 

field meter.

Alternative Methods for Servicing a Monitor

Other modifications to the standard protocol may be 

desirable based on data-quality objectives, site characteristics, 

and field conditions. The optimal method for determining 

fouling is to record pre-cleaned and cleaned-sensor readings at 

the field site in a stable environment. In hazardous field situations, such as working from a boat in choppy waters, it may 

be difficult to perform accurate calibration checks. It may be 

more practical to exchange the sonde with a clean, calibrated 

sonde. One example of an alternative method for servicing a 

monitor is water-quality sonde exchange, in which fouling is 

determined by pre-cleaned and cleaned-sensor readings at the 

site but calibration checks are made off site, and the sonde is 

replaced with another clean, calibrated sonde. Once the sonde 

is cleaned, calibration checks can be performed elsewhere if 

care is taken in transporting the sonde; however, calibration of 

replacement sondes should be performed at or near the field 

site, especially if DO is being measured. If conditions prevent 

field calibration, the replacement sonde should be calibrated 

in the office or laboratory, and a minimal one-point calibration check must be performed on site or in a nearby mobile 

laboratory. 

Data-quality objectives may require even further 

modifications of the standard protocol. In this case, the sonde 

is carefully removed from the monitoring location, and precleaned and cleaned-sensor readings are made off site in a less 

hazardous environment to determine fouling, and calibration 

checks are made for calibration drift. All modifications to the 

steady-state protocol introduce the possibility of immeasurable 

errors into the process and may preclude identifying calibration drift error and fouling error separately. Consideration 

of these errors should be taken into account when rating the 

accuracy of the record (see Final Data Evaluation).

Monitor Calibration Criteria

When calibration checks reveal only a small amount 

of calibration drift, it may not be necessary to recalibrate 

the instrument (Gordon and Katzenbach, 1983). Under field 

conditions, the operating accuracy of the equipment has limits. 

Within these acceptable limits (calibration criteria), adjustments to calibration may not improve overall data accuracy. 

The calibration criteria for water-quality monitors (table 7) are 

based on stabilization criteria defined by Wilde and Radtke 

(2005). The criteria take into consideration the lower accuracy 

of some continuous water-quality sensors. In practice, 

a calibration check of cleaned sensors using calibration 

Table 7. Calibration criteria for continuous water-quality 

monitors.

[±, plus or minus value shown; °C, degree Celsius; μS/cm, microsiemens per 

centimeter at 25 °C; %, percent; mg/L, milligram per liter; pH unit, standard 

pH unit; turbidity unit is dependent on the type of meter used]

Measurement

Calibration criteria 

(variation outside the value shown 

requires recalibration)

Temperature ± 0.2 oC 

Specific conductance ± 5 μS/cm or ±3% of the measured value, 

whichever is greater

Dissolved oxygen ± 0.3 mg/L 

pH ± 0.2 pH unit

Turbidity ± 0.5 turbidity unit or ± 5% of the measured 

value, whichever is greater

16  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
standards is compared to the calibration criteria. If calibration 

drift is within the calibration criterion, the sensor is considered 

stable and recalibration is not required.

Field Cleaning of Sensors

During the cleaning process, care should be taken 

to ensure that the electrical connectors are kept clean and 

dry. Water in the connector pins can cause erratic readings. 

For this reason, a container of compressed air is a useful 

component of field-cleaning supplies. Most commercially 

available temperature sensors can be cleaned with a detergent 

solution and a soft-bristle brush. Film on the sensor that resists 

removal usually can be removed by soaking the sensor in a 

detergent and water solution (Ficken and Scott, 1989), but the 

manufacturer’s recommended cleaning procedures must be 

followed carefully for multiparameter sensor systems.

Radtke and others (2005) recommend cleaning specific 

conductance sensors thoroughly with de-ionized water before 

and after making a measurement. Oily residue or other 

chemical residues (salts) can be removed by using a detergent 

solution. Specific conductance sensors can soak in detergent 

solution for many hours without damage. Oil or other residues 

can be removed by dipping the sensor in a solvent or diluted 

hydrochloric acid solution (5 percent), but the manufacturer’s 

recommendations must be checked before using acid solution 

or solvents on sensors. The sensor must never be in contact 

with acid solution for more than a few minutes. Carbon and 

stainless-steel sensors can be cleaned with a soft brush, but 

platinum-coated sensors must never be cleaned with a brush.

Routine cleaning of DO sensors involves using a 

soft-bristle brush to remove silt from the outside of the 

sensor, wiping the membrane with a damp, lint-free cotton 

swab (available at local electronics stores), and rinsing 

with de-ionized water. The sensor usually is covered with a 

permeable membrane and filled with a potassium chloride 

solution. The membrane is fouled easily and typically will 

need to be replaced every 2 to 4 weeks. When the membrane 

is replaced, the potassium chloride solution must be rinsed 

out of the sensor with de-ionized water followed by several 

rinses with potassium chloride solution before the sensor is 

refilled. The membrane must be replaced with care so that the 

surface of the membrane is not damaged or contaminated with 

grease, and no bubbles are trapped beneath the membrane. The 

surface of the membrane should be smooth, and the membrane 

should be secured tightly with the retaining ring. The sensor 

must be stored in water for a minimum of 2 to 4 hours, preferably longer, to relax the membrane before installation and 

calibration. The time required to relax the membrane requires 

either replacing the DO sensor membrane with a pre-relaxed 

membrane and recalibrating or replacing the membrane and 

revisiting the site for calibration later. The retaining ring must 

be replaced annually or more frequently to prevent loss of 

electrolytes. Replacing the retaining ring when membranes are 

changed ensures a tight seal.

The gold cathode of the DO sensor also can be fouled 

with silver over an extended period of time, and a special abrasive tool usually is required to recondition the sensor. A fouled 

anode, usually indicated by the white silver electrode turning 

gray or black, can prevent successful calibration. As with the 

cathode, the sensor anode usually can be reconditioned following the manufacturer’s instructions. Following reconditioning, 

the sensor cup must be rinsed, refilled with fresh potassium 

chloride solution, and a new membrane installed.

The pH electrode must be kept clean, and the liquid junction (if applicable) must be free flowing in order to produce 

accurate pH values (Radtke and others, 2003). The body of 

the electrode should be thoroughly rinsed with de-ionized 

water before and after use. In general, this is the only routine 

cleaning needed for pH electrodes; however, in cases of 

extreme fouling or contamination, the manufacturer’s cleaning 

instructions must be followed. 

Turbidity sensors are extremely susceptible to fouling; 

thus, frequent maintenance trips may be necessary to prevent 

fouling of the turbidity sensor in an aquatic environment 

high in sediment, algae accumulation, larvae growth, or other 

biological or chemical debris. Mechanical cleaning devices, 

such as wipers or shutters that remove or prevent accumulation, are available for modern sensors. In environments that 

cause severe algal fouling, however, algae can accumulate on 

the wiper pad preventing complete removal of algal fouling 

from the optical lens and resulting in erratic turbidity data. 

Similarly, inorganic or organic debris can lodge in shutters, 

allowing fouling to accumulate and preventing efficient operation of the sensor. If the turbidity sensor is not equipped with a 

mechanical cleaning device that removes solids accumulation 

or a shutter that prevents accumulation on the lens before 

readings are recorded, reliable data collection is very difficult.

Sensors first should be inspected for damage, ensuring 

that the optical surfaces of the probe are in good condition. 

The wiper pad or other cleaning device also should be 

inspected for wear and cleaned or replaced if necessary. 

Before placing the turbidity sensor in standards, the optic 

lens should be carefully cleaned with alcohol by using a 

soft cloth to prevent scratching (or as recommended by the 

manufacturer), rinsed three times with turbidity-free water, 

and carefully dried. If the readings are unusually high or 

erratic during the sensor inspection, entrained air bubbles may 

be present on the optic lens and must be removed.

Field Calibration of Sensors

A water-quality monitoring sensor or sonde should be 

calibrated in the laboratory before installation at a field location and checked for calibration at the field site. Calibration 

in the laboratory or the field is done only by using calibration 

standards of known quality. During field visits, calibration of 

all sensors should be checked with two standard solutions that 

bracket the range of expected environmental conditions and 

a third standard near the ambient environmental conditions 

before any adjustments are made to the monitor calibration. 

Water-Quality Monitoring Station Operation 17
Field calibration is performed if the cleaned-sensor readings 

obtained during the calibration check differ by more than 

the calibration criteria (table 7). Spare monitoring sondes or 

sensors are used to replace water-quality monitors that fail 

calibration after troubleshooting steps have been applied (see 

Troubleshooting Procedures). All calibration equipment and 

supplies must be kept clean, stored in protective cases during 

transportation, and protected from extreme temperatures.

Temperature Sensors

Modern temperature sensors are quite sturdy and 

accurate, and the manufacturers generally make no provisions 

for field calibration of the temperature sensor. Temperature 

readings are compared between the sensors and thermometers 

that have been certified by the local USGS Water Science 

Center with an NIST-traceable thermometer as described by 

Radtke and others (2004). Temperature accuracy is especially 

important because of the effect of temperature on the performance of other sensors. Before a field trip, the Water Science 

Center certification labels on thermometers or thermistors 

should be checked to ensure that the certification is current 

and that the thermometer or thermistor is appropriate for use 

in extreme field conditions.

The temperature sensor and the calibrated field thermistor are placed adjacent to each other, preferably in flowing 

water or an insulated bucket of water. If a liquid-in-glass 

thermometer is used, it must be the total-immersion type. 

Sufficient time for temperature equilibration must elapse 

before a reading is made. The two temperature sensors must 

be read and the temperatures recorded instantaneously. If the 

monitoring temperature sensor fails to agree within +0.2 °C, 

troubleshooting steps must be taken; if troubleshooting fails, 

the sensor must be replaced. The faulty sensor or sonde should 

be returned to the manufacturer for proper calibration, repair, 

or replacement.

Specific Conductance Sensors

Calibration of specific conductance sensors should be 

checked with three calibration standard solutions of known 

quality before any adjustments are made, thus providing 

data for possible three-point calibration drift corrections (see 

Application of Data Corrections). Calibration checks must be 

made with two standard solutions that bracket the expected 

range of environmental conditions and a third standard solution near the ambient specific conductance value of the water. 

In addition, the zero response of the dry sensor in air should be 

checked and recorded to ensure linearity of sensor response at 

low values. If sensor-cleaning processes fail to bring a specific 

conductance sensor within the calibration criteria (table 7), 

the sensor must be recalibrated. Field calibration differs 

among the types of instruments, but most sensors generally are 

calibrated with only one or two standards. The manufacturer’s 

calibration procedures should be followed. 

Expiration dates and lot numbers for the standard 

solutions must be recorded and the standard solution bottles 

allowed to equilibrate to the temperature of the aquatic 

environment (by immersing in the water for 15 to 30 minutes). 

After three calibration standard solutions are checked and 

recorded (without making any adjustments), the monitor is 

recalibrated, if necessary, by using the appropriate calibration 

standard solutions and following the manufacturer’s calibration procedures. The sensor, thermistor or thermometer, 

and measuring container must be rinsed three times with 

a standard solution. Gentle tapping will ensure that no air 

bubbles are trapped on the sensor. Fresh standard solution 

is poured into the calibration cup; the temperature setting 

is adjusted, if necessary; the specific conductance values, 

calibration standard values, and temperature are read and 

recorded in the field notes and the monitoring instrument log. 

A temperature correction may be necessary if the monitor does 

not have automatic temperature correction (Radtke and others, 

2005).

Standard solution that has been used is discarded into a 

waste container, and the procedure is repeated using a second 

or third standard solution to check linearity of sensor response. 

If the sensor readings differ from the standards by more than 

5 μS/cm or 3 percent, whichever is greater, the calibration 

sequence must be repeated. If the second calibration sequence 

still differs by more than the calibration criteria, troubleshooting techniques must be attempted (see Troubleshooting 

Procedures). If these steps fail, the sonde or monitoring sensor 

must be replaced and the backup instrument calibrated.

Dissolved Oxygen Sensors

Dissolved oxygen in water is related to temperature, 

atmospheric pressure, and salinity. Calibration of DO 

sensors should be checked at 100-percent saturation and 

with a fresh zero-DO solution before any adjustments are 

made. Lewis (2005) provides detailed steps for four different 

calibration procedures: (1) air-calibration chamber in water, 

(2) air-saturated water, (3) air-calibration chamber in air, and 

(4) iodometric (Winkler) titration. The first three procedures 

calibrate DO to 100-percent saturation by an amperometric 

method. Based on measured temperature and atmospheric 

pressure, the saturated DO is obtained from a reference table 

developed by Weiss (1970) as described in U.S. Geological 

Survey (1998) and reproduced by Lewis (2005). An interactive program also is available for producing a table of DO 

saturation values (http://water.usgs.gov/software/dotables.

html) to the nearest 0.1 or 0.01 mg/L over user-defined ranges 

of temperature and barometric pressure and a table of salinitycorrection factors over user-defined ranges of specific conductance. Ambient atmospheric pressure must be determined 

with a calibrated pocket barometer to the nearest 1 mm of 

mercury. The fourth method, the iodometric titration method, 

measures DO in an unknown sample directly (by a dye color 

change upon reduction of available oxygen) to determine the 

sample concentration to which the DO sensor is calibrated. 

The appropriate procedure depends on the type of monitoring 

equipment. The manufacturer’s calibration procedures must be 

18  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
followed closely to achieve a calibrated accuracy of 

+0.3 mg/L concentration of DO. Calibration of field barometers should be checked before each field trip, preferably by 

checking with an official weather station (Lewis, 2005).

Most DO sensors can be calibrated only with a one-point 

calibration, usually at 100-percent saturation, although some 

sondes have the capability of a two-point calibration, at 

zero-percent and 100-percent saturation. For the sondes that 

are calibrated only at 100-percent saturation, the DO sensor 

response is checked in a zero-DO sodium-sulfite solution. A 

fresh zero-DO standard solution should be prepared before 

each field trip, as described by Lewis (2005). 

Calibration in the field presents a problem because 

replacement of the Teflon® membrane may be required 

frequently, and the replaced membrane must be allowed to 

“relax” in water for 2−4 hours before calibration (Lewis, 

2005). One solution to this problem is to carry into the field 

clean and serviced spare DO sensors, stored in water (or 

moist, saturated air). The replacement DO sensors then can 

be calibrated in the field, thus avoiding an interruption in the 

record and a return site visit. 

Luminescent-based DO sensors are calibrated by the 

manufacturer, and the manuals indicate that calibration may 

not be required for up to a year. When calibrated, the user 

should follow the manufacturer’s guidance. Regardless of the 

manufacturer’s claims, the user must verify the correct operation of the sensor in the local measurement environment. The 

standard protocol for servicing should be used for luminescent-based DO sensors to quantify the effects of fouling and 

calibration drift. Lewis (2005) advises users to make frequent 

calibration checks and to recalibrate as frequently as required 

to meet the specific data-quality objectives. Recalibration 

should not be necessary if calibration checks show the sensor 

to be in agreement with the calibration criteria (table 7).

pH Sensors

Field calibration of pH sensors often is a time-consuming 

process that requires patience. Expiration dates for the pH-4, 

7, and 10 buffer solutions must be checked, and spare pH 

sensors or backup sondes will need to be prepared in case 

replacement of the sensors is required.

Upon arrival at the field site, tightly capped buffer solutions are immersed in the aquatic environment to allow time 

for temperature equilibration, usually 15 to 30 minutes. Prior 

to replacement or calibration of the pH sensor, the cleaned 

sensor should be checked for calibration drift in all three 

buffer solutions. If the pH sensor readings exceed the calibration criteria (table 7), the monitoring sensor must be calibrated 

as described by the manufacturer’s instructions. 

Temperature effects on pH buffer solutions vary with 

individual solutions, and the temperature-correction factor 

must be verified with the manufacturer. Examples of common 

pH buffer solution variances with temperature are given in 

table 8. The pH sensor reading must be standardized to the 

temperature-corrected pH value. 

For most sondes, a two-point calibration is used. A 

relative zero is established with a pH-7 buffer and the slope is 

set with a pH-4 or pH-10 buffer. A three-point calibration may 

be used for some sondes. To begin calibration, the pH sensor, 

thermistor or thermometer, and calibration cup are rinsed with 

pH-7 buffer solution, which is then discarded along with all 

subsequent rinsates in a waste container. Fresh pH-7 buffer 

solution is poured into the rinsed calibration cup, and the 

instruments are allowed to equilibrate for at least 1 minute 

before the buffer solution is discarded and fresh pH-7 buffer 

solution is poured into the calibration cup. The solution is 

slowly swirled in the calibration cup, ensuring that the pH 

sensor and thermistor are fully immersed and that values 

have stabilized. The temperature, pH, and associated millivolt 

reading (if available) are measured and recorded, along with 

lot numbers and expiration dates of the pH buffers. This standardization process is repeated with fresh pH-7 buffer solution 

until two successive values of the temperature-adjusted pH-7 

readings are obtained.

The pH sensor, thermistor or thermometer, and calibration cup are rinsed with de-ionized water, and the standardization process is repeated with a pH-4 or pH-10 buffer solution 

to establish the response slope of the pH sensor. A buffer that 

brackets the expected range of pH values in the environment 

should be selected. The second temperature-corrected pH 

value, temperature, millivolt readings, lot numbers, and 

expiration dates are recorded, and the pH sensor, thermistor or 

thermometer, and calibration cup are rinsed with de-ionized 

water. The pH-7 buffer solution is then used to rinse, fill, and 

check the pH-7 calibration measurement. If the pH sensor 

reading is 7 +0.1 pH units, the slope adjustment has not 

affected the calibration. If the accuracy standard is not met, 

the calibration and slope adjustment steps must be repeated. 

If repeated calibration and troubleshooting steps fail, the pH 

sensor or monitoring sonde must be replaced.

Table 8. Example of the effects of temperature on pH calibration standard solutions.

[oC, degree Celsius; all pH values are in standard pH units. Temperaturecompensation values for pH may vary with buffer manufacturers; refer to 

manufacturer’s specifications for actual buffer values]

Temperature 

(°C)

pH buffer solution nominal value

4.01 7.00 10.00

0 4.00 7.14 10.30

5 4.00 7.10 10.23

10 4.00 7.07 10.17

15 4.00 7.04 10.11

20 4.00 7.02 10.05

25 4.01 7.00 10.00

30 4.01 6.99 9.96

35 4.02 6.98 9.92

40 4.03 6.98 9.88

Water-Quality Monitoring Station Operation 19
Once the slope-adjustment step is completed satisfactorily, the third buffer solution can be used as a check for 

calibration range and linearity of electrode response. The 

temperature and pH values are read and recorded along 

with the lot numbers and expiration dates of the pH buffers; 

however, the +0.1 pH accuracy should not be expected to 

be achieved over the full range from pH-4 to pH-10 for a 

monitoring sensor. The third buffer should be within +0.2 pH 

unit value.

Waters with specific conductance values less than 

100 μS/cm may require special low-ionic strength buffers and 

pH sensors. The extra preparations, precautions, and troubleshooting steps necessary for using these buffers and sensors to 

measure low-ionic strength waters are described in Busenberg 

and Plummer (1987).

Turbidity Sensors

Field inspection or calibration of the turbidity sensor is 

made by using approved calibration turbidity and calibration 

verification solutions and by following the manufacturer’s 

calibration instructions as described in Anderson (2004). 

Turbidity standard solutions with various ranges are available 

commercially, and most sensor manufacturers recommend 

either formazin-based or SDVB-polymer standards for calibrating turbidity sensors. Formazin-based standard solutions 

can be diluted by using a dilution formula; however, errors 

may be introduced during the dilution process, thus reducing 

the accuracy of the standard solution. Turbidity-free water, 

used in the preparation of standard solutions, dilution, and 

rinsing, should be prepared as described in Anderson (2004). 

Checking or calibrating the turbidity sensor must occur in an 

environment in which stable readings can be obtained. Such an 

environment minimizes movement of the standard solutions, 

wind, or direct sunlight as much as possible. Care should be 

taken to avoid interference from the bottom of the calibration 

vessel. Follow the manufacturer’s recommendations for 

calibration.

Calibration of the turbidity sensor should be checked in 

three standard solutions before any adjustments are made. If 

the sensor readings exceed the calibration criteria (the greater 

of +5 percent or 0.5 turbidity unit) during the inspection 

process, the sensor must be calibrated. A three-point calibration process is recommended, covering the expected range of 

values, although some instruments may be limited to calibration with only one or two standards. If instrument calibration 

allows only a two-step process, two primary standard solutions 

covering the expected range must be used for calibration 

and a third midpoint standard solution is used to check for 

linearity. Similarly, if the instrument calibration requires 

only turbidity-free water and one standard solution, another 

midpoint standard solution must be used to check for linearity.

Large particles, leaves, twigs, or other natural debris 

may interfere with the measurement of true turbidity by 

causing spikes in the data. Most turbidity sondes designed 

for continuous deployment have a filtering algorithm coded 

in the instrument software that eliminates such spikes. The 

data-processing algorithms may be programmed in the 

sensor software with no options for user input. User-defined 

variables, such as time constants and spike thresholds, may 

be permitted using proprietary algorithms. Anderson (2004) 

describes some instruments that have the capability of providing such statistics as maximum, minimum, mean, median, 

range, and variance of multiple readings over a time span 

of a few seconds. These statistics can be useful for reducing 

variability in recorded turbidity readings, for understanding 

sources of turbidity, or for diagnostic purposes. Anderson 

(2004) cautions that algorithms that are intended to reduce 

spikes in instantaneous data can provide a smoother signal 

than simple instantaneous measurements; however, because 

the algorithms may not be published, these data must be used 

with care and in consideration of the data-quality objectives of 

the study. Because signal averaging to smooth the data output 

alters the instrument response to changes in turbidity readings, 

true changes in turbidity may not be measured. 

Troubleshooting Procedures

When a field parameter cannot be calibrated with 

standard solutions, the hydrographer must determine if the 

problem is with the sensor or the monitor and make the 

necessary corrections to ensure that the monitor is operational. 

The hydrographer should carry spare sensors and sondes so 

that troubleshooting can be accomplished at the time of the 

service visit. Troubleshooting in the field can prevent the 

need for extra trips and greatly reduce record loss and the 

amount of time spent in processing records in the office. A 

successful service trip results in a properly calibrated and fully 

functional monitor. Some of the more common problems that 

are encountered in the field when servicing monitors are listed 

in table 9.

20 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Table 9. Troubleshooting problems with water-quality monitors.

[DO, dissolved oxygen; %, percent]

Symptom Possible problem Likely solution

Water temperature

Thermistor does not read accurately Dirty sensor Clean sensor.

Erratic monitor readings Poor connections at monitor or sensor Tighten connections.

Monitor slow to stabilize Dirty sensor Clean sensor.

Readings off scale Failure in electronics Replace sensor or monitor.

Specific electrical conductance

Will not calibrate Standard solutions may be old or contaminatedUse fresh standard solutions.

Electrodes dirty Clean with soap solution.

Air trapped around sensor Thrust sensors up and down and tap gently to 

expel air.

Weak batteries Replace batteries.

Erratic monitor readings Loose or defective connections Tighten or replace connections.

Monitor requires frequent calibration Broken cables Replace cables.

Replace monitor.

Dissolved oxygen

Meter drift or excessive time for monitor to 

stabilize

Temperature compensator has not equilibrated with temperature of streamWait for temperature equilibration.

Fouled sensor Clean or recondition.

Stirrer or pulse mechanism not working 

properly

Check for obstructions or replace.

Erratic monitor readings Bad connection at monitor or sensor Tighten connections.

Fouled sensor Clean or recondition.

Monitor slow to stabilize Gold cathode tarnished Buff with ink eraser or recondition sensor.

Fouled membrane Recondition sensor and replace membrane.

Silver anode blackened Replace sensor and soak fouled sensor in 

3-percent ammonia for 24 hours.

Monitor will not zero Zero-DO solution contains oxygen Add additional sodium sulfite to zero-DO 

solution.

Zero-DO solution is old Mix a fresh solution.

Monitor will not calibrate Membrane damaged Replace membrane.

Electrolyte diluted Replace membrane and electrolyte.

pH

Meter will not calibrate Buffers may be contaminated Replace buffers.

Faulty sensor Replace sensor.

Slow response time Dirty sensor bulb Clean sensor.

Water is cold or of low ionic strength Be patient.

Erratic readings Loose or defective connections Tighten connections.

Defective sensor Replace sensor.

Turbidity

Unusually high or erratic readings Entrained air bubbles on the optical sensor Follow manufacturer’s directions.

Damaged sensor Replace sensor.

Dirty sensor Clean, following manufacturer’s directions.

Water in connections Dry connector and reinstall.

Water-Quality Monitoring Station Operation  21
Requirements for Field Notes and Instrument 

Logs

Field notes and instrument logs are the basis for 

the accurate and verifiable computation of water-quality 

monitoring records. Legible, detailed, and in-depth field notes 

and instrument logs are essential for accurate and efficient 

record processing. Minimum field-note requirements for 

water-quality monitors include the following items:

Station number and name

Name(s) of data collector(s)

Date, time, and time datum of each set of measurements

Serial or “W” numbers of field meters and monitor 

Lot numbers and expiration dates of calibration 

standard solutions

Purpose of the site visit

Horizontal and vertical locations of sensors in the cross 

section 

Recorded monitor values, field values, and corresponding time and time datum (pre-cleaned, cleaned, calibration checks, calibrations, and final in-stream readings)

Cross-section survey data and(or) vertical-profile data 

(locations of vertical points, measured field parameter 

values, and measurement times), and monitor values 

before and after the cross-section survey

Pertinent gage-height data

Remarks that describe channel and site conditions, 

condition of the sensors, and any other pertinent 

observations

Battery voltage of monitor at arrival and departure (was 

battery replaced?)

Notes on sensor changes or replacements, troubleshooting performed, and other remarks or observations that 

may aid in further processing of the record

The use of field-note forms that include these items 

encourages consistency and helps to avoid the costly omission 

of critical information. The USGS standard field form for 

continuous water-quality monitors contains a comprehensive 

checklist for data collection at many water-quality monitoring sites (Attachment 1). The current version of the USGS 

standard field form can be accessed by USGS personnel at 

http://water.usgs.gov/usgs/owq/Forms.html. Modifications of 

this form or alternative forms must be approved by the USGS 

Water Science Center water-quality specialist or designated 

reviewer.

•

•

•

•

•

•

•

•

•

•

•

•

•

An instrument logbook must be maintained for each 

field meter and water-quality monitor, and all pertinent 

information regarding the monitor and field meter must be 

recorded in the appropriate logbook. Details of instrument 

calibration—both field and laboratory calibrations—are one of 

the most important pieces of recorded information. Calibration 

information can be recorded initially on field forms or in field 

notebooks, but the information then must be copied into the 

instrument logbook. Repair or replacement of sondes, sensors, 

membranes, or modification to the sonde software must be 

recorded in the instrument logbook. The instrument logbook 

must contain a complete record of all maintenance in the field, 

the laboratory, or by the manufacturer. Permanent instrument 

logs contain critical calibration and maintenance information 

that documents instrument performance throughout the service 

life of the instrument. Calibration information that is important 

to log for record processing includes

Sensor repair or replacement;

Calibration dates, times, time datum, and temperatures;

Calibration standard values, expiration dates, and lot 

numbers;

Initial and final monitor-calibration data; and

Field meter calibration values.

Field notes and calibration information should be clearly 

written, and all notations must be self explanatory. The goal 

is to have sufficient information for another individual to be 

able to independently compute the record with similar results. 

Clear notes simplify the record computation and final review 

processes.

Record Computation

The record-computation process verifies the data and 

documents its quality. Accurate field notes and calibration 

logs are essential in processing the record. The hydrographer 

who services the water-quality monitor typically computes the 

water-quality data record, writes the manuscript that describes 

the data, updates the station description and station analysis, 

and prepares the data-record review package.

The primary steps in processing the record are an initial 

data evaluation, removal of erroneous data, application of 

data corrections, and a final data evaluation. The initial data 

evaluation should begin immediately upon completion of the 

field trip to ensure that all necessary information is available 

and to check for possible instrument malfunctions that may 

not have been observed in the field. Immediate corrections 

to obviously erroneous data are required for data that are 

transmitted to the Internet. Application of data corrections 

and record computation are essential within a few weeks of 

servicing a monitor, or more frequently if the data are being 

•

•

•

•

•

22  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
served to the public over the Internet. Record computation and 

final review of the record ensure that the data are accurate and 

ready for publication. 

Data-Processing Procedures

The processing of water-quality monitoring records must 

be completed in a timely manner according to Water Science 

Center or project quality-assurance plan policies. Complete 

and accurate field notes reduce the amount of time required 

to process the data and are an essential part of the process. 

Corrections to data must not be made unless the cause(s) of 

error(s) can be validated or explained in the field notes or by 

comparison with information from adjacent stations (G.F. Ritz, 

Kenneth Butcher, Janine Ferarese, David Grey, John Kuzmiak, 

and William Payne, U.S. Geological Survey, written commun., 

1998). Data processing includes six procedures: (1) initial data 

evaluation, (2) application of data corrections, (3) application 

and evaluation of cross-section corrections, (4) final data 

evaluation, (5) record computation, and (6) final record review. 

The first four procedures are discussed in this section, and 

the last two procedures are combined and discussed in a later 

section (see Preparation of the Review Package).

Initial Data Evaluation

The initial data evaluation is conducted to verify 

the accurate transfer of 

raw field data (instrument 

readings) to the database 

and to evaluate and identify 

erroneous data. A variety 

of formats is available 

to store raw field data, 

depending on the recording 

equipment and the means 

of downloading data from 

the recording equipment. 

Many monitoring sites 

have transmitters that 

provide daily satellite 

downloads of data from 

the field; other monitoring 

sites have data loggers that 

record data for retrieval 

during site visits. Conversion of raw data into a 

standard format for NWIS 

storage is accomplished 

by use of the on-line 

computer program Device 

Conversion and Delivery 

System (DECODES). 

After data are stored in 

NWIS, primary data tables and plots are produced for review 

using the Automated Data Processing System (ADAPS). See 

Attachment 2 for an example of an ADAPS primary data table.

Sensors, recorders, transmitters, receivers, relays, 

transmission systems, or unforeseen events can all produce 

erroneous data. Therefore, data that are automatically 

downloaded, decoded, and reported on the Internet should be 

reviewed daily to remove obvious erroneous data. Regardless 

of how data are recorded and downloaded, the record should 

be processed and plotted immediately after the service visit to 

confirm the accurate transfer of data and to detect instrument 

or sensor error. Missing data (for example, because of instrument or transmission problems) should be documented but not 

estimated (refer to “Use of the program HYDRA to estimate 

or modify unit values in ADAPS,” USGS Office of Water 

Quality Technical Memorandum 2005.03, dated August 8, 

2005, at http://water.usgs.gov/admin/memo/QW/qw05.03.

html). Statistics for periods containing missing data should be 

calculated with care (see Publication Criteria). Figure 5 shows 

an example of a truncated continuous specific conductance 

record caused by exceedance of the data-logger maximum 

default setting of 330 μS/cm. This condition was not noticed 

during routine site visits; and because the record was not 

computed, plotted, and reviewed following service visits, the 

truncated maximum values were not observed until the end of 

the water year. The maximum extremes for the affected record 

Figure 5. Specific conductance record at Duwamish River at Tukwila, Washington, September 5–10, 

1998, was truncated at 330 microsiemens per centimeter, which was the default maximum setting for the 

data logger.

Record Computation  23

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Figure 5. Specific conductance record at Duwamish River at Tukwila, Washington, September 5–10, 1998, was truncated

at 330 microsiemens per centimeter, which was the default maximum setting for the data logger.

340

320

300

280

260

240

220

200

180

160

140

120

2400 0600 1200

5

1800 2400 0600 1200

6

1800 2400 0600 1200

7

1800 2400

SEPTEMBER 1998

0600 1200

8

1800 2400 0600 1200

9

1800 2400 0600 1200 2400

10

1800
were lost, and only minimum daily values could be published. 

A statement was added to the manuscript station description 

explaining that maximum values may have been exceeded 

during periods of missing record (see Data-Qualification 

Statements for details). 

Turbidity sensors have a maximum reporting level that is 

limited by the type of optical sensor and the signal-processing 

electronics. During periods of runoff when high quantities 

of sediment are present and the clarity of the stream has 

been greatly reduced, turbidity increases substantially. It is 

not uncommon for the turbidity sensor to record values at 

its maximum reporting limit. When a turbidity sensor has 

reached its maximum signal output, the same maximum data 

value is recorded repeatedly over time (fig. 6). This period 

can be for a few hours or days depending on the stream 

runoff. Additionally, not all sensors have the same maximum 

reporting level. Maximum turbidity readings commonly range 

from 1,000 to 2,000 NTUs, even with sensors made by the 

same manufacturer. It should be noted, however, that turbidity 

readings that are truncated at a high value may indicate that 

the wiper has parked incorrectly on the optical sensor or the 

sensor may be buried in sediment (fig. 7). The maximum 

sensor response may differ from the reported maximum limit 

(see Data Reporting section).

Great emphasis has been placed on the relation of 

variations in the five water-quality properties (temperature, 

specific conductance, DO, pH, and turbidity) and discharge 

variations, but other event-related changes are equally 

important and can be factored into the relation only through 

historical measurements, field experience, and first-hand 

on-site observation. Some examples include changes in air 

temperature, periods of sustained cloud cover, chemical spills, 

increased photosynthesis (influenced by a variety of factors), 

increased wind conditions, combined sewer overflows, forest 

fires in the watershed, road construction, and ice formation.

As mentioned previously, temperature, specific conductance, DO, pH, and turbidity have complex interdependencies. 

Understanding these relations is a necessary component 

of accurate water-quality record computation. Periodically 

visiting the upstream areas of the drainage basin and recording 

evidence of changes in land-use activities can help explain 

observed changes in water quality.

Application of Data Corrections 

The application of data corrections allows recorded data 

to be adjusted for instrument calibration drift and sensorFigure 6. Turbidity values at the Little Arkansas River at Highway 50 near Halstead, Kansas, October 2003.

24  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data

10,000

5,000

1,000

500

100

50

20

10

5

1

6 7 8 9 10 11 12 15

OCTOBER 2003

13 14 16 17 18 19 20 21 22 23 24 25

TURBIDITY, IN FORMAZIN NEPHELOMETRIC UNITS (FNU)

10,000

5,000

1,000

500

100

50

20

10

5

1

DISCHARGE, IN CUBIC FEET PER SECOND

Discharge

Turbidity (computed)

Sensor maximum response

was approximately 1,685 FNU.

Maximum daily values for

Oct. 9–11 reported as

greater than 1,700 FNU

Erroneous data (measured)

caused by sediment buildup

after the wiper was turned 

off to prevent parking spike 

during the recession

Erroneous data (measured) —

spikes caused by wiper

“parking” on the sensor. Wiper

was turned off
fouling errors that occurred during the interval between 

servicing visits because of environmental or instrumentation 

effects, and for other factors such as cross-section variability 

or calculated parameters (that is, chlorophyll a from fluorescence measurements). Errors occur primarily from fouling of 

sensors and from sensor calibration drift. A data correction is 

required when the combined absolute values for fouling and 

calibration drift error exceed the criterion for water-quality 

data corrections (table 10):

ET Ef Ed = + ,

where

ET = total error,

Ef = sensor fouling error, and 

Ed = calibration drift error.

For example, if the fouling error for a DO sensor is 

+0.2 mg/L and the calibration error (the theoretical value for 

100-percent DO saturation and actual reading in 100-percent 

DO saturation) is – 0.2 mg/L, the sum of the absolute value of 

the errors (0.4 mg/L) exceeds the data-correction criterion for 

DO (± 0.3 mg/L; table 10). Thus, the data must be corrected. 

These data-correction criteria are minimum requirements 

for data corrections. More stringent data-correction criteria 

may be needed to meet the data objectives of the project or 

Figure 7. Turbidity values at the North Fork Ninnescah River above Cheney Reservoir, Kansas, October 2004.

Table 10. Criteria for water-quality data corrections.

[±, plus or minus value shown; °C, degree Celsius; μS/cm, microsiemens per 

centimeter at 25 °C; mg/L, milligram per liter; pH unit, standard pH unit; 

turbidity unit is dependent on the type of meter used]

Measured 

field parameter

Data-correction criteria 

(apply correction when the sum of 

the absolute values for fouling and 

calibration drift error exceeds 

the value listed)

Temperature (may affect 

other field parameters) ±0.2 oC

Specific conductance ±5 μS/cm or ±3% of the measured 

value, whichever is greater

Dissolved oxygen ±0.3 mg/L 

pH ±0.2 pH unit

Turbidity ±0.5 turbidity units or ±5% of the 

measured value, whichever is greater

(1)

Record Computation  25

3,000

1,000

500

100

50

20

10

2

5

1

TURBIDITY, IN FORMAZIN NEPHELOMETRIC UNITS

2400 1200

25

2400 1200

26

2400 1200

27

2400 1200

28

OCTOBER 2004

2400 1200

29

2400 1200

30

2400 1200

31

2400

150

140

130

120

110

100

90

80

70

60

50

DISCHARGE, IN CUBIC FEET PER SECOND

Discharge

Turbidity (measured)

Turbidity

(computed)

Erroneous data because sensor 

was buried in sand

Erroneous data caused by 

wiper parking too close to the 

optical sensor
specific hydrologic and chemical environments. The decision 

to apply data corrections when total errors are smaller than the 

data-correction criteria or to correct for other factors, such as 

cross-section or vertical variability, is left to the professional 

judgment of the hydrographer.

The data-correction procedure is the same whether the 

site is serviced using the standard protocol under steady-state 

or rapidly changing environmental conditions. However, if 

environmental conditions change slowly (without fluctuating) 

during servicing, the procedure for determining data corrections for sensor fouling varies slightly to account for the 

change in environmental conditions. Methods for applying 

data corrections are described in detail below.

All data corrections have a starting date and time and 

an ending date and time that delineate the data correction 

interval. A data correction period typically begins and ends 

on a servicing date. Calibration drift is assumed to occur 

at a constant rate throughout the correction period. Sensor 

fouling commonly begins as soon as the sonde is deployed in 

the aquatic environment; however, if certain environmental 

or hydrologic events, such as a rise in gage height (affecting 

turbidity, for example) or increase in temperature (affecting 

DO, for example), can be identified as significant fouling 

events, the event may be used as the start or end date for a 

fouling correction. Thus, a zero correction is applied at the 

start of the correction interval, the full correction is applied 

at the end of the correction interval, and corrections to the 

data between the start and end dates are linearly interpolated 

(Bartholoma, 2005). 

Data corrections sometimes are started at a time in 

between site visits. An example of a case in which a specific 

conductance record was affected by fouling that begins to 

severely affect the record occurred on March 21 (fig. 8). A 

data correction was applied to the record that began with a 

zero correction on March 21 and culminated in an 18-percent 

correction on April 1 at 1300 hours.

Data corrections for fouling, calibration drift, and crosssection variability are based on measurements made during 

servicing or cross-section surveys. In ADAPS, corrections are 

applied using one of three data-correction sets (Bartholoma, 

2005). Data-correction set one is for sensor fouling, datacorrection set two is for calibration drift, and data-correction 

set three is for “other” data adjustments. Each data-correction 

set may be used to apply constant data corrections, or one-, 

two-, or three-point variable data corrections (Bartholoma, 

2005). Variable data corrections are prorated by time over 

the correction interval. Two- and three-point variable data 

corrections also are prorated by amount of correction over the 

Figure 8. Example of fouling of specific conductance sensor.

26  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data

390

380

370

360

350

340

330

320

310

300

290

80

75

70

65

60

55

50

45 2 4 6 8 10 12 14

0% correction 0% to 18% correction

16

MARCH

2002

APRIL

18 20 22 24 26 28 30 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Site visit

March 5 at

1300

Site visit

March 31 at

1300

DISCHARGE, IN CUBIC FEET PER SECOND

Discharge

Specific conductance

(measured)

Specific conductance

(computed)
15

14

13

12

11

10

9

8

7

6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

SEPTEMBER 2005

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Measured

Computed

WATER TEMPERATURE, IN DEGREES CELSIUS

New datalogger

(wrong offset)

Replaced

datalogger

(correct offset)

range of recorded values and may be based on absolute error 

or percentage error.

A constant data correction is rarely used for water-quality 

records but could be appropriate if, for example, it was known 

that an error in calibration caused recorded values to be in 

error by a constant, known 

amount. Each recorded 

value would be adjusted 

by a single, constant value 

within the correction 

period. An example of a 

situation that needed a constant correction occurred at 

South Fork Tolt River near 

Carnation, Washington, 

when a new data logger 

was installed and incorrectly programmed with 

an offset that resulted in a 

+1.4 difference in recorded 

temperatures (fig. 9). A 

constant data correction 

is applied in ADAPS by 

entering an input and 

correction pair with a 

starting date (table 11; 

fig. 9). To designate that the 

correction continues over 

time without proration, the 

same input and correction 

pair are entered as an 

additional correction 

with the ending date as 

the starting date. This is 

immediately followed 

by a new correction pair 

of 0.0, which resets the 

correction to zero.

Most often, a oneor two-point variable 

data correction is needed. 

For variable data corrections, the amount of 

correction is prorated by 

time over the correction 

interval. For two-point 

variable data corrections, 

the amount of correction 

also is prorated over the 

range of recorded values. 

When the range of 

recorded values is small, 

a one-point variable 

data correction is used 

to linearly interpolate 

the amount of correction 

from zero at the beginning of the correction interval to its 

maximum value at the end of the correction interval. For 

example, a calibration drift or fouling correction of –50 to 

a specific conductance value of 500 μS/cm is not unusual. 

This one-point variable data correction would be prorated 

Table 11. Example of a constant data correction.

[USGS, U.S. Geological Survey; WA, Washington; C, Celsius; CORR, correction; PDT, Pacific Daylight Time zone]

 CORRECTION SET 2 

 USGS 12148000 SOUTH FORK TOLT RIVER NEAR CARNATION, WA 

 Temperature, water, in degrees C WATER YEAR: 2005

 DATES VALID FROM: 10/01/2004 00:00 TO 09/30/2005 23:59

 Enter one of the commands from the menu

********************************************************************************

 START DATE TIME DATUM INPUT CORR INPUT CORR INPUT CORR

 END DATE TIME DATUM COMMENT

 

 6:2005/09/13 1045 PDT 0.0 0.0 ________ ________ ________ ________

 /__/__ ____ ______ _____________________________________________________

 7:2005/09/13 1100 PDT 0.0 -1.4 ________ ________ ________ ________

 /__/__ ____ ______ _____________________________________________________

 8:2005/09/28 1030 PDT 0.0 -1.4 ________ ________ ________ ________

 /__/__ ____ ______ _____________________________________________________

 9:2005/09/28 1115 PDT 0.0 0.0 ________ ________ ________ ________

 /__/__ ____ ______ _____________________________________________________

Figure 9. Example of a constant data correction of –1.4 degrees Celsius applied in September 2005 to 

correct for programming error that produced an incorrect offset at South Fork Tolt River (12148000) near 

Carnation, Washington.

Record Computation  27
over time, beginning with a correction of zero at the start of 

the interval and ending with a correction of –50. One-point 

variable data corrections are applied in ADAPS by entering 

one pair of input and correction values for the beginning and 

another pair of input and correction values for the end of the 

correction period (table 12).

Past practice in the USGS has been to apply one-point 

variable data corrections to water-quality records; however, 

such a correction would have a significant and unintended 

effect on the corrected record if the range of recorded 

measurements was large. In the previous example, if specific 

conductance were to drop as low as 100 μS/cm toward the 

end of the correction period when the correction was near its 

maximum (–50), the correction would result in an adjusted 

value of about 50 μS/cm—a 50-percent adjustment. The more 

accurate approach when the range of recorded values is large 

is to apply a data correction based on percentage error. This is 

a two-point variable data correction based on percentage error. 

When the range of recorded values is large, a two-point 

correction is used to (1) linearly interpolate the amount of 

the correction by time from zero at the beginning of the 

correction period to the maximum value at the end of the 

correction period, and (2) linearly interpolate the amount 

of the correction, based on percentage error, over the range 

of recorded values. In the above example, a maximum 

–10-percent correction (–50 divided by 500) is applied to 

recorded measurements so that a 100-μS/cm value recorded 

near the end of the correction period is adjusted by about 

10 percent to 90 μS/cm. The validity of this concept is most 

evident for environmental extremes, such as 0 °C or zero DO, 

when one-point variable data corrections may give unreasonable (negative) results compared to two-point variable data 

corrections based on percentage. It is recommended that a 

percentage error approach be used for all specific conductance 

records and for temperature, DO, or turbidity records that have 

a large range of environmental sensor values. Most pH records 

have a small range of environmental values, and a one-point 

variable data correction is appropriate with little or no loss in 

accuracy.

Two-point variable 

data corrections based 

on percentage error are 

applied in ADAPS by 

using two input and 

correction pairs for the 

beginning and two input 

and correction pairs for 

the end of the correction 

period that bracket the 

range of recorded values 

during the correction 

interval (table 13). To 

apply the –10-percent data 

correction in the previous 

example, assuming 

recorded data range 

between 50 and 

1,000 μS/cm, two datacorrection pairs (0; 0) and 

(1,000; 0) are entered into 

correction set 1 in line 1 to 

indicate a zero correction 

for the range 0 to 1,000 μS/cm at the beginning of the service 

interval, and two data-correction pairs (0; 0) and (1,000; –100) 

are entered in line 2 to indicate a –10-percent correction for 

the range 0 to 1,000 μS/cm for the end of the correction period 

(table 13). The third line contains two input pairs that reset the 

data corrections to zero, representing the cleaned sensor and 

the start of a new data-correction period.

A three-point variable data correction is used when a data 

correction is required because the sensor response is not linear, 

as determined by calibration checks. Specific conductance, 

temperature, DO, turbidity, and pH sensors are designed by the 

manufacturer to respond linearly. The linearity of temperature 

sensors is nearly synonymous with accuracy. Modern specific 

conductance sensors, for example, are designed by the 

manufacturer to read zero in air, and DO sensors are designed 

to read zero in an oxygen-free environment (in nitrogen gas or 

oxygen-free water). The slope of sensor response from zero is 

linear for specific conductance and DO. Even though the pH 

scale is logarithmic, pH sensors are set to pH-7 at zero millivolts and are designed to respond on a linear millivolts scale 

that is translated to standard pH units. Some turbidity sensors 

have calibration internally set by the manufacturer; whether 

preset or calibrated by the user, however, turbidity sensors are 

calibrated in the field with a zero-turbidity solution and one or 

two higher calibration standard solutions such that the sensor 

responds linearly in the range of environmental values. The 

Table 12. Example of a one-point variable data correction.

[USGS, U.S. Geological Survey; US, United States; Specific cond, specific conductance; μS/cm @ 25C, microsiemens per 

centimeter at 25 degrees Celsius; CORR, correction; PRV, previous; PDT, Pacific Daylight Time zone]

 USGS 12345000 ANYWHERE RIVER AT ANYTOWN,US 

 Specific cond at 25C, in uS/cm @ 25C WATER YEAR: 2005

 DATES VALID FROM: 10/01/2004 00:00 TO 09/30/2005 23:59

 Enter one of the commands from the menu

********************************************************************************

 START DATE TIME DATUM INPUT CORR INPUT CORR INPUT CORR

 END DATE TIME DATUM COMMENT

PRV: None 

 

--------------------------------------------------------------------------------

 1:2005/05/06 1040 PDT 0 0 _______ _______ _______ _______

 /__/__ ____ ______ activated sonde 

 2:2005/06/03 1225 PDT 0 –50 _______ _______ _______ _______

 /__/__ ____ ______ fouling correction 

 3:2005/06/03 1233 PDT 0 0 _______ _______ _______ _______

 /__/__ ____ ______ after cleaning 

--------------------------------------------------------------------------------

28  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
linear response of sensors is an important factor in considering 

how data corrections should be applied to continuous waterquality records. When sensor calibration response is nonlinear, 

strong consideration should be given to replacing the sensor.

An example of 

specific conductance 

sensor response (fig. 10) 

shows how the response 

linearity can be 

quantified using linear 

regression. In general, 

if the coefficient of 

determination (r2) of 

the regression is less 

than 90 percent (r2 less 

than 0.90), a three-point 

variable data correction 

may be warranted; 

however, care should be 

used with this method 

because linear regression can give misleading 

results when the regression line is defined by 

only three data points. 

Linearity of temperature 

sensor response can be 

examined by comparing 

the sensor at five points 

with an NIST-traceable 

mercury-in-glass thermometer as described 

in Radtke and others 

(2004). Linear response 

of turbidity sensors can 

be examined by comparing sensor readings in 

three standard solutions. 

Sensor linearity of pH 

electrodes is measured by 

the electrode response in 

millivolts (slope), which 

can be read on some 

continuous water-quality 

monitors. Because DO 

can only be checked for 

calibration at zero and 

saturation, this method is 

not useful for testing the 

linearity of DO sensor 

response.

A three-point 

variable data correction 

is applied in ADAPS 

similarly to the two-point 

variable data correction 

except that three input and correction pairs are entered into 

the ADAPS data correction set to describe the nonlinear 

relation of the sensor response to calibration. An example of 

a three-point variable data correction is shown graphically in 

Table 13. Example of a two-point variable data correction based on percentage error.

[USGS, U.S. Geological Survey; WA, Washington; Specific cond, specific conductance; μS/cm @ 25C, microsiemens per 

centimeter at 25 degrees Celsius; CORR, correction; PRV, previous; PDT, Pacific Daylight Time zone]

 CORRECTION SET 1 

 USGS 13351000 PALOUSE RIVER AT HOOPER, WA 

 Specific cond at 25C, in uS/cm @ 25C WATER YEAR: 2002

 DATES VALID FROM: 10/01/2001 00:00 TO 09/30/2002 23:59

 Enter one of the commands from the menu

********************************************************************************

 START DATE TIME DATUM INPUT CORR INPUT CORR INPUT CORR

 END DATE TIME DATUM COMMENT

PRV: None 

 

--------------------------------------------------------------------------------

 1:2002/05/06 1040 PDT 0 0 1000 0 ________ ________

 /__/__ ____ ______ activated sonde 

 2:2002/06/03 1225 PDT 0 0 1000 -100 ________ ________

 /__/__ ____ ______ fouling correction 

 3:2002/06/03 1233 PDT 0 0 1000 0 ________ ________

 /__/__ ____ ______ after cleaning 

--------------------------------------------------------------------------------

Figure 10. Linearity of specific conductance sensor response, Palouse River at Hooper, Washington, 

September 10, 2002, showing best-fit regression.

Record Computation  29

0

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

0 100 200 300 400 500 600

y = −0.0148 x −2.27

r 2 = 0.89

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

SENSOR DRIFT ERROR, IN PERCENT

Figure 10. Linearity of specific conductance sensor response, Palouse River at Hooper, Washington,

September 10, 2002 showing best-fit regression.

EXPLANATION

Specific

conductance

 standard

Sensor

reading

Sensor drift

error

(percent) Average

99

252

488

102

270

532

-3.03

-7.14

-9.02

-6.40
figure 11. Two standards have calibration errors that exceed 

the data-correction criteria (±5 μS/cm or ±3 percent of the 

measured value, table 10). The third standard has a calibration 

error (2.5 percent) that does not exceed the data correction 

criteria. In figure 11 and table 14, corrections are shown for 

all three standards. The quality of the record at 667 μS/cm 

and greater is excellent (see Publication Criteria) and does 

not require a data correction, but applying a correction for 

the range between 429 and 667 μS/cm does affect the record. 

Thus, it is necessary to apply a correction for the value of 

667 μS/cm in order to correct data for the range between 429 

and 667 μS/cm. The hydrographer and reviewers must use 

knowledge and experience in making decisions about when to 

apply data correction.

Figure 11. Graphic representation of a three-point data correction for nonlinear 

calibration drift.

100

200

300

400

500

600

700

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

CALIBRATION DRIFT CORRECTION, IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

Figure 11. Graphic representation of a three-point data correction for non linear

calibration drift.

-30 -20 -10 0 10 20 30

CALIBRATION CHECK

Standard solution

120

400

650

Reading

130

429

667

Correction

(in percent)

-10 (7.7%)

-29 (6.8%)

-17 (2.5%)

30 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Corrections for Fouling

Sensor-fouling error can result from a variety of sources 

and is determined by the difference between sensor measurements in the environment before and after the sensors are 

cleaned. Assuming steady-state conditions in the environment 

in which the sensor is being checked, the calculation for 

fouling error is as follows: 

Ef Ma Mb = – ,

where

Ef = sensor fouling error,

Ma = monitor reading after the sensor is cleaned, and

Mb = monitor reading before the sensor is cleaned.

Environmental conditions rarely are stable. Water 

temperature, pH, and DO often have daily cycles based on 

thermal warming and cooling, biological photosynthesis, 

and respiration. Field parameters also change in response 

to dilution or input from upstream tributaries, ground-water 

discharge or recharge, and precipitation. If environmental 

conditions change while the monitor is being serviced but do 

not fluctuate, the fouling error must be determined by modifying equation 2 to account for the change in the environment 

during servicing:

Ef Ma Mb ( ) – Fe Fs = – ( ) – ,

where

Ef = sensor fouling error,

Ma = monitor reading after the sensor is cleaned, 

Mb = monitor reading before the sensor is cleaned,

Fs = field meter reading at the start of servicing, and

Fe = field meter reading at the end of servicing.

(Because (Fe – Fs) = 0 when field readings do not change during servicing, equation 3 reduces to equation 2 under steadystate conditions.) For example, if the DO sensor reading before 

cleaning is 10.2 mg/L, and the sensor reading after cleaning 

is 10.4 mg/L, and the field meter changes during the servicing 

period from 10.6 to 10.3 mg/L (without any fluctuations), the 

fouling error, using equation 3, is as follows:

Ef = (10.4 – 10.2) – (10.3 – 10.6)

Ef = (0.2) – (–0.3) 

Ef = 0.5

Notice that the apparent change in the sensor readings of 

0.2 mg/L (from 10.2 to 10.4 mg/L) when the sensor is cleaned 

is less than the actual fouling error of 0.5 mg/L when the 

environmental change is factored in.

Fouling is assumed to occur at a constant rate and 

typically is assumed to begin at the start of the service interval 

when the sensor was last cleaned. It may be possible, however, 

Table 14. Example of a three-point variable data correction for nonlinear calibration drift.

[USGS, U.S. Geological Survey; US, United States; Specific cond, specific conductance; μS/cm @ 25C, microsiemens per 

centimeter at 25 degrees Celsius; CORR, correction; PRV, previous; PDT, Pacific Daylight Time zone]

 CORRECTION SET 2 

 USGS 12345000 ANYWHERE RIVER AT ANYTOWN,US 

 Specific cond at 25C, in uS/cm @ 25C WATER YEAR: 2005

 DATES VALID FROM: 10/01/2001 00:00 TO 09/30/2002 23:59

 Enter one of the commands from the menu

********************************************************************************

 START DATE TIME DATUM INPUT CORR INPUT CORR INPUT CORR

 END DATE TIME DATUM COMMENT

PRV: None 

 

--------------------------------------------------------------------------------

 1:2001/10/04 1515 PDT 130 0 429 0 667 0

 /__/__ ____ ______ ____________________________________________________ 

 

 2:2001/11/07 1525 PDT 130 -10 429 -29 667 -17

 /__/__ ____ ______ ____________________________________________________ 

 

 3:2001/11/07 1530 PDT 130 0 429 0 667 0

 /__/__ ____ ______ ____________________________________________________ 

 

--------------------------------------------------------------------------------

(2)

Record Computation  31

(3)
to identify a specific event, 

such as a rise in stream 

stage, change in temperature, or sudden change in 

sensor reading that marks 

the onset of fouling. From 

equation 1, if the absolute 

value of the fouling error 

(Ef) plus the absolute value 

of the calibration drift 

error (Ed; described below) 

exceed the data-correction 

criteria (table 10), a data 

correction is required. 

Because fouling takes 

place gradually, it typically 

is prorated from the start to 

end date of the correction 

period as illustrated in 

figure 12 for specific 

conductance. ADAPS 

data-correction set 1 

(sensor fouling) should be 

used for applying fouling 

data corrections.

If the range in 

environmental values is 

small, a one-point variable 

data correction may be appropriate. When using a one-point 

variable data correction, the fouling correction equals the 

fouling error:

Cf Ef Ma Mb ( ) – Fe Fs = = – ( ) – ,

where

Cf = fouling correction, 

Ef = fouling error,

Ma = monitor reading after the sensor is cleaned,

Mb = monitor reading before the sensor is cleaned,

Fs = field meter reading at the start of servicing, and 

Fe = field meter reading at the end of servicing.

For example, if the DO sensor reading before cleaning is 

10.1 mg/L, and the sensor reading after cleaning is 10.5 mg/L, 

and the field meter reading is 10.6 mg/L during the servicing 

period (without change), the data correction for fouling, using 

equation 4, is as follows:

Cf = (10.5 – 10.1) – (10.6 – 10.6) 

Cf = 0.4

Record computation can be simplified by inserting this 

equation into a spreadsheet and using the spreadsheet as a 

measurement summary form (Attachment 3). In this example, 

the fouling correction exceeds the data-correction criteria.

When the range of values is large over the service 

interval, it is appropriate to apply a two-point variable data 

correction based on percentage error:

%Cf 100

Ma Mb ( ) – Fe Fs – ( ) –

Mb

------------------------------------------------------ ⎝ ⎠ ⎛ ⎞ = ,

where

%Cf = fouling correction, in percent,

Ma = monitor reading after the sensor is cleaned,

Mb = monitor reading before the sensor is cleaned,

Fs = field meter reading at the start of servicing, and

Fe = field meter reading at the end of servicing.

To calculate the percentage error using the above example and 

equation 5: 

%Cf 100 ( ) 10.5 10.1 – – ( ) 10.6 10.6 –

10.1 --------------------------------------------------------------------- ⎝ ⎠ ⎛ ⎞ =

%Cf 100 ( ) 0.4 – ( ) 0

10.1 ------------------------- ⎝ ⎠ ⎛ ⎞ =

%Cf = 4.0

Figure 12. Data corrections for fouling of a specific conductance sensor at Palouse River at Hooper, 

Washington, May–June 2002.

(4)

(5)

32  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

300

280

260

240

220

200

180

160

140

120

100

6 7 8 9 10 11 12 14 16

MAY

2002

JUNE

13 15 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3

Specific conductance

(measured)

Specific conductance

(computed)

Data missing due to

faulty wiring Site visit
Modern temperature sensors provide continuous 

temperature corrections to specific conductance, DO, pH, 

and turbidity values, all of which are temperature dependent. 

Because of the complex interdependencies of temperature 

and these other measured field parameters, a malfunctioning 

thermistor can invalidate some or all of the other recorded field 

parameters. DO concentrations, for example, change 3 percent 

for each temperature change of 1 °C (Lewis, 2005). Specific 

conductance also can vary as much as 3 percent per 1 °C. 

Thus, the record for a faulty temperature sensor may have to 

be deleted along with associated records that depend on the 

temperature sensor, such as DO, pH, and specific conductance. 

It is fortunate that temperature sensors are the least prone to 

fouling of any sensor. The fouling data correction for temperature sensors, if needed, typically is a one-point variable data 

correction; however, if the range in environmental values of 

temperature is large, a two-point variable data correction based 

on percentage error may be more appropriate.

In general, specific conductance sensors are sturdy, easy 

to clean, and reliable. The fouling correction recommended 

for specific conductance generally is a two-point variable data 

correction based on percentage error.

The DO fouling error correction for the appropriate part 

of the service interval when fouling occurred generally is a 

one-point variable data correction in ADAPS. If a large range 

in DO concentrations was recorded, however, a two-point 

variable data correction based on percentage error may be 

more appropriate. The determination of onset and magnitude 

of fouling error for DO may range from straight forward (for 

example, onset of a flood) to complex (for example, gradual 

growth of an algal coating).

A fouling correction for turbidity generally is applied as 

a one-point variable data correction. If the range in values of 

turbidity is large, however, a two-point variable data correction 

based on percentage error may be more appropriate. The optic 

mechanism on the turbidity sensor is extremely sensitive to 

accumulation of sediment or algae. If a turbidity sensor is 

equipped with a wiper or shutter that is properly maintained, 

the need for a fouling correction can be minimized.

Continuous records of DO and turbidity often are the 

most difficult to maintain within acceptable limits for accuracy 

and precision because of the susceptibility of these sensors 

to fouling. The time between service visits may need to be 

shortened to maintain adequate quality of the DO or turbidity 

record.

Fouling of a pH sensor can be similar to the fouling of 

a DO membrane. The pH sensor must be cleaned delicately in 

order to avoid scratching the glass surface. The recommended 

data correction for pH fouling is a one-point variable data 

correction.

Corrections for Calibration Drift

Calibration drift error is the result of an electronic drift in 

the sensor reading from the last time the sensor was calibrated 

and is determined by the difference between cleanedsensor readings in standard solutions or buffers and the true, 

temperature-compensated value of the standard solution or 

buffer. Using the formula that describes the standard protocol 

for slowly changing conditions, the percentage correction is as 

follows:

Ed Vs Vc = – ,

where

Ed = calibration drift error,

Vs = value of a calibration standard solution or 

buffer of known quality (for DO, the standard 

value is represented by the DO 100-percent 

saturation value), and,

Vc = sensor reading in the calibration standard 

solution.

Calibration drift is assumed to occur at a constant rate 

during the service interval. If the calibration drift plus fouling 

error is within the data-correction criteria (table 10), corrections are not required, but if the calibration drift plus fouling 

error exceeds the data-correction criteria (table 10), a correction is required. ADAPS data-correction set two (calibration 

drift) should be used to apply calibration drift data corrections 

(table 14). Before any corrections are made, proper operation 

of the monitor must be verified for correct calibration ranges, 

sufficient battery voltage, and any other variables that may 

affect operation. The calibration drift correction equals 

calibration drift error and is computed as follows: 

Cd Ed Vs Vc = = – ,

where

Cd = calibration drift correction

Ed = calibration drift error,

Vs = value of a calibration standard solution or 

buffer of known quality (for DO, the standard 

value is represented by the DO 100-percent 

saturation value), and,

Vc = sensor reading in the calibration standard 

solution.

When the correction is applied as a linear interpolation 

over the correction interval, it is called a two-point variable 

data correction in ADAPS (Bartholoma, 2005) and is 

illustrated for specific conductance in figure 13 and for DO in 

figure 14.

(6)

(7)

Record Computation  33
Figure 13. Two-point variable data correction for calibration drift of a specific conductance sensor 

at Palouse River at Hooper, Washington, August–September 2002.

Figure 14. Two-point variable data correction for calibration drift of a dissolved oxygen sensor at 

Little Arkansas River near Sedgwick, Kansas, March–April 2004.

34  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

6 7 8 9 10

AUGUST

2002

SEPTEMBER

20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5

Specific conductance

(measured)

Specific conductance

(computed)

Site visit

380

370

360

350

340

330

320

310

300

290

Site visit

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

6 7 8 9 10 11 12 13

MARCH

2004

APRIL

22 23 24 25 26 27 28 29 30 31 1 2 3 4 5

Dissolved oxygen

(measured)

Dissolved oxygen

(computed)

22

20

18

16

14

12

10

8

6

4
If the range in recorded values is small, a one-point 

variable data correction may be appropriate. As with fouling 

corrections, however, the appropriate data correction for 

calibration drift generally is a two-point variable data correction based on percentage error such that 

%Cd 100

Vs Vc –

Vc

----------------- ⎝ ⎠ ⎛ ⎞ = ,

where

Vs = value of a calibration standard solution or 

buffer of known quality (for DO, the standard 

value is represented by the DO 100-percent 

saturation value), and 

Vc = sensor reading in the calibration standard 

solution. 

If calibration checks with NIST-traceable calibration 

standard solutions or buffers verify that sensor response is not 

linear, the sensor may be faulty and may need to be repaired 

or replaced. Linearity of sensor response can be graphically 

displayed by plotting observed sensor response against 

actual values of the standards; for example, observed specific 

conductance sensor response can be plotted against specific 

conductance standard solution values, as described earlier and 

shown in figure 10. Similar plots of sensor response can be 

displayed for any field parameter that is measured with three 

or more standard solutions (or observations, in the case of 

temperature comparisons). If the sensor response is not linear, 

a three-point variable data correction should be used rather 

than a one- or two-point variable data correction. 

To apply a three-point variable data correction, three 

pairs of data corrections must be entered in ADAPS as value 

adjustments for a measured field parameter (table 14). This 

value-dependent correction is applied in situations where, at 

the beginning and(or) ending of the correction interval, sensor 

calibration percentage error is different for different standards, 

which reflects a nonlinear sensor response. The equivalent 

situation for computation of streamflow in ADAPS is called 

a variable shift (Bartholoma, 2005). A graphic representation 

of a three-point variable data correction based on percentage error for specific conductance calibration is shown in 

figure 11. Three-point variable data correction can be used 

for any field parameter and is the recommended technique 

for corrections related to instrument calibration drift if the 

sensor response is not linear. Use of a three-point variable 

data correction implies that the sensor response is not linear. 

Calibration data that reflect nonlinear sensor response may 

be caused by a malfunctioning sensor, contaminated standard 

solutions, or errors in calibration. Careful attention should 

be given to correcting potential errors and observing sensor 

response during subsequent servicing. If the sensor response 

continues to be nonlinear, consideration should be given to 

replacing the sensor. Improvements in electronic devices in 

the past two decades have greatly reduced the amount of true 

electronic calibration drift. 

Other Corrections

The most common “other” data corrections to continuous 

water-quality records are adjustments to stream water-quality 

records based on correlations between point measurements 

and cross-section average measurements. This category of 

data corrections also may be appropriate for data corrections 

that involve ancillary laboratory measurements, such as 

chlorophyll sensors or in-situ nitrate analyzers. Cross-section 

corrections may not be applicable for environments with poor 

mixing, upstream tributaries, point and nonpoint discharges, 

or other natural conditions that negate the use of cross-section 

corrections. In these environments it can be extremely 

difficult, if not impossible, to characterize variability with 

any degree of confidence or predictability. For these reasons, 

cross-section corrections may not be applicable for urban sites 

monitored for regulatory compliance or for sites that have 

widely varying coefficients that have no consistency or predictability. Project or data-quality objectives may require that 

measured values at the sonde location be adjusted because of 

variability in measured values in the cross section or because 

of a significant difference between values recorded at the 

sonde location and average or median values measured in the 

cross section. Such cross-section adjustments may be needed 

to provide better estimates of constituent loads or regression 

estimates (Christensen and others, 2000), or simply to provide 

more accurate calculated values if the water-quality monitor 

cannot be located at a point that is representative of the stream. 

If the variability of discharge-weighted values of measured 

field parameters compared to the measured values at the 

sonde locations exceeds the data-correction criteria (table 10), 

cross-section data correction may be needed (see Evaluation 

of Cross-Section Data Corrections). ADAPS data-correction 

set three (other) should be used for other data corrections, 

such as cross-section corrections. The analysis and statistical 

summaries of cross-section variability should be documented 

in the annual station analysis (see Station Analysis).

Evaluation of Cross-Section Data Corrections

If the measurement point is not representative of the 

stream, the measurement point should be relocated to a more 

representative measuring point in the cross section. If a 

representative measuring point cannot be found, consideration 

should be given to moving the monitor to a better site. Location of the monitoring site, however, may be required 

for regulatory reasons, cooperator’s objectives, or other 

(8)

Record Computation  35
data-quality objectives. Another option that should be 

considered before applying cross-section corrections is 

downgrading the rating of accuracy for a monitoring record 

(see Publication Criteria). If cross-section data show that 

the site is not representative of particular flows or seasons, 

these particular portions of the record could be downgraded in 

ratings of accuracy. Professional judgment and observations in 

the field logs will aid in evaluating the need for the application 

of cross-section corrections. Details for application of crosssection corrections are described in Attachment 4.

Computation of Salinity

The separate data processing of both specific conductance and salinity records is time consuming and can lead to 

errors. Salinity should be calculated from fully processed and 

corrected continuous specific conductance records. If specific 

conductance values have been compensated to 25 °C and 

water depths are sufficiently shallow making pressure corrections unnecessary, salinity can be calculated from processed 

specific conductance records using standard equations. 

Schemel (2001), following principles of the 1978 Practical 

Salinity Scale, simplified the general equation for salinity (S) 

described by Lewis (1980) for the case of a single temperature 

(25 °C) and atmospheric pressure (760 mm):

S K1 K2 R1/2 ( ) × K3 ( ) × R K4 R3/2 ( ) × K5 R2 ( ) ×

K6 R5/2 ( ) × ,

+ + + +

+

=

where

K1 = 0.0120,

K2 = – 0.2174,

K3 = 25.3283,

K4 = 13.7714,

K5 = – 6.4788, and

K6 = 2.5842.

The variable R is the ratio of specific conductance at 25 °C to 

standard seawater (salinity equals 35) at 25 °C (53.087 millisiemens per centimeter). Salinity expressed by PSS is a dimensionless value, although it is commonly reported as practical 

salinity units and is nearly equivalent to parts per thousand.

Salinity can be processed from corrected specific 

conductance records and entered into the NWIS ADAPS 

database (Bartholoma, 2005) using a rating table. The following 30-point rating table (table 15) defines the conversion of 

specific conductance in microsiemens per centimeter to salinity in practical salinity units for measurements at atmospheric 

pressure (Schemel, 2001).

Before the development of the Practical Salinity Scale, 

salinity commonly was reported in parts per thousand. The 

values in practical salinity units and parts per thousand 

are nearly equivalent. The following 30-point rating table 

(table 16) defines the conversion of specific conductance in 

microsiemens per centimeter to salinity in parts per thousand 

for measurements at atmospheric pressure (Miller and others, 

1988).

Table 15. Rating table for conversion of specific conductance, in microsiemens per centimeter, to salinity, in practical salinity units.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; psu, practical salinity units; (90860), ADAPS parameter code for salinity in psu]

Specific 

conductance, 

in μS/cm

Salinity, in psu 

(90860)

Specific 

conductance, 

in μS/cm

Salinity, in psu 

(90860)

Specific 

conductance, 

in μS/cm

Salinity, in psu 

(90860)

100 0.046 11,000 6.233 38,000 24.099

300 0.142 13,000 7.464 41,000 26.220

500 0.240 15,000 8.714 44,000 28.364

700 0.340 17,000 9.981 47,000 30.532

1,000 0.492 20,000 11.911 50,000 32.722

2,000 1.016 23,000 13.873 53,000 34.935

3,800 2.001 26,000 15.865 56,000 37.172

5,000 2.679 29,000 17.885 59,000 39.430

7,000 3.836 32,000 19.931 62,000 41.712

9,000 5.022 35,000 22.003 65,000 44.016

(9)

36  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Computation of Percentage of Dissolved Oxygen 

Saturation

Many sondes equipped with DO sensors also may record 

DO percentage of saturation values that are calculated by the 

sonde software. The separate processing of records for both 

continuous DO and continuous DO percentage of saturation is 

time consuming and can lead to inconsistent data corrections 

between the two records. If data corrections are applied to 

the DO record, similar corrections must be applied to the DO 

percentage of saturation record. The USGS recommends that 

DO percentage of saturation be calculated from fully processed and corrected continuous records of DO, temperature, 

and specific conductance. This calculation can be facilitated 

by using computer programs specifically developed to use 

ADAPS-computed DO records to calculate DO percentage of 

saturation using the equations of Weiss (1970). The programs 

and instructions required for this process are posted for USGS 

personnel at http://sr.water.usgs.gov/qw/qwmonitors/.

Final Data Evaluation

Final data evaluation consists of reviewing the data 

record, checking data corrections, and making any needed 

final corrections. When review is completed, the data are 

verified for publication and rated for quality. Data that cannot 

be verified or are rated as unacceptable are retained for 

record checking and review purposes but are not published 

or stored in ADAPS (except in raw or measured format). All 

data must be archived following policies established in the 

quality-assurance plan of each Water Science Center.

Maximum Allowable Limits for Reporting Continuous Data

Systematic adoption of a standardized final dataevaluation process, including maximum allowable limits and 

publication criteria, are important components in finalizing a 

Water Science Center’s water-quality records. Many USGS 

Water Science Centers have established quality-control 

limits for correcting data. These commonly are referred to 

as “maximum allowable limits.” The concept is simple: if 

the recorded values differ from the corrected values by more 

than the maximum allowable limits, the corrected data are not 

reported or stored in the database. For consistency within the 

USGS and to inform data users of the maximum allowable 

limits, the limits are established at 6–10 times the calibration 

criteria for all standard continuous-monitoring data-collection 

activities (table 17).

Table 16. Rating table for conversion of specific conductance, in microsiemens per centimeter, to salinity, in parts per thousand.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ppt, parts per thousand; (00480), ADAPS parameter code for salinity in ppt]

Specific 

conductance, 

in μS/cm

Salinity, in ppt 

(00480)

Specific 

conductance, 

in μS/cm

Salinity, in ppt 

(00480)

Specific 

conductance, 

in μS/cm

Salinity, in ppt 

(00480)

100 0.040 11,000 6.229 38,000 24.124

300 0.131 13,000 7.463 41,000 26.252

500 0.226 15,000 8.714 44,000 28.405

700 0.324 17,000 9.983 47,000 30.582

1,000 0.474 20,000 11.914 50,000 32.783

2,000 0.997 23,000 13.878 53,000 35.008

3,800 1.984 26,000 15.872 56,000 37.257

5,000 2.664 29,000 17.895 59,000 39.530

7,000 3.826 32,000 19.945 62,000 41.826

9,000 5.016 35,000 22.022 65,000 44.146

Table 17. Maximum allowable limits for continuous waterquality monitoring sensors.

[±, plus or minus value shown; oC, degree Celsius; %, percent; mg/L, milligram per liter; pH unit, standard pH unit. Data corrections that exceed the 

maximum allowable limits should not be stored in the database]

Measured field 

parameter

Maximum allowable limits for waterquality sensor values

Temperature ±2.0 °C

Specific conductance ±30% 

Dissolved oxygen ±2.0 mg/L or 20%, whichever is greater

pH ±2 pH units

Turbidity ±3.0 turbidity units or ±30%, whichever 

is greater

The maximum allowable limits shown in table 17 are 

considered minimum standards, and Water Science Centers 

are encouraged to establish stricter requirements. Even with 

the establishment of maximum allowable limits, professional 

judgment by the hydrographer is still needed in record 

Record Computation  37
processing. For example, a decision must be made regarding at 

what point in the data-collection interval the error-correction 

value exceeded the maximum allowable limit. Tools, such 

as the ADAPS “Display expanded shifts and corrections 

(SHIFT_TABLE)” described by Bartholoma (2005), can assist 

the hydrographer in identifying this point (see Publication 

Criteria). In the cases of calibration drift, gradual fouling, 

or event-related fouling, reasonable judgment can be made 

regarding the time in the record interval when the maximum 

allowable limits were exceeded. The hydrographer computing 

the record and the record reviewer must agree on this determination and note the time when the maximum allowable limits 

were exceeded on the station analysis sheet. Values between 

this point and the end of the data-correction interval should not 

be published.

It generally is understood that the more publishable 

record there is for a site, the more consistently the data 

objectives can be met for accuracy and precision. Therefore, 

it follows that it may take some time to finalize the appropriate intervals between service visits at new installations to 

minimize inaccurate data and missing record. Decisions about 

the costs of more frequent service visits in relation to potential 

decreases in accuracy and precision in the record must be 

considered carefully for every water-quality monitoring site in 

relation to various seasons and flows.

Publication Criteria

An assessment of the overall accuracy of the record is 

required for the publication of all continuous water-quality 

records in the USGS annual data reports or in other publication formats. The accuracy rating is based on data values 

recorded before any data corrections are made (table 18). After 

the record has been evaluated and data corrections applied 

(based on table 10), one of four accuracy classifications 

ranging from excellent to poor is applied to each measured 

field parameter. 

The accuracy of the daily record can be calculated in 

some cases by using the SHIFT_TABLE program in ADAPS, 

as described by Bartholoma (2005). Accuracy ratings that are 

measured in concentration or applicable units (water temperature, pH, and perhaps DO or turbidity) can be applied directly 

by using the expanded shift and corrections for the range of 

environmental concentrations or values during the period of 

the rating record. Details for this procedure and for ratings that 

are measured in percentages are described in Attachment 5.

Additional consideration for rating the accuracy of the 

record must be based on an assessment of the amount of 

publishable record, the amount of data that have been corrected, and the quality-control (or lack of) data used during the 

monitor operation. The amount of publishable record indicates 

how well the instrument or sensor functioned during the 

period of record. Less confidence should be placed in a record 

that has frequent data gaps or insufficient quality-control data. 

A record that requires frequent, substantial data corrections 

may indicate that sensor fouling is a major source of error, or 

the sensor has calibration-stability problems. Downgrading the 

quality of the record may be appropriate in cases of frequent 

data gaps. Missing data or data that do not meet the criteria for 

maximum allowable limits may have a varied range of effects 

on the accuracy of the published data, depending on the situation; the decision to publish the data is left to the professional 

judgment of the hydrographer. To publish a daily value for any 

field parameter, the following minimum requirements apply:

1. To publish a daily mean or median value on days when 

less than 100 percent of the discrete time interval data are 

collected, both the expected daily maximum and minimum values must be present. Reporting daily mean values 

when less than 100 percent of the unit values are available is a decision based on professional judgment of the 

Table 18. Accuracy ratings of continuous water-quality records.

[≤, less than or equal to; ±, plus or minus value shown; °C, degree Celsius; >, greater than; %, percent; mg/L, milligram per liter; pH unit, standard pH unit]

Measured 

field parameter

Ratings of accuracy

(based on combined fouling and calibration drift corrections applied to the record)

Excellent Good Fair Poor

Water temperature ≤±0.2 °C >±0.2–0.5 °C >±0.5 – 0.8 °C >±0.8 °C

Specific conductance ≤±3% >±3–10% >±10 – 15% >±15 %

Dissolved oxygen ≤±0.3 mg/L or ≤±5%, 

whichever is greater

>±0.3–0.5 mg/L or 

>±5–10%, whichever 

is greater

>±0.5 – 0.8 mg/L or 

>±10 – 15%, whichever 

is greater

>±0.8 mg/L or 

>±15%, whichever is 

greater

pH ≤±0.2 units >±0.2–0.5 units >±0.5 – 0.8 units >±0.8 units

Turbidity ≤±0.5 turbidity units or 

≤±5%, whichever is 

greater

>±0.5–1.0 turbidity units 

or >±5 –10%, whichever is greater

>±1.0 – 1.5 turbidity units 

or >±10–15%, whichever is greater

>±1.5 turbidity units or 

>±15%, whichever is 

greater

38  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
hydrographer and generally is determined by data trends, 

knowledge of site characteristics, and the influence of 

hydrologic events.

2. Daily maximum and minimum values may be published 

independently if they occur during the expected times and 

if the reported values can reasonably be expected to be 

the true extremes (Novak, 1985). Additionally, there must 

be sufficient adjacent values to confirm that the value is a 

daily extreme, and there should be no inflows or effluents 

upstream from the sensor that would disrupt a normal 

daily pattern. The proportion of missing data should not 

be the sole criterion for publication, but knowledge of site 

characteristics and the consequences of flow and hydrologic events on the measured field parameters should be 

taken into consideration in defining measured extremes of 

the physical phenomena being measured. The following 

criteria are suggested as a starting point for making decisions on publishing daily extremes:

Water temperature—Presence of a minimum of 

25 percent of the unit values, and flow that is gradually 

changing or at a steady state.

Specific conductance—Presence of a minimum of 

50 percent of the unit values, and flow that is gradually 

changing or at a steady state. Consider the relation of 

specific conductance to streamflow.

Dissolved oxygen—Presence of a minimum of 

25 percent of the unit values, and flow that is gradually 

changing or at a steady state. Consider the relation of 

dissolved oxygen to temperature and pH.

pH—Presence of a minimum of 25 percent of the unit 

values, and flow that is gradually changing or at a 

steady state.

Turbidity—Presence of a minimum of 50 percent of 

the unit values, and flow that is gradually changing or 

at a steady state. Consider the relation of turbidity to 

specific conductance and streamflow.

The current station-description format for the USGS 

annual data report requires a description of the specific service 

interval ratings (Novak, 1985). The record-rating system 

described in table 18 must be systematically applied to all 

continuous water-quality monitoring data published in USGS 

reports. Accuracy ratings are listed in the “REMARKS” 

section of the manuscript for USGS annual data reports (see 

Data Reports). The appropriate rating and accuracy of each 

water-quality monitoring record also must be included on the 

station analysis form (see Preparation of the Review Package).

The calculation of daily mean pH values in ADAPS 

should be discontinued in USGS water-quality data-collection 

programs and replaced by the calculation of median daily pH 

values or a more accurate computational method for calculating mean pH values. The correct calculation involves the 

•

•

•

•

•

conversion of pH units to hydrogen-ion activity, calculation 

of a mean activity value, and conversion back to pH units, as 

described by Radtke and others (2003). 

Preparation of the Review Package

Typically, the same individual who services the waterquality monitoring station also computes the water-quality 

monitoring record, writes the water-quality station analysis 

that describes annual operation of the station, makes necessary 

changes in the station description, and prepares a package of 

auxiliary information to aid in the review of the record.

Record Checking

All data used in producing the final water-quality record 

must be checked thoroughly for completeness and accuracy 

before final review and publication. The hydrographer 

who is responsible for computing the water-quality record 

should review the record, followed by a second independent 

review for completeness and accuracy by an experienced 

hydrographer. All field data must be verified for accuracy of 

transcription from field sheets; all data corrections must be 

checked to assure that the correct values were used for a data 

correction; and all dates and numbers in the station manuscript 

must be checked for accuracy. Final inspection and review of 

the water-quality record should be made by the USGS Water 

Science Center water-quality specialist or designated reviewer.

Station Description

Specific information about each field site (station) is 

documented in a station description (Attachment 6). The 

station description contains specific information about the 

station location, history, and operation and should be placed 

in a protective field folder and carried into the field. Any 

changes in location, instrumentation, operation, or land use 

must be recorded in the station description. For some stations, 

service intervals may be a week or less to meet data-quality 

objectives. The measurement frequency must be included in 

the station description and in the USGS annual data report 

manuscript, as described by Novak (1985). At sites that also 

have a streamflow gage, this information can be merged with 

the existing gaging-station description. A complete description 

is prepared for each new station and is revised only when 

changes in location occur or when changes are substantial 

enough to affect how the continuous monitor is operated 

or how the data are interpreted. Although the contents of a 

continuous water-quality station description can vary, the 

station description must include the following at a minimum: 

Location of station

Station history

Drainage area (including basin characteristics)

•

•

•

Record Computation  39
Channel characteristics (including any unique or 

unusual features of the installation)

Station description, equipment, and benchmarks

Methods for making discharge and water-quality 

measurements

Historical extremes

Cross-section surveys (discharge and water quality)

Purpose and cooperation

Maps, photographs, and permits

Safety hazards, detailed road log, and access remarks

Date and initials of preparer and reviewer(s)

The station description should enable a person unfamiliar 

with the site to find the site, know what equipment was used 

and for what periods, and where sensors are and were located 

throughout the history of the site. A person should also be able 

to track major land-use changes that may affect the continuous 

water-quality data at the site based on information in the 

station description (Attachment 6).

Station Analysis

Each continuous-monitoring station requires an annual 

station analysis (Attachment 7). The purpose of the station 

analysis is to document conditions at the site, how the record 

was compiled and manipulated, and how this information was 

used in analyzing the accuracy and completeness of the record 

for the water year. Information from field notes, instrument 

logs, ADAPS output, and the station description are used to 

evaluate the accuracy and completeness of the record. Records 

of cross-section surveys and extreme values must be updated 

annually in the station analysis (Attachment 7), and analysis 

of cross-section statistical summaries must be included in the 

station analysis as part of the data-evaluation process. At a 

minimum, the station analysis must include the following:

Sampling location—Station name and number, year, 

field parameter measured, and measurement frequency

Instrumentation—Type of monitoring sensor(s) and 

recorder, location of sensor(s) or intakes, any other 

special instrumentation or features, and dates sensors 

were changed or replaced

Published records—Identification of the field 

parameter that is published, as well as any data that 

are collected and not published

Site characteristics—Brief description of the composition of the channel and any unique or unusual 

features that may affect the quality of the record

•

•

•

•

•

•

•

•

•

•

•

•

•

Record—Statement of the completeness of the record 

and how the raw measured record is decoded or 

manipulated to provide the constituent values used 

to compute the final records (Note all missing data 

and the reason it is missing. Include any data that are 

collected but not published. Include the final rating of 

accuracy for the period of record (or specific service 

intervals) in chronological order.)

Calibration—How and when the instrument was 

calibrated or checked, including statement if 

corrections were needed for the data

Computations—Statement of how corrections were 

applied to the data, a list of extreme correction values 

that were applied to the recorded values during the 

year, and causes for correc­tion(s) (Any treatment of 

unusual or atypical data should be documented.)

Cross-section surveys—How, where, and when the 

cross section was measured, the number of verticals, 

and the amount of variation throughout the cross 

section (State whether the location of the sen­sor(s) or 

water-supply intake is representative of the stream.)

Vertical profiles—How, where, and when the vertical 

profile was measured, the number and depth of 

measurements, and the amount of variation throughout 

the profile (State whether the location of the sensor(s) 

or water-supply intake is representative of the stream.)

Remarks—Any additional information about the site, 

data collected, or general statements that do not fit in 

any other section

Date and name (or initials)—Identity of preparer and 

reviewer(s)

The station analysis should enable a reviewer to 

reconstruct what happened at the site, how the record was 

processed, why data corrections were applied, and the reasoning behind the accuracy ratings assigned. 

Documentation

The USGS methods for collecting, correcting, and 

processing continuous water-quality monitoring data have 

been defined, and USGS publication standards have been 

established. Systematic application of monitoring-station 

operation, the record-rating system, and a standard recordreview process are part of the necessary quality assurance in 

producing and documenting complete and accurate waterquality monitoring records. The use of DECODES for data 

conversion and ADAPS for data processing ensures national 

consistency and a valid data trail. Additionally, participation 

in the USGS National Field Quality Assurance (NFQA) 

Program (Stanley and others, 1998) ensures the proficiency of 

•

•

•

•

•

•

•

40 Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
the analyst and the instruments used in making measurements. 

Participation in the NFQA Program provides additional 

required documentation of field-instrument accuracy and 

reliability of the instruments used in making measurements.

Level of Review Required

At a minimum, the record-review package is examined 

annually for completeness and accuracy by a second individual (a senior colleague or section supervisor). If real-time 

data are transmitted to the Internet, data should be reviewed 

more frequently (U.S. Geological Survey, 2000). The decisions must be documented in the review package if areas of 

interpretation exist (for example, when to start or terminate a 

data correction). The completed record-review package is then 

inspected by the Water Science Center water-quality specialist 

or designated reviewer for completeness and accuracy.

Contents of the Review Package

Review of a continuous water-quality record requires 

analysis of tables of the measured field parameter; graphs of 

the measured field parameter (fig. 15); ancillary information, 

such as graphs of streamflow (fig. 16); graphs of related field 

parameters; and ancillary files used in making data corrections. Review of measured physical parameters is improved 

by analysis of the associated streamflow. Similarly, analysis 

of measured field parameters may be enhanced by review of 

related measured field parameters. For example, review of a 

continuous DO record may be enhanced by a graph of related 

continuous temperature record (fig. 17). The ADAPS station 

analysis, which provides a summary of all data corrections 

made during the year and lists all periods of missing record 

(table 19), is a useful report for the review process in addition 

to the ADAPS end-of-year table, which summarizes minimum 

and maximum field parameter values for the year (table 20). 

The data-calibration and correction criteria, maximum 

allowable limits, and ratings of accuracy have been combined 

into one table for easy reference in the record-computation and 

review process (Attachment 8).

Figure 15. Example of a review graph of measured and computed values of specific conductance at 

Palouse River at Hooper, Washington, August 2002.

Record Computation  41

370

360

350

340

330

320

310

300

290

280

AUGUST 2002

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Measured

Computed
Figure 16. Example of a discharge hydrograph at U.S. Geological Survey streamgaging station 13351000, 

Palouse River at Hooper, Washington.

Figure 17. Example of a review graph of computed values of related water-quality field parameters at U.S. 

Geological Survey streamgaging station 12121500, Puyallup River at Puyallup, Washington, August 2001.

42  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data

8,000

5,000

2,000

1,000

500

200

100

50

20

10 14

OCT. NOV. DEC. JAN. FEB. MAR.

2002 WATER YEAR

APR. MAY JUNE JULY AUG. SEPT.

28 14 28 14 28 14 28 14 28 14 28 14 28 14 28 14 28 14 28 14 28 14 28

MEAN DISCHARGE, IN CUBIC FEET PER SECOND

Water temperature

Dissolved oxygen

11.0

10.5

10.0

9.5

9.0

8.5

8.0

18

17

16

15

14

13

12

11

10

9

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

WATER TEMPERATURE, IN DEGREES CELSIUS

AUGUST 2001

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Table 19. Example of an ADAPS station analysis report.

[WA, Washington; USGS, U.S. Geological Survey; NAD27, North American Datum of 1927: NGVD29, National Geodetic Vertical Datum of 1929; cond, conductance; 25C, 25 degrees Celsius; μS/cm @ 25C, microsiemens per centimeter at 25 degrees Celsius; Corr, correction; PDT, Pacific Daylight Time zone]

U.S. DEPARTMENT OF THE INTERIOR - U.S. GEOLOGICAL SURVEY - WATER RESOURCES

STATION NUMBER 13351000 PALOUSE RIVER AT HOOPER, WA SOURCE AGENCY USGS STATE 53 

COUNTY 075

LATITUDE 464531 LONGITUDE 1180852 NAD27 DRAINAGE AREA 2500.00 CONTRIBUTING DRAINAGE 

AREA 2500 DATUM 1040.8 NGVD29

 Date Processed: 2003-09-23 10:54 By rjwagner

 Station Analysis Report for Water Year 2002

 Computation Type: daily values only

 CORRECTIONS:

 Specific cond at 25C, in uS/cm @ 25C

 

 Correction set # 1 

 Start Date / End Date / Comments SET Input Corr Input Corr Input Corr

 2002/05/06 @ 10:40:00 PDT 1 100 0 1000 0

 ----/--/-- @ --:--:--

 activated onde

 2002/06/03 @ 12:25:00 PDT 1 0 0 1000 -130

 ----/--/-- @ --:--:--

 Fouling correction

 2002/06/03 @ 12:33:00 PDT 1 0 0 1000 0

 ----/--/-- @ --:--:--

 after cleaning

 2002/09/10 @ 10:05:00 PDT 1 0 0 1000 0

 ----/--/-- @ --:--:--

 2002/10/01 @ 09:03:00 PDT 1 0 0 1000 60

 ----/--/-- @ --:--:--

 Fouling correction

 

 Correction set # 2 

 Start Date / End Date / Comments SET Input Corr Input Corr Input Corr

 2002/08/20 @ 13:08:00 PDT 2 100 0 1000 0

 ----/--/-- @ --:--:--

 2002/09/10 @ 10:05:00 PDT 2 0 0 1000 -80

 ----/--/-- @ --:--:--

 2002/09/10 @ 10:09:00 PDT 2 0 0 1000 0

 ----/--/-- @ --:--:--

 

 Correction set # 3 

 RATINGS & SHIFTS:

 DAILY VALUES

 Gaps in Record

 10-01 To 04-23

 05-09 To 05-17

 Estimated Record

 None

Record Computation  43
Table 20. Example of an ADAPS end-of-year summary report.

[WA, Washington; USGS, U.S. Geological Survey; NAD27, North American Datum of 1927: NGVD29, National Geodetic Vertical Datum of 

1929; DD, data descriptor; cond, conductance; 25C, 25 degrees Celsius; μS/cm @ 25C, microsiemens per centimeter at 25 degrees Celsius; 

HH, hour; MM, minute; SS, second]

U.S. DEPARTMENT OF THE INTERIOR - U.S. GEOLOGICAL SURVEY - WATER RESOURCES

STATION NUMBER 13351000 PALOUSE RIVER AT HOOPER, WA SOURCE AGENCY USGS 

STATE 53 COUNTY 075

LATITUDE 464531 LONGITUDE 1180852 NAD27 DRAINAGE AREA 2500.00 CONTRIBUTING DRAINAGE 

AREA 2500 DATUM 1040.8 NGVD29

 Date Processed: 2005-07-01 13:29 By rjwagner

 INPUT DD: Specific cond at 25C, IN uS/cm @ 25C

 PROVISIONAL DATA FOR WATER YEAR ENDING SEPT. 30, 2002

 ANNUAL MAXIMUM, BASED ON 159 EQUIVALENT DAYS OF RECORD.

 TIME Specific cond at 25C

 DATE HH MM SS (uS/cm @ 25C)

---------- ---------------- --------------

 MAXIMUM INSTANTANEOUS Specific cond at 25C

 09/30/2002 06:00:00 364

 MINIMUM INSTANTANEOUS Specific cond at 25C

 04/02/2002 05:00:00 0

 04/03/2002 00:00:00 0

 MAXIMUM DAILY Specific cond at 25C

 09/29/2002 355

 MINIMUM DAILY Specific cond at 25C

 05/06/2002 134

44  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
The record-review package includes the following 

figures, forms, or tables (examples are included in this report 

and in the accompanying attachments): 

1. Station-description form (Attachment 6)

2. USGS water-quality monitor field-inspection forms 

(Attachment 1) 

3. Annual field-measurement summary form (Attachment 3) 

4. Unit-value graphs for review of each water-quality field 

parameter (figure 15)

5. Data correction for fouling table from ADAPS (for 

example, table 13)

6. Data correction for calibration drift table from ADAPS 

(for example, tables 11 and 14)

7. Water-quality three-point variable data-correction table 

from ADAPS (table 14)

8. Primary computations table from ADAPS (Attachment 2)

9. ADAPS output from “Expanded Shifts/Corrections” used 

to rate accuracy of record (Attachment 5)

10. Computed (type 3) daily-values tables (see Data Reporting) 

11. Discharge hydrograph (figure 16)

12. ADAPS end-of-year summary (table 20)

13. ADAPS station analysis report (table 19)

14. Station analysis form (Attachment 7)

15. Draft manuscript station description for the USGS annual 

data report (see Data Reporting) 

Data Reporting

It is USGS policy that, with rare exception, all data 

collected by the USGS must be published or made publicly 

available. Although paper reports are the traditional means of 

publication, data are currently (2006) being made available 

in other electronic formats, such as computer disks, compact 

disks-read only to memory (CD-ROMs), or on the Internet. 

The same requirements for checking the completeness and 

accuracy of the record apply to all forms of publication.

Data Reports

All nonproprietary water-quality data collected during 

the water year are published in the USGS annual data report, 

“Water Resources Data, [State name], Water Year ___,” or in 

individual project data reports. Approval for publishing hydrologic data is in accordance with applicable USGS publication 

policies (Alt and Iseri, 1986; U.S. Geological Survey, 

2003a, b). USGS policy (U.S. Department of the Interior, 

1992) states that data and information are to be published; 

however, publication is not limited to paper media (U.S. 

Geological Survey, 1990; U.S. Department of the Interior, 

1993). Electronic formats for public distribution include 

computer storage media, such as CD-ROMs and the Internet.

Long-term deployment of continuous water-quality monitors generates large amounts of unit-value data. Measurements 

of water-quality field parameters may be set to record from 

24 to 96 unit values (or more) per day, producing more data 

values than can easily be published in data tables. Because 

environmental extremes in field parameters are important 

for understanding and describing biological processes and 

documenting adherence to regulatory criteria, it is desirable 

to publish both the daily minimum and maximum values 

of field parameters. Additionally, it is desirable to publish 

the daily statistical central tendency of unit values (mean or 

median). Thus, these measurements are processed statistically 

and published as daily records of minimum, maximum, and 

either mean or median (table 21). Measurements of pH should 

be published as minimum, maximum, and median (or mean 

hydrogen-ion activities can be calculated as described by 

Radtke and others, 2003). Because of the large variability 

commonly observed in measurements of turbidity, the decision 

to publish mean or median daily turbidity values depends 

on the project and data-quality objectives. It may not be 

appropriate to publish or calculate daily median, maximum, 

and minimum data for tidally influenced sites where the tidal 

period (about 24.2 hours) is superimposed on the daily period 

making daily statistics less meaningful.

Data Reporting  45
Table 21. Example of an ADAPS daily-values table.

[WA, Washington; USGS, U.S. Geological Survey; NAD27, North American Datum of 1927; NGVD29, National Geodetic Vertical 

Datum of 1929; MAX, maximum; MIN, minimum; data shown only for June through September of 2002]

 U.S. DEPARTMENT OF THE INTERIOR - U.S. GEOLOGICAL SURVEY - WATER RESOURCES

 STATION NUMBER 13351000 PALOUSE RIVER AT HOOPER, WA SOURCE AGENCY USGS STATE 53 

COUNTY 075 LATITUDE 464531 LONGITUDE 1180852 NAD27 DRAINAGE AREA 2500.00 CONTRIBUTING 

DRAINAGE AREA 2500 DATUM 1040.8 NGVD29

 Specific conductance, water, unfiltered, microsiemens per centimeter at 25 degrees Celsius 

 WATER YEAR OCTOBER 2001 TO SEPTEMBER 2002 

DAY MAX MIN MEAN MAX MIN MEAN MAX MIN MEAN MAX MIN MEAN

 JUNE JULY AUGUST SEPTEMBER

 1 189 166 178 287 279 282 314 300 307 347 336 341

 2 200 180 188 293 287 289 318 309 315 346 335 340

 3 204 171 184 293 290 291 331 315 325 345 324 338

 4 181 174 178 299 290 294 336 328 333 331 308 322

 5 192 181 186 300 288 293 341 325 333 331 308 321

 6 198 180 192 297 289 293 343 326 335 338 319 330

 7 201 197 198 305 294 298 342 335 339 341 331 338

 8 208 196 205 308 292 299 343 323 336 343 335 340

 9 211 207 209 297 288 294 335 316 329 343 336 340

 10 215 208 211 293 284 288 324 311 317 343 336 340

 11 220 214 215 292 286 289 319 308 314 343 335 338

 12 221 213 217 295 288 291 328 308 320 344 335 340

 13 222 216 219 304 295 300 333 322 330 345 338 341

 14 225 204 222 310 303 306 339 330 335 344 338 341

 15 232 206 223 314 308 311 341 333 336 344 338 342

 16 239 222 232 314 308 311 339 334 336 347 339 344

 17 243 228 237 314 308 311 340 333 336 349 342 345

 18 242 222 235 315 310 313 341 334 338 351 344 349

 19 243 223 237 318 309 314 341 331 335 355 348 352

 20 248 228 242 319 311 315 342 333 336 357 348 353

 21 250 223 239 322 304 316 341 330 334 354 331 346

 22 241 221 235 322 312 317 337 328 331 331 306 318

 23 250 241 244 322 309 316 334 328 330 311 302 307

 24 273 250 261 319 303 312 335 328 332 316 308 311

 25 282 269 275 317 303 310 332 326 330 321 309 316

 26 274 257 264 317 303 310 334 321 329 328 317 323

 27 269 260 266 317 306 311 337 315 327 344 325 338

 28 275 269 273 320 310 314 336 321 329 355 343 353

 29 276 271 274 325 311 318 341 326 335 363 355 360

 30 281 274 277 324 304 315 343 333 339 364 354 361

 31 --- --- --- 317 304 311 345 336 341 --- --- ---

 MONTH 282 166 227 325 279 304 345 300 330 364 302 338

46  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Data-Qualification Statements

The accuracy of published water-quality monitoring 

records is defined by the rating of the record (see Publication 

Criteria; table 18), and this accuracy is indicated in a statement in the “REMARKS” paragraph of the manuscript station 

description (table 22). Different accuracies may be attributed 

to different parts of a given record. Additional comments 

regarding missing data and the quality of recorded data also 

may be noted in the “REMARKS” paragraph. For example, 

comments should be noted if daily extremes may have 

occurred during periods of missing record (see Publication 

Criteria; Novak, 1985). 

Reporting Parameter Codes and Units

All continuous time-series data must be stored in NWIS. 

Data from all temperature, specific conductance, pH, and DO 

sensors are essentially comparable and can be stored under 

the appropriate NWIS parameter code (table 23). (See “Data 

Rounding Convention” section in Bartholoma (2005) for a 

discussion of significant figures and rounding conventions in 

data reporting.) Numerous methods and instruments can be 

used to measure turbidity. Because of differences in design 

and technology, Gray and Glysson (2003) suggest that a set 

of turbidity reporting units be used to differentiate between 

various instruments and methodologies. Turbidity data should 

be stored and reported in NWIS using parameter codes and 

measurement reporting units that are specific to the instrument 

type used (Anderson, 2004; table 23). Because ADAPS 

currently does not have the capability of storing method 

codes, as described by Anderson (2004), it is necessary to 

document the turbidity instrument make and model number 

in the ADAPS data descriptor (Bartholoma, 2005). It also is 

necessary to describe the turbidity instrument make and model 

in the instrumentation section of the station description and in 

the manuscript station description for the annual data report. 

During intervals when recorded turbidity measurements 

are verified as exceeding the sensor-reporting limit because 

of stream turbidity, the daily maximum must be reported as 

greater than the manufacturer’s reporting limit or as greater 

than a value near the maximum sensor response that has been 

observed. For example, maximum turbidity measurements 

in figure 6 were observed at approximately 1,685 FNU. 

Table 22. Example of a manuscript station description.

[WA, Washington; NAD27, North American Datum of 1927; °C, degree Celsius; mg/L, milligram per liter]

PALOUSE RIVER BASIN

13351000 PALOUSE RIVER AT HOOPER, WA

(National Water-Quality Assessment station)

WATER-QUALITY RECORDS

PERIOD OF RECORD.--Water years 1959-71, 1993-current year. 

PERIOD OF DAILY RECORD.-- 

SPECIFIC CONDUCTANCE: October 1961 to September 1971, August 1993 to September 1994, April 2002 to current year. 

WATER TEMPERATURE: October 1961 to September 1971, August 1993 to current year. 

SUSPENDED SEDIMENT DISCHARGE: October 1961 to September 1971, October 1992 to September 1999; November 2000 to March 

2001 (discontinued). 

INSTRUMENTATION.--Water-quality monitor since August 1993. Electronic data logger with 60-minute recording interval except for period 

Nov. 15, 1994, to Oct. 20, 1995, when the recording interval was 72 minutes. 

REMARKS.--Specific conductance record excellent for the period except Aug. 5 to 18, which is good; and Apr. 1, May 19, June 2, 4, Sept. 

29, which is fair. Temperature record excellent. In October 1996, station became a Central Columbia Plateau National Water-Quality 

Assessment Program (NAWQA) surface-water quality trend site. 

EXTREMES FOR PERIOD OF DAILY RECORD.-- 

SPECIFIC CONDUCTANCE: Maximum recorded, 447 microsiemens Aug. 14, 1994, but may have been higher during periods of missing 

record; minimum recorded, 131 microsiemens observed May 6, 2002, but may have been lower during periods of missing record. 

WATER TEMPERATURE: Maximum recorded 32.5 °C (rounded) July 24, 1994, but may have been higher during periods of missing 

record; minimum recorded, 0.0 °C for several days during winter months. 

SEDIMENT CONCENTRATION: Maximum daily mean, 10,100 mg/L Feb. 8, 1996; minimum daily mean, 1 mg/L Jan. 4, 2001. 

SEDIMENT DISCHARGE: Maximum daily, 527,000 tons Feb. 9, 1996; minimum daily, 0.04 tons Aug. 16, 20, 1994. 

EXTREMES FOR CURRENT YEAR.-- 

SPECIFIC CONDUCTANCE: Maximum 380 microsiemens Sept. 27; Minimum 88 microsiemens Feb. 2. 

WATER TEMPERATURE: Maximum 28.7 °C July 21; minimum, 0.2 °C Nov. 2

Data Reporting  47
The maximum could be reported as greater than (>) the 

manufacturer’s reporting limit (for example, >1,400 FNU). 

Reporting as greater than the individual instrument maximum 

limit, or as >1,700 FNU in this example, is recommended. 

The maximum sensor response can be determined by carefully 

holding a wet tissue or towel completely over the optical 

sensor after calibration and observing the reading. 

Salinity should be reported in practical salinity units, a 

dimensionless unit that uses the PSS (Lewis, 1980). Salinity, 

reported in practical salinity units, is based on measurements 

of conductivity, temperature, and pressure and is nearly the 

same as direct measurements of salinity in parts per thousand.

Archiving of Records

In accordance with USGS policy, all original data 

that (1) are published or (2) support published scientific 

analyses must be archived (Hubbard, 1992; U.S. Geological 

Survey, 1992). Original data from automated data-collection 

stations, laboratories, outside sources, and non-automated 

field observations are unmodified data collected or received 

in conventional units (engineering units, generally with a 

decimal). Original data must be preserved in its original form, 

regardless of how the data may be modified later (Hubbard, 

1992). Recent USGS policy (U.S. Geological Survey, 1999) 

reaffirms the requirement of preserving original, unaltered 

field data and further requires that the data be preserved in 

computer-readable (electronic) digital format. In addition, 

paper copies of digital field-recorded time-series data are 

no longer required. All USGS personnel responsible for the 

collection, analysis, manipulation, and storage of water-quality 

monitoring data must ensure that the specified requirements of 

archiving original electronic data are implemented.

In addition to electronic field data, original water-quality 

monitoring data on paper may include field notes, field 

measurements, calibration notes, analytical service requests, 

and water-quality analytical printouts. These data are archived 

when the project is completed or terminated, or if data are 

more than 7 years old. It is the responsibility of the Water 

Science Center Director to ensure that project files are entered 

into the Water Science Center archive and are organized 

and complete. The archive should be well documented and 

maintained by appointed personnel in the Water Science 

Center.

Table 23. Parameter codes and reporting units for measured field parameters at water-quality monitoring sites.

[NWIS, National Water Information System; °C, degree Celsius; μS/cm, microsiemens per centimer at 25 °C; mg/L, milligram per liter; FNU, formazin nephelometric units; FBU, formazin backscatter units; FNMU, formazin nephelometric multibeam units; >, greater than; <, less than; ppt, parts per thousand; psu, 

practical salinity units. Turbidity units are specified for each type of measurement instrument. All light wavelengths for turbidity methods have spectral output 

typically near infrared (780−900 nanometers)]

Field parameter NWIS parameter code Reporting units Reporting conventions

Temperature 00010 °C To nearest 0.1 °C.

Specific conductance 00095 μS/cm Three significant figures, whole numbers only.

pH 00400 Standard pH units To nearest 0.1 standard pH unit.

Dissolved oxygen 00300 mg/L To nearest 0.1 mg/L.

Dissolved oxygen,

percent saturation 00301 mg/L To nearest whole number.

Turbidity, formazin nephelometrica 63680 FNU 0–10, to nearest 0.1 turbidity unit;

10–100, to nearest 1 turbidity unit;

>100, to nearest 10 turbidity unit.

Turbidity, backscatter 63682 FBU

Turbidity, formazin, multibeam 63684 FNMU 

Salinity 00480 ppt

<1, to nearest 0.1 ppt;

1–10, to nearest 0.1 ppt;

10–100, to nearest 1 ppt.

Salinity, calculated from specific 

conductance at 25 °C 90860 psu

<1, to nearest 0.1 psu;

1–10, to nearest 0.1 psu;

10–100, to nearest 1 psu.

a

Submersible-type sensors currently (2006) in use for monitoring environmental turbidity typically are reported in FNU. See Anderson (2004) for a description of the method codes that apply to particular instrument manufacturers and turbidity parameter codes for other measurement technologies.

48  Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Operation, Record Computation, and Data
Summary

Recording systems that measure physical and chemical 

water-quality field parameters at discrete time intervals 

provide a nearly continuous record of water quality. A 

common configuration is the four-parameter water-quality 

monitoring system that collects temperature, specific conductance, dissolved oxygen, and pH data, although systems can 

be configured to measure other properties, such as turbidity or 

fluorescence. The sensors that are used to measure these water 

properties require careful field observation, cleaning, and 

calibration. Producing an accurate final water-quality record 

requires thorough and consistent procedures for the computation, publication, and archiving of the data. 

This report provides guidelines for U.S. Geological 

Survey personnel in site- and monitor-selection considerations; 

sensor inspection and calibration methods; field procedures; 

data evaluation, correction, and computation; and record 

review and data-reporting processes. Emerging sensor technology broadens the variety of measurable chemical constituents, 

allows lower detection limits, and provides increased stability 

and accuracy. Recent improvements make it possible to access 

near real-time water-quality monitoring data on the Internet. 

Such technological progress will continue to improve applications and allow quality-control procedures to be refined even 

further.

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References  51

Attachments

1. U.S. Geological Survey continuous water-quality monitor field form.

2. ADAPS primary data tables: (a) example of a standard format ADAPS primary data table, 

and (b) example of a historical format ADAPS primary data table.

3. Output from Excel spreadsheets for data-correction calculations for temperature, specific 

conductance, dissolved oxygen, and pH, and a correlation test for sensor linearity.

4. Evaluation and application of a cross-section data correction.

5. Examples of accuracy rating using “Expanded shifts/corrections” output: (a) “Expanded 

shifts/corrections” daily output, (b) “Expanded shifts/corrections” hourly output for rating 

transition days, and (c) final rating of accuracy based on “Expanded shifts/corrections” 

output.

6. Example of a station description for the Yampa River near Maybell, Colorado.

7. Example of a station analysis for specific conductance on the Yampa River near Maybell, 

Colorado.

8. Calibration criteria, data-correction criteria, maximum allowable limits, and ratings of 

accuracy.

1 Monitor form ver. 3.0 

Station No. ______________________________________ Station Name ________________________________________ 

Monitor Inspected By ____________________________ Date _____________ Watch Time ________ Time Datum _______ 

Gage Ht ___________ (Rising, Falling, Steady, Peak) Channel Conditions ________________________________________ 

Monitor Make/Model __________________________________________ Monitor Serial No. __________________________ 

Field Meter Make/Model _______________________________________ Field Meter Serial No. _______________________ 

Weather Cold Cool Warm Hot Rain Mist Sleet Snow Humid Dry Cloudy Pt Cloudy Overcast Clear Windy Gusty Breeze Calm 

Comments:

MONITOR FOULING CHECKS

Before Cleaning After Cleaning 

Time _______ Time _______ 

Recorded/ 

Live Value 

Field 

Meter

Recorded/ 

Live Value 

Field 

Meter

Temp (°C)

pH (units)

DO (mg/L) 

SC (ǐS/cm)

Turbidity ( FNU FNMU FBU ) 

 Method code ______

Other _____________ 

Parameter 

U.S. GEOLOGICAL SURVEY 

CONTINUOUS WATER-QUALITY MONITOR FIELD FORM 

February 2006 

SPECIFIC CONDUCTANCE

Calibration Criteria: the greater of 5 ǐS/cm or 3% of 

measured value

Calibration Check 

Time _______ 

Recalibration 

Time _______ 

STD VALUE STD

LOT NO. 

EXP.

DATE 

STD

TEMP

SC

READING 

Error

%

STD

TEMP

SC

READING 

Error

%

Cell

range = 

Comments:

Reading in air = 

(should be zero) 

STD

TYPE

KCl; NaCl 

CALIBRATION DRIFT CHECKS

TEMPERATURE

Calibration Criteria: ± 1 percent or ± 0.5 C for 

liquid-filled thermometers; ± 0.2 C for thermisters 

Calibration Check 

Time _______ 

Recalibration 

Time _______ 

Comments:

Station No.___________________ 

Attachment 1. USGS continuous water-quality monitor field form.

1–1
2 Monitor form ver. 3.0 

pH

Calibration Criteria: ± 0.2 pH units

Calibration Check 

Time _______ 

Recalibration 

Time _______ 

pH

BUFFER 

THEORETICAL 

pH

FROM 

TABLE 

BUFFER LOT NO. BUFFER 

EXP DATE 

TEMP pH

READING

ERROR

%

TEMP pH

READING

ERROR

%

SLOPE MILLIVOLTS 

pH 7 

pH ___ 

pH ___ 

Comments:

DISSOLVED OXYGEN

Calibration Criteria: ± 0.3 mg/L

Calibration Check 

Time _______ 

TEMP

oC

BARO 

PRES

mm Hg 

DO

TABLE 

READING 

mg/L

SALINITY 

CORR. 

FACTOR 

DO

READING 

ERROR

%

Reading 

in zero DO 

sol’n

TEMP

oC

BARO 

PRES

mm Hg 

DO

TABLE 

READING 

mg/L

SALINITY 

CORR. 

FACTOR 

DO

READING 

ERROR

%

Reading 

in zero 

DO sol’n 

SALINITY: SALINITY

CORRECTION

APPLIED? Y N

DO CHARGE: DO GAIN: Date Barometer Calibrated: 

COMMENTS:

Recalibration 

Time _______ 

TURBIDITY

Calibration Criteria: ± 0.5 Turbidity Units or ± 5% 

Calibration Check 

Time _______ 

Recalibration 

Time _______ 

Lot no. or 

Date Prepared 

CONC 

______

TEMP

oC READING 

ERROR

%

 TEMP 

oC READING 

ERROR

%

Stock Turbidity 

Standard

Zero

Standard (DIW) 

Standard 1 

Standard 2 

Standard 3 

Turbidity Sensor Limit : Comments:

Station No. __________________________ 

FINAL READINGS Time _________ 

Parameter Recorded/

Live Value 

Field Meter Parameter Recorded/

Live Value 

Field Meter 

Temp (°C) Turbidity ( FNU FNMU FBU ) 

METHOD CODE ______ pH (units)

DO (mg/L) Other _____________

SC (ǐS/cm)

1–2
3 Monitor form ver. 3.0 

CROSS SECTION COMPARISON AT ________ CFS 

CROSS-SECTION SURVEY INFORMATION

Method EWI EDI Other _________________________________ No. of Verticals __________ 

Measurement Location: __________ ft upstream downstream of monitor 

Stream Mixing: Excellent Good Fair Poor 

Comments __________________________________________________________________________________ 

Station Time

Ft from 

left bank 

(00009) or 

ft from 

right bank 

(72103) 

Depth to 

bottom at 

meas. loc. 

ft

(81903)

Measurement 

depth 

ft

(00003) 

Temp 

°C

(00010) 

pH

units

(00400) 

SC

ǐS/cm

(00095) 

DO

mg/L

(00300) 

Turbidity 

_______

( ) 

Other 

_______

( ) 

NWIS

Record No. 

Gage ht = 

At monitor 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

At monitor 

Gage ht = 

Parameter Cross-section median Point value Indicated coefficient 

Water Temp °C °C

pH units units

SC ǐS/cm ǐS/cm

DO mg/L mg/L

Turbidity ________ _______

Other __________ 

CROSS-SECTION COMPARISON AT _______________ CFS 

Station No. ________________________ 

1–3
4 Monitor form ver. 3.0 

Reference (Field) 

Meter(s) Make/Model Serial No. Corr. Factor Applied? 

 Multi-Meter None Yes No

 Temperature None Yes No 

 Conductivity None Yes No 

 pH None Yes No 

 Dissolved Oxygen None Yes No 

 Turbidity (1) None Yes No 

 Turbidity (2) None Yes No 

Other None Yes No 

COMMENTS/OBSERVATIONS:

MAINTENANCE RECORD FOR CONTINUOUS MONITOR

Correction factors applied to field meter readings? YES NO 

Battery changed? YES NO Voltage ____________volts 

Sensors cleaned? YES NO Type of fouling ____________________________________________________________ 

Calibration check: WT SC pH DO TURB Recalibrated: WT SC pH DO TURB 

Sensor changed? SC YES NO Sensor ID ___________________________ 

 pH YES NO Sensor ID ________________________ 

 DO YES NO Sensor ID ___________________________ 

 Turbidity YES NO Sensor ID ___________________________ 

 Sonde Changed? YES NO New Sonde No. _______________ Old Sonde No. _________________

DO Membrane changed? YES NO Date Changed: ______________ Membrane allowed to relax _______ hrs 

Comments ________________________________________________________________________________________________ 

Station No. __________________________ 

Turbidity method codes are available at: http://water.usgs.gov/owq/FieldManual/Chapter6/6.7_contents.html 

1–4
Attachment 2-a. Example of a standard format ADAPS primary data table. 

 U.S. DEPARTMENT OF THE INTERIOR - U.S. GEOLOGICAL SURVEY - WATER RESOURCES STATION NUMBER 13351000 PALOUSE RIVER AT HOOPER, WA SOURCE AGENCY USGS STATE 53 COUNTY 075 LATITUDE 464531 LONGITUDE 1180852 NAD27 DRAINAGE AREA 2500.00 CONTRIBUTING DRAINAGE AREA 2500 DATUM 1040.8 NGVD29 Date Processed: 2004-11-08 11:28 By rjwagner Primary Computations of Specific cond at 25C Specific cond at 25C, in uS/cm @ 25C Time zone: PST DST used Stored statistics: Maximum, Minimum, Mean, Median PROVISIONAL DATA FOR WATER YEAR ENDING SEPT. 30, 2002 working ------- (00095) Specific cond at 25CuS/cm @ 25C ------- FLAGS DATE MAX CORR TIME | MIN CORR TIME | MEAN CORR ---- ---------- ------- ------- ---- ------- ------- ---- ------- ------- 09-01-2002 347 -17 0900 | 336 -16 0100 | 341 -17 09-02-2002 346 -18 1100 | 335 -18 2300 | 340 -18 09-03-2002 345 -19 0800 | 324 -19 2400 | 338 -19 09-04-2002 331 -20 0900 | 308 -19 2300 | 322 -20 09-05-2002 331 -21 1100 | 308 -19 0000 | 321 -21 09-06-2002 338 -23 0900 | 319 -22 0000 | 330 -23 09-07-2002 341 -25 0800 | 331 -24 0000 | 338 -25 09-08-2002 343 -27 1000 | 335 -26 0000 | 340 -27 09-09-2002 343 -28 1100 | 336 -29 2300 | 340 -28 09-10-2002 343 -30 0800 | 336 -29 0000 | 340 -13 

Attachment 2. ADAPS primary data tables.

2–1
Attachment 2-b. Example of a historical format ADAPS primary data table.

U.S. DEPARTMENT OF THE INTERIOR - U.S. GEOLOGICAL SURVEY - WATER RESOURCES STATION NUMBER 13351000 PALOUSE RIVER AT HOOPER, WA SOURCE AGENCY USGS STATE 53 COUNTY 075 LATITUDE 464531 LONGITUDE 1180852 NAD27 DRAINAGE AREA 2500.00 CONTRIBUTING DRAINAGE AREA 2500 DATUM 1040.8 NGVD29 Date Processed: 2004-11-08 11:26 By rjwagner Primary Computations of Specific cond at 25C Specific cond at 25C, in uS/cm @ 25C Time zone: PST DST used Stored statistics: Maximum, Minimum, Mean, Median PROVISIONAL DATA FOR WATER YEAR ENDING SEPT. 30, 2002 working ------- (00095) Specific cond at 25CuS/cm @ 25C ------- Value At Indicated Hours Maximum Minimum Maximum Minimum 0000 0100 0200 0300 0400 0500 0600 0700 Value Value Mean Corr. Corr. 0800 0900 1000 1100 1200 1300 1400 1500 Flag Date <Time> <Time> Value <Time> <Time> 1600 1700 1800 1900 2000 2100 2200 2300 ---- ---------- ------- ------- ------- ------- ------- ------- ------- ------- ------- ------- ------ - ------- ------ 09-01-2002 347 336 341 -17 -16 337 336 337 338 341 344 345 346 <0900> <0100> <0900> <0100> 346 347 346 345 344 343 342 342 340 339 339 339 337 337 338 337 09-02-2002 346 335 340 -18 -18 337 336 336 337 338 340 341 342 <1100> <2300> <1100> <2300> 344 345 345 346 344 344 344 345 341 340 339 339 337 338 337 335 09-03-2002 345 324 338 -19 -19 335 336 337 337 338 341 342 343 <0800> <2400> <0800> <2400> 345 344 344 343 342 341 341 338 338 336 335 332 331 331 329 327 09-04-2002 331 308 322 -20 -19 324 322 322 322 324 326 327 329 <0900> <2300> <0900> <2300> 330 331 331 329 325 328 326 323 321 319 317 316 314 312 310 308 09-05-2002 331 308 321 -21 -19 308 308 309 312 315 319 322 326 <1100> <0000> <1100> <0000> 327 329 330 331 330 327 323 321 323 321 321 321 320 318 319 319 09-06-2002 338 319 330 -23 -22 319 320 323 325 328 330 333 335 <0900> <0000> <0900> <0000> 336 338 338 337 331 333 333 331 326 329 328 328 328 328 330 330 09-07-2002 341 331 338 -25 -24 331 332 333 336 337 337 338 340 <0800> <0000> <0800> <0000> 341 341 341 341 339 341 339 339 338 338 338 336 337 337 337 336 09-08-2002 343 335 340 -27 -26 335 337 337 339 341 341 341 342 <1000> <0000> <1000> <0000> 342 342 343 343 343 342 342 342 341 339 340 340 339 338 338 337 09-09-2002 343 336 340 -28 -29 338 338 338 339 339 340 341 342 <1100> <2300> <1100> <2300> 342 342 342 343 343 337 341 340 342 340 340 339 338 337 337 336 09-10-2002 343 336 340 -30 -29 336 337 338 338 339 340 341 342 <0800> <0000> <0800> <0000> 343 343 343 342 342 340 340 340 339 338 338 338 338 337 338 338

2–2
VERSION 2.2 04/28/2005

TEMPERATURE – Calculations of data corrections for temperature are based on meter-tometer comparisons of a Water Science Center-certified field thermistor with the continuous 

monitor. The certified field thermistor should be checked at least every 4 months against an 

NIST-traceable thermometer. Corrections are arithmetic, but if a large range in environmental values occurs, consideration should be given to making percentage corrections.

SPECIFIC CONDUCTANCE – Data corrections for fouling are percentages based on 

pre-cleaned and cleaned-sensor readings. Data corrections for calibration drift are based 

on the average deviation from expected readings of the cleaned monitor in three standard 

solutions.

DISSOLVED OXYGEN – Data corrections for fouling are percentages based on precleaned and cleaned-sensor readings. Data corrections for calibration drift are based only 

on the deviation from 100-percent saturation readings. If environmental values are in a 

lower range (near zero), consideration should be given to making data corrections based on 

zero-DO readings. If environmental values are in a higher range (much greater than saturation), consideration should be given to making data corrections based on ancillary information, such as Winkler-DO readings or the field meter. 

pH – Data corrections for fouling are arithmetic, based on pre-cleaned and cleaned-sensor 

readings. Data corrections for calibration drift are based on the assumption of linear sensor 

response and the average deviation from expected readings of the cleaned monitor in three 

standard solutions. If sensor response is not linear, a three-point data-correction curve 

(V-shift) should be used.

CROSS SECTION – Space is included in these spreadsheets for making cross-section data 

corrections, but no formulas have been entered. It is recommended that cross-section data 

corrections not be made but rather locate a better site or possibly just downgrade the quality of record. (See attachment 4 for additional information on cross-section corrections.)

V-SHIFT TEST – An Excel correlation test is included to give the users a tool that may 

help determine sensor linearity. This tool does not always work, however, and should be 

used with care. This tool can be used to help the user better understand sensor linearity. 

For example, if the three SC standard solutions in the example read 102, 259, and 503, the 

average is about + 3% departure from 100% linearity and indicates a much better response 

than the example. The V-shift test, however, gives an R2 of 0.0656. By experimenting with 

the test, it is possible to have a better idea of the kind of sensor response that can be 

expected in the field.

Attachment 3. Output from Excel spreadsheets for data-correction calculations for temperature, specific conductance, 

dissolved oxygen, and pH, and a correlation test for sensor linearity.

3–1
Station No. Station Name: Water Year:

erofeB

retfA

erofeB

retfA

7.3 8.3 8.3 8.5 1.0 0.2

TEMPERATURE

Corrections

noitcerroC

Meter-tometer

correction

(after)

naeM

gnidaerrotinoM

Meter-tometer

correction

(before)

Monitor

cleaning

Field

reading

Remarks

(Meter to meter corrections found before and after cleaning should be applied at the times they were observed)

desunoitcerrocnoitces-ssorC

etaD

Time

(before/after)

revresb0

Crosssection

correction

3–2
Fouling + drift

(absolute)

erofeB

retfA

erofeB

retfA

4/23/2002 BAS 162 161

5/6/2002 1035/40 BAS 134 134 131 131 0.0 0.0% 99 101 -2.0% 252 257 -1.9% 488 494 -1.2% -1.7% 1.7%

6/3/2002 1225/33 BAS 196 170 164 164 -26.0 -13.3% 99 101 -2.0% 252 256 -1.6% 488 495 -1.4% -1.7% 14.9%7/8/2002 1105/10 BAS 299 298 291 291 -1.0 -0.3% 99 101 -2.0% 252 258 -2.3% 488 501 -2.6% -2.3% 2.6%8/6/2002 1221/28 BAS 340 342 337 337 2.0 0.6% 99 100 -1.0% 179 182 -1.6% 488 495 -1.4% -1.4% 1.9%

9/10/2002 1005/09 BAS 373 376 359 359 3.0 0.8% 99 107 -7.5% 252 270 -6.7% 488 532 -8.3% -7.5% 8.3%

10/1/2002 0903/05 BAS 341 360 368 368 19.0 5.6% 99 101 -2.0% 252 256 -1.6% 488 497 -1.8% -1.8% 7.4%

Cross-section

correction Fouling correction

Monitor cleaning Field reading

noitcerrocgniluoF

tnecreP

noitcerroc

Percent

correction

applied

separately if

> 3%

etaD

emiT

revresbO

desunoitcerrocnoitces-ssorC

[%, percent; SC, specific conductance, in microsiemens per centimeter at 25 degrees Celsius; +, plus; >, greater than or equal to. Formulas for fouling correction, (E9-D9) - (G9-F9); percentage correction applied, H9/D9; calibration percentage drift correction, (J9-

K9)*100/K9. Current version of the form is available for USGS personnel at http://sr.water.usgs.gov/qw/qwmonitors/ ]

gnidaerrotinoM

tfirdtnecreP

noitcerroc

noitces-ssorC

noitcerroc

dradnatsCS

gnidaerrotinoM

noitces-ssorC

naem

gnidaerrotinoM

SPECIFIC CONDUCTANCE

Drift correction

dradnatsCS

gnidaerrotinoM

tfirdtnecreP

noitcerroc

dradnatsCS

tfirdtnecreP

noitcerroc

tnecrepegarevA

noitcerroc

Remarks

3–3
ATTACHMENT 3

_

DISSOLVED OXYGEN 12505450 GRANGER DRAIN AT GRANGER, WA 2003 WATER YEAR WATER TEMPERATURE

Fouling + drift

(absolute)

erofeB

retfA

erofeB

retfA

8.64 9.00 8.66 8.65 0.37 4.28% 0.00 0.10 -0.10 9.44 9.82 -0.38 -3.9% 8.2%

8.64 8.75 8.66 8.65 0.12 1.39% 1.4%

etaD

emiT

revresb0

noitcerrocnoitces-ssorC

desu

DISSOLVED OXYGEN

Drift correction

Zero

DO

gnidaerrotinoM

noitcerroctfirD

ODrelkniW

noitcerroctfirD

tnecreP

noitcerroc

Fouling correction

Monitor

cleaning

Field

reading gn

iluoF

noitcerroc

tnecreP

noitcerroc

Remarks noit

ces-ssorC

naem

gnidaerrotinoM

gnidaerrotinoM

noitcerroctfirD

Cross-section

correction

noitces-ssorC

noitcerroc

noitarutas%001

gnidaerrotinoM

Correction

applied

(separately)

3–4
2003 WATER YEAR WATER TEMPERATURE 12505450 GRANGER DRAIN AT GRANGER, WA

Fouling + drift

(absolute)

erofeB

retfA

erofeB

retfA

8.30 7.30 7.30 7.30 -1.00 4.00 4.01 -0.01 7.00 7.05 -0.05 10.00 10.20 -0.20 -0.09 1.09

etaD

emiT

revresb0

noitcerrocnoitces-ssorC

desu

gnidaerrotinoM

noitcerroctfirD

Cross-section

correction

noitces-ssorC

noitcerroc

dradnatsHp

gnidaerrotinoM

Correction

applied

(separately)

Remarks noi

tces-ssorC

naem

gnidaerrotinoM

Monitor cleaning Field reading

noitcerrocgniluoF

pH

Drift correction

dradnatsHp

gnidaerrotinoM

noitcerroctfirD

dradnatsHp

noitcerroctfirD

tfirdegarevA

noitcerroc

Fouling correction

3–5
Average

99 102 -3.03

252 270 -7.14 -6.40488 532 -9.02

Explanation

cificepS

ecnatcudnoc

dradnats

gnidaerrosneS

rorretfirdrosneS

)tnecrep(

Use average percent drift

or if R2 < 0.90, three-point data correction

y = -0.0148 x -2.27

R2 = 0.8918

-10

-1-2-3-4-5-6-7-8-9

0

0 100 200 300 400 500 600

Specific Conductance (μS/cm)

)egatnecrep(noitcerroCtfirD

3–6
Recognizing the interdependence of water-quality physical properties under varying flow regimes is essential to the value of a continuous water-quality monitoring 

program, and understanding these relations is essential in the appropriate application 

of discharge-dependent corrections. The purpose of cross-section corrections is to 

adjust the measurements of the monitoring equipment to more accurately reflect 

conditions in the entire stream cross section, from bank to bank and surface to 

bottom. The principal value of such adjusted measurements is realized when concurrent discharge measurements permit the computations of constituent loads transported 

past the station. Primary tables and plots from ADAPS that represent drift and fouling 

corrections should be made before applying corrections to the cross section. The 

application of cross-section corrections is intended to improve the accuracy and representativeness of the water-quality measurements; however, cross-section corrections 

should be made only if the variability in the cross section exceeds the data-correction 

criteria (table 10) and if sufficient cross-section surveys have been made to accurately 

define the variability of the physical property. If the relation between recorded values 

and the measured cross-section values is fairly consistent and predictable, a one- or 

two-point data correction can be applied to the recorded data. As with fouling and 

drift corrections, cross-section corrections are based on the cross-section values 

measured during servicing, and the correction is prorated from the previous servicing. 

The decision to use a one- or two-point data correction depends on the range of 

environmental values during the service interval. If the cross-section variability of 

the measured physical property is fairly consistent and predictable, another option for 

cross-section corrections is to apply data corrections using an ADAPS rating table 

(Bartholoma, 2005) of discharge and the measured physical property.

Corrections to the cross section are based on field measurements taken both 

horizontally and vertically in the stream cross section. Several considerations govern 

the use of corrections. Cross-section measurements must be made under a variety 

of flow conditions and all seasonal conditions; once a baseline of cross-section 

measurements under all seasons and flows has been established, measurements 

should be made at least twice annually to confirm that the correction applied to the 

water-quality measurements truly represents the median or mean cross-section values. 

Corrections should not be applied to water-quality measurements beyond the range 

of discharge measurements nor during periods of unsteady flow. If the correction for 

a measured physical property is consistent across a range of discharges and seasons, 

the correction of the measurement values by simple adjustment is warranted. Seasonal 

changes in water quality may be representative of the hydrologic system, but they also 

may be a result of local conditions.

Attachment 4. Evaluation and application of a cross-section data correction.

4–1

The accuracy of the daily record can be calculated in many cases by using the 

“Expanded shift or corrections” feature (the SHIFT_TABLE program) in ADAPS, as 

described by Bartholoma (2005). Accuracy ratings that are measured in concentration 

or applicable units (water temperature, pH, and perhaps DO or turbidity) can be 

applied directly by using the expanded shift and corrections for the range of environmental concentrations or values during the period of rating record. Accuracy ratings 

that are measured in percentages (specific conductance, and perhaps DO or turbidity) 

can be calculated easily if the data percentages have been applied linearly throughout 

the range of data, by specifying one or more columns (that are multiples of 10) in 

the “Table column values selection” of SHIFT_TABLE (see table 5a). Using a value 

of 100 in the “Data Corrections Table” produces values that correspond directly to 

percentages if the value 100 is bracketed by the data corrections during the period of 

record being rated. For dissolved oxygen records, using the value 10 (if the value 10 

is bracketed by the data corrections during the period of record being rated) is easily 

converted to percentages by moving the decimal point to the right of the number.

Data-quality or project objectives may require precise analysis of accuracy, 

especially for short-term deployment (several days or weeks) of water-quality 

monitors. In such cases, accuracy categories may be defined in time increments as 

small as hours or minutes (see table 5b). In general, however, it is sufficient to rate 

the accuracy intervals by days for long-term deployment of water-quality monitors. 

On transition days, usually the day of servicing, the preferred approach of rating is 

to categorize the entire day as the lowest category that occurred during the day (see 

table 5c). The maximum correction for the day thus corresponds to the lowest rating 

of accuracy for the day and should be applied to the daily record.

The current (2006) version of the SHIFT_TABLE program (NWIS 4.5) combines all data corrections to provide the net correction applied to unit values. Proper 

application of the accuracy rating requires that the absolute value of net corrections 

be applied to rate the quality of the unit value records. Thus, if two or more data 

corrections are used during the same or overlapping time intervals, they both must be 

positive or negative. If not, the accuracy ratings must be calculated manually. Alternatively, the absolute values of corrections for fouling and drift can be entered into 

a test (or “dummy”) station to ascertain the ratings of accuracy. If both positive and 

negative data corrections occur during separate time intervals, the SHIFT_TABLE 

program can be used.

Attachment 5. Example of accuracy rating using “Expanded Shifts/Corrections” output.

5–1
Table 5a. “Expanded Shifts/Corrections” daily output.

[WA, Washington; USGS, U.S. Geological Survey; NAD27, North American Datum of 1927; NGVD29, National Geodetic 

Vertical Datum of 1929; Specific cond at 25C, in uS/cm @ 25C, specific conductance in microsiemens per centimeter at 

25 degrees Celsius]

STATION NUMBER 13351000 PALOUSE RIVER AT HOOPER, WA SOURCE AGENCY USGS 

STATE 53 COUNTY 075

LATITUDE 464531 LONGITUDE 1180852 NAD27 DRAINAGE AREA 2500.00 

CONTRIBUTING DRAINAGE AREA 2500 DATUM 1040.8 NGVD29

 Date Processed: 2004-11-08 11:51 By rjwagner

EXPANDED DATA CORRECTION TABLE

Specific cond at 25C, in uS/cm @ 25C

 DATE/TIME 100

 -------------------- -------

 08/19/2002 00:00:00 0.0

 08/20/2002 00:00:00 0.0

 08/21/2002 00:00:00 -0.2

 08/22/2002 00:00:00 -0.6

 08/23/2002 00:00:00 -0.9

 08/24/2002 00:00:00 -1

 08/25/2002 00:00:00 -2

 08/26/2002 00:00:00 -2

 08/27/2002 00:00:00 -2

 08/28/2002 00:00:00 -3

 08/29/2002 00:00:00 -3

 08/30/2002 00:00:00 -4

 08/31/2002 00:00:00 -4

 09/01/2002 00:00:00 -4

 09/02/2002 00:00:00 -5

 09/03/2002 00:00:00 -5

 09/04/2002 00:00:00 -6

 09/05/2002 00:00:00 -6

 09/06/2002 00:00:00 -6

 09/07/2002 00:00:00 -7

5–2
Table 5b. “Expanded Shifts/Corrections” hourly output for rating transition days.

[Transition day, rating of accuracy changes from one category to another. The first example changes from excellent to good]

 

 DATE / TIME 100

----------------------------- ---------

08/29/2002 00:00:00 -3

08/29/2002 01:00:00 -3

08/29/2002 02:00:00 -3

08/29/2002 03:00:00 -3

08/29/2002 04:00:00 -3

08/29/2002 05:00:00 -3

08/29/2002 06:00:00 -3

08/29/2002 07:00:00 -3

08/29/2002 08:00:00 -3

08/29/2002 09:00:00 -3

08/29/2002 10:00:00 -3

08/29/2002 11:00:00 -3

08/29/2002 12:00:00 -3

08/29/2002 13:00:00 -3

08/29/2002 14:00:00 -3

08/29/2002 15:00:00 -3

08/29/2002 16:00:00 -3

08/29/2002 17:00:00 -4

08/29/2002 18:00:00 -4

08/29/2002 19:00:00 -4

08/29/2002 20:00:00 -4

08/29/2002 21:00:00 -4

08/29/2002 22:00:00 -4

08/29/2002 23:00:00 -4

08/30/2002 00:00:00 -4

 DATE / TIME 100

-------------------- -------

09/06/2002 00:00:00 -6

09/06/2002 01:00:00 -6

09/06/2002 02:00:00 -6

09/06/2002 03:00:00 -6

09/06/2002 04:00:00 -6

09/06/2002 05:00:00 -6

09/06/2002 06:00:00 -6

09/06/2002 07:00:00 -6

09/06/2002 08:00:00 -6

09/06/2002 09:00:00 -6

09/06/2002 10:00:00 -6

09/06/2002 11:00:00 -6

09/06/2002 12:00:00 -6

09/06/2002 13:00:00 -7

09/06/2002 14:00:00 -7

09/06/2002 15:00:00 -7

09/06/2002 16:00:00 -7

09/06/2002 17:00:00 -7

5–3
Table 5c. Final rating of accuracy based on “Expanded Shifts/Corrections” output.

[Specific cond at 25C, in uS/cm @ 25C, specific conductance in microsiemens per centimeter at 25 degrees Celsius; 

E, excellent; G, good; F, fair]

 

EXPANDED DATA-CORRECTION TABLE

Specific cond at 25C, in uS/cm @ 25C

 DATE / TIME 100

 -------------------- ------

08/19/2002 00:00:00 0.0 E

08/20/2002 00:00:00 0.0 E

08/21/2002 00:00:00 -0.2 E

08/22/2002 00:00:00 -0.6 E

08/23/2002 00:00:00 -0.9 E

08/24/2002 00:00:00 -1 E

08/25/2002 00:00:00 -2 E

08/26/2002 00:00:00 -2 E

08/27/2002 00:00:00 -2 E

08/28/2002 00:00:00 -3 E

08/29/2002 00:00:00 -3 E -> *G

08/30/2002 00:00:00 -4 G

08/31/2002 00:00:00 -4 G

09/01/2002 00:00:00 -4 G

09/02/2002 00:00:00 -5 G

09/03/2002 00:00:00 -5 G

09/04/2002 00:00:00 -6 G

09/05/2002 00:00:00 -6 G

09/06/2002 00:00:00 -6 G -> *F

09/07/2002 00:00:00 -7 F

__________________ 

* Rating was modified based on lowest rating during the day.

5–4
Revised 10/28/02 RGC

QW Revision: 11/19/03 TAS

United States Department of the Interior

GEOLOGICAL SURVEY

Water Resources Discipline

Grand Junction, Colorado

FIELD STATION DESCRIPTION

09251000 Yampa River near Maybell, CO

LOCATION.—Lat 40°30’10”, long 108°01’45” (Maybell Quadrangle, CO, scale 1:24,000), in SE 1/4 NW 1/4 sec. 

2, T.6 N., R.95 W., Moffat County, Hydrologic Unit 14050002, on left bank 60 ft downstream from bridge on 

U.S. Highway 40, 2.0 mi downstream from Lay Creek, and 3.0 mi east of Maybell.

ESTABLISHMENT AND HISTORY.—Established April 1, 1904 by the State Engineer. Reestablished June 1, 

1910 and April 1, 1916. April 17, 1904 to August 18, 1905, vertical staff gage; August 18 to October 31, 1905, 

June 12, 1910 to November 30, 1911, chain gage; April to November 1912, vertical staff at Thornburgh bridge 

about 14 mi downstream from the present gage at a different datum. April 24, 1916 to November 3, 1917 chain 

gage; November 4, 1917 to March 8, 1937, water-stage recorder at Fraker Fork Bridge 700 ft downstream at 

datum 0.92 ft higher prior to October 1, 1932. 

Water-quality Samples—November 1950 to current year.

Continuous water-quality monitoring—November 1950 to August 1973, July 1975 to current year, daily record 

of specific conductance and water temperature. December 1950 to May 1958, October 1975 to September 

1976, October 1977 to September 1978, October 1981 to September 1982, daily record of suspended-sediment 

discharge. November 1998 to current year, daily record of pH.

HYDROLOGIC CONDITIONS.—The Yampa River begins in the Flat Tops and flows north to Steamboat Springs, 

then flows west to Craig and beyond to its confluence with the Green River. The 3,410-square-mile basin 

ranges in elevation from over12,000 ft at the continental divide to 5,900 ft at the gage. The upper part of the 

drainage is mountainous while the lower part is rolling foothills. Vegetation is very sparse above timberline, 

moderately to heavyily wooded with conifers and aspen from timberline down to about 8,000 ft, lightly to 

moderately covered with pinon, juniper, and sage from about 8,000 ft down to about 6,500 ft, and lightly 

covered with sage and grass below 6,500 ft. Stream banks in the drainage are typically lined with willows, 

brush, and cottonwood trees. Surface geology in the upper part of the basin consists of Tertiary basalts and 

Cretaceous shales, sandstones, and coal beds. The lower part of the basin consists mostly of Cretaceous 

shales, sandstones, and coal beds, and Tertiary sandstone and siltstone, with a few Tertiary intrusions scattered 

throughout the area. There is a small exposure of Paleozoic limestone, sandstone, and shale in Juniper Canyon 

a few miles upstream of the gage. Major tributaries of the Yampa River include the Elk River, Elkhead Creek, 

and the Williams Fork River. Sediment yields have been estimated from 0.1 to 0.2 ac-ft/mi2/yr in the upper 

drainage to 0.2 to 0.5 ac-ft/mi2/yr in the lower part of the drainage (USGS Open-File Report 83-76). Most 

streamflow occurs in late spring as a result of snowmelt.

DRAINAGE AREA.—3,410 mi2, approximate, from U.S. Forest Service maps.

GAGE.—A Sutron 8200 data-collection platform (DCP) with satellite telemetry and accubar pressure sensor are 

housed in a 2x2x3-ft metal shelter mounted on a 21-inch corrugated metal pipe set in concrete, located on left 

bank 60 ft downstream from new highway bridge. The DCP is set to a wire-weight gage (check bar elevation 

23.18 ft) installed on the downstream side of the new highway bridge, near the left bank, on August 18, 1998. 

The DCP records stage, water temperature, specific conductance, and pH every 30 minutes. The DCP transmits 

recorded data every four hours. Power is supplied by a 12-volt 26-amp-hour solar-recharged battery.

Attachment 6. Example of a station description for the Yampa River near Maybell, Colorado.

6–1
The datum of the gage is 5900.23 ft above NGVD 1929, from topographic map. Supplemental equipment 

consists of a three-parameter YSI Model 600XL (2-in. diameter) continuous water-quality monitor which sends 

water temperature, specific conductance, and pH readings to the DCP. The sonde is located in a 4-in. plastic 

pipe 60 ft upstream from the gage under the bridge, except during low flows and winter when the sonde is moved 

into deeper water and weighted to the bottom for protection from ice. A nitrogen tank is stored inside the 21-in. 

pipe shelter. A National Weather Service weather station is located near the gage shelter with an air-temperature 

sensor and precipitation gage connected to the Sutron 8200. No sampling equipment is stored in the gage shelter.

Water-quality Instrumentation History—July 1975-1979 or 80?, a Smoot continuous specific conductance and 

water-temperature monitor; 1980 to September 3 1997, a USGS minimonitor, continuous specific conductance 

and water temperature; September 3, 1997 to April 9, 1998, a Hydrolab Reporter, continuous specific 

conductance and water temperature; April 9, 1998 to November 20, 1998, a Hydrolab MiniSonde, continuous 

specific conductance and water-temperature; November 20, 1998 to present, a YSI 600XL continuous monitor, 

continuous specific conductance, water temperature, and pH.

 

REFERENCE AND BENCH MARKS.—

RM 1 (base) is a standard brass cap set in concrete, located 35 ft shoreward and 20 ft downstream from left bank 

downstream end of new highway bridge, elevation 16.87 ft. 

RM’s 2 and 3 were destroyed during bridge construction during fall and winter of 1997–98. 

RM 4 is top of 3/4-in. pipe 22 ft shoreward and 7 ft downstream from gage shelter,

 elevation 17.80 ft.

RM 5 is top of 1/2-in. pin 8 ft streamward and 1 ft downstream from gage shelter,

 elevation 10.07 ft.

Other elevations—The nut holding the orifice in the orifice pipe cap, elevation 0.09 + 0.05 ft; check-bar elevation 

23.18 ft.

 Levels were last run 8/13/2002.

CHANNEL, CONTROL, AND CROSS-SECTION.—The channel is straight for about 500 ft upstream and 

1,000 ft downstream from the gage. There is one channel at all stages. Streambed is composed of cobbles and 

boulders. The banks are fairly clean of vegetation. The left bank is steep and will overflow at a stage of about 

10.5 ft; the overflow area has moderate grass and sage brush growth. The right bank slopes gradually and is 

lined with willows at a stage of about 8.0 ft. Behind the willows is a gradually sloping hay field. The control for 

extreme low flow is cobbles and gravel at the gage (the outside and orifice are in separate pools at these extreme 

low flows). Control for low and medium stages is a boulder riffle 700 ft downstream from the gage, which is 

relatively stable but may be altered by high flows or spring ice breakup. High flow control is the channel. During 

the summer, light to moderate algae growth occurs. Complete ice cover occurs most winters.

Cross-section physical property variability—Variability for all physical properties is rated as well mixed to 

slightly variable excluding values near the banks in shallow flow. Late summer variability for water temperature 

(0.5–1.0 OC), dissolved oxygen (1–2 mg/L), and pH (0.2–0.4 units) can be significant in low-flow sections. 

Specific conductance variability is higher during early spring (when Lay Creek is running) and late summer 

during extreme low flow (<30 ft3/s). 

DISCHARGE MEASUREMENTS.—Low-flow measurements are made by wading 150 ft below the gage (most 

uniform cross section) or 60 ft upstream at the monitor. A wading measurement of 894 ft3/s (3.03 ft gage height) 

was made on July 14, 1999 by J.B. Foster, 150 ft below the gage, with minor difficulty. Footing is generally good 

but cobbles may be slippery when algae is present. Medium and high flows are measured from the downstream 

side of the bridge 60 ft upstream from the gage. The bridge is marked in 5-ft increments (double marks at 10-

ft intervals and triple marks at 100-ft intervals) with the initial point (0) at the right-bank end of the bridge and 

296 ft at the left-bank end of the bridge. A bridge crane is recommended. A 50-lb weight is sufficient for all 

but the highest flows. At the bridge section, most of the flow is in the left center part of the channel, velocities 

exceed 6.0 ft/s, and depths may exceed 7.0 ft during high flow. Velocities are uniform except for the minor 

effect of bridge piers. Ice measurements are made at the wading section. Caution should be used when the

6–2
river is completely covered with ice, and during spring breakup. Ice may be thin and the river may be 

deep due to backwater, even when discharge is low. Measurement accuracy usually is good for all open-water 

measurements.

MONITOR EXTREMES.—

Specific Conductance:

Maximum: 1,630 microsiemens, July 21, 2002.

Minimum: 78 microsiemens, June 1-2, 1994.

Water temperature:

Maximum: 33.0 oC, Aug. 29, 1976.

Minimum: 0.0 oC, frequently during winter months.

pH:

Maximum: 9.2, July 19, 2003.

Minimum: 7.6, August 8, 2001, June 1, 2002.

SAMPLING METHODS AND EQUIPMENT.—Procedures used for the collection and processing of waterquality and biological samples are described in USGS Techniques of Water-Resources Investigations Book 9 

National Field Manual (NFM) for the Collection of Water-Quality Data and in the Quality-Assurance Plan (QA 

Plan) for Water-Quality Activities in the USGS Colorado Water Science Center (January 2003). Any deviations 

from the NFM or the QA Plan are described in the station analysis. 

Samples are collected using the equal-width increment (EWI) method when the depths and velocities are 

sufficient to collect an isokinetic sample. Sampler types are the DH–81 and DH–95 TM or D–95 TM with either 

a 1/4- or 5/16-in. Teflon nozzle and a plastic cap and bottle. Single- or multiple-vertical samples are collected 

using an open 3-L plastic bottle when the minimum conditions for isokinetic samples are not met. Single 

vertical samples should be collected out in a main-flow section and the collection point documented.

EWI samples are composited and processed in a plastic churn splitter; whole-water samples are drawn off 

through the spigot and dissolved samples are filtered through a 0.45-micron Gelman capsule filter. 

Specific conductance (SC) and pH meters are calibrated at least once daily. If a temperature change of >3 oC 

or greater occurs between sites, the meters are recalibrated. Samples for the determination of SC and pH are 

drawn off through the spigot of the churn prior to the filtering of the water sample. Dissolved-oxygen meters 

are calibrated at each site. Typically, the thermistor of the SC meter or a calibration-checked VWR alcohol 

thermometer are used for the determination of water temperature. Dissolved oxygen and water temperature 

are measured in-situ. Measurements are made at the centroid of flow. When well-mixed conditions occur, 

measurements are made in a flowing section near the bank. All field meters and field-meter thermistors are 

calibrated and checked in accordance with the NFM and QA Plan.

Discharge is measured at the site prior to, during, or after the collection of water samples using standard methods 

as described in USGS Water-Supply Paper 2175, volume 1(Rantz and others, 1982). The discharge also can be 

obtained using the current rating with the most recent shift applied. These computed discharges are finalized at 

the end of the water year after the surface-water record has been worked and checked.

FLOODS.—No data on floods outside the period of record. Maximum discharge observed for the period of record, 

25,100 ft3/s, May 17, 1984, gage height 12.42 ft.

POINT OF ZERO FLOW.—0.49 + 0.2 ft, September 9, 1999, measured with wading rod.

WINTER FLOW.—Complete ice cover during most winters.

6–3
REGULATION AND DIVERSION.—Natural flow of the stream is affected by transbasin diversions, numerous 

storage reservoirs, and diversions upstream from the station for irrigation of about 65,000 acres upstream and 

about 800 acres downstream from the station.

ACCURACY.—Discharge records usually are good except for periods of estimated record, which are poor. 

Continuous water-quality record accuracy is reported yearly in the station analysis. Most data are rated good to 

excellent.

COOPERATION AND PURPOSE.—USGS-Upper Colorado River Compact Station, long-term streamflow data. 

U.S. Bureau of Reclamation monitors salinity trends by periodic water-quality sampling. Colorado River Water 

Conservation District, continuous real-time monitoring of water temperature, specific conductance, and pH. The 

specific conductance record is used to monitor salinity trends. The U.S. Fish and Wildlife Service uses the water 

temperature and pH record as part of endangered fish recovery studies.

ROAD LOG (in miles)

From Craig, CO:

 0.0 From intersection of U.S. Highway 40 and Colorado Highway 13

 in west end of Craig. Drive west on U.S. 40.

26.1 Cross bridge over Yampa River. Gage is on the left bank 60 ft

 downstream from the bridge.

From Maybell, CO:

 0.0 From U.S. Highway 40 in Maybell, drive east.

 3.0 Gage is on the left bank downstream from the bridge.

From Meeker, CO: Note! This road may be impassable after heavy snow.

0.0 Intersection of Colorado Highways 13 and 64 two miles south

 of Meeker. Take Highway 64 west.

 1.1

1.1 Intersection of Colorado Highway 64 and Rio Blanco County Road 7.

 Take County Road 7 north.

 15.7

16.8 Moffat County line. Pavement ends. Rio Blanco County Road 7

 becomes Moffat County Road 57.

 11.8

28.6 Pavement begins again.

 11.4

40.0 Intersection of Moffat County Road 57 and U.S. Highway 40.

 Turn right (east) on Highway 40.

 1.2

41.2 Bridge over Yampa River. Gage is on left bank below bridge.

6–4
6–5
JOB HAZARD ANALYSIS

Station: 09251000 Yampa River near Maybell, Colorado

Prepared by: 

J.R. Dungan 

Date: December 1, 2002 

Reviewed by 2nd line supervisor:

Policy exemptions at this site: Use of a Personal Floatation Device (PFD) is exempted at this site when flow 

conditions permit the use of a pygmy meter. Use of the exemption must conform to conditions discussed in District 

Memorandum, PFD Exemption Memorandum dated January 22, 2002. If you have not read the memorandum or 

have not had in-water training you must wear a PFD. 

 JOB/Job Steps Potential hazards

TRAVEL TO SITE

Roads are both paved and dirt 

/gravel. 

GAGE INSPECTION

Check flows (DCP) prior to 

leaving on field trip. 

WADING MEASUREMENTS

Maximum wade is 1,510 cfs, ght 

3.56; expect depths of 2.8 feet 

and velocities of 3.2 feet per 

second. Ice measurements are 

required in the winter. 

BRIDGE MEASUREMENTS

Bridge has significant traffic. 

Poor driving conditions during 

winter due to ice/snow. Getting 

stuck, rolling your vehicle or running 

off the road. Getting lost. 

Rattlesnakes, drowning, insect bites 

and stings, encounters with cattle. 

Falls down steep bank. 

Drowning, ankle or knee sprains, 

falling. Slippery cobbles on stream 

bed. Breaking through thin ice; 

getting hit by chunks of ice during 

spring ice break up; hypothermia; 

falling on ice. Afternoon thunder 

storms in the summer. 

Getting struck by a vehicle or flying 

debris from the roadway. Falling 

over the bridge rail and drowning. 

Being struck by the bridge crane. 

Getting large debris caught on 

suspension cable. 

Observe rules of the road, drive 

defensibly and according to road 

conditions. 

Be observant for animals and snakes. 

Open gage with caution. Inspect the 

inside of the shelter for wasps or 

spiders and check for presence of 

rodent feces. Be aware of livestock in 

area. If wasps or spiders are present, 

use a long distance insect spray. If 

the presence of rodent feces is 

detected, follow the cleanup 

procedures as outlined in the USGSWRD Hanta Virus Infection Prevention 

Plan. Be careful traversing steep 

bank. 

Know your wading limits. Typically a 

person can wade up to a factor of 9; 3 

foot depths times 3 feet per second 

velocity. Consider the footing on the 

streambed. Continually test stream 

footing and probe the bottom with a 

wading rod. Use an ice bar to test ice 

conditions. Continually check for 

floating ice chunks when ice is 

breaking up. Do not enter the stream 

during an electrical storm. 

Traffic control devices/signs are 

necessary. Wear highly visible 

reflective vests. Do not lean over 

bridge rail. Use sufficient counter 

weight for the bridge crane. The 

suspension reel must be equipped 

with a breakaway cable. 

 Approval by 1st line supervisor:

RW Boulger, Data Chief, signed 

copy on file 

Recommended Protective Clothing and Equipment: PFD, foul weather gear, cell phone, waders, wasp spray, 

winter cold weather gear, ice cramp-ons, multiple traffic cones, men working sign, ice bar, change of clothes, 

disinfectant supplies for Hanta-virus.

Recommended Safe Job Practices

6–6
TRAFFIC PLAN FOR:

Nature of work: 2-3 times per year, high-flow discharge measurements by USGS personnel from south (downstream) shoulder of bridge. Shoulder will be closed approximately 

1.5 hours during measurements. 

Conditions:  Low volume, low speed.  Lanes 10 ft wide, shoulders 2.5 ft wide. 

Gage

                                                                                                      Creek                        Shelter 

                           200 ft      200 ft

Survey  Crew

Survey 

Crew 

             200 ft          200 ft             shoulder    buffer                                                             taper 

Survey 

Crew 

Survey 

Crew 

6–7

WATER-QUALITY MONITOR STATION ANALYSIS

2002 WATER YEAR

specific conductance

09251000

Yampa River near Maybell, CO

EQUIPMENT.—Refer to the “Gage” paragraph in the station description for instrument information and sensor 

location. No equipment were changed or replaced during the period covered by this analysis.

SITE CHARACTERISTICS.—The streambed is composed of large cobbles and small boulders. Moderate algal 

blooms occur during summer months that may affect sensor fouling. At extreme low flows the sensor pool may 

become isolated from the main flow. Early lowland snowmelt runoff from tributaries affects stream mixing 

along right bank, shallow sections. Ground water seeps at the left bank also may affect mixing but only within 

1 ft of the bank. During low flows in the summer, sections near the right bank are shallow; solar warming and 

photosynthesis may significantly affect water temperature, dissolved oxygen, and pH. Suspended sediment 

concentration during snowmelt runoff period generally is low and does not usually foul the sensor significantly.

RECORD.—The record is the satellite telemetry data that are processed using DECODES software. Data are logged 

at a 30-minute interval. The record is complete for the year except as noted. The sonde was removed for a short 

period in March–April to avoid ice damage. Based on criteria in Water-Resources Investigations Report, 00–

4252, table 9, page 23, this record is rated excellent 

(< 3% error) except for periods when flow is less than 30 ft3/s, July 6–9, 11, 19, 20, 22-27; August 1–9, 13–16, 

22; September 11–15, which are rated poor.

Unit values were deleted:

During instrument servicing;

The first part of November, problem unknown;

The first part of December, the sensor was ice-affected;

Periods in July, August, and September when the flow was 7 ft3/s or less. At a flow of 3.6 ft3/s, the sonde 

pool was observed to be isolated from the main flow. See the unit value plots for discharge and specific 

conductance.

Daily values for partial days caused by transmission errors, servicing the sonde, ice-affected values, or 

isolated flow were updated where data existed during the expected time for the occurrence of the maximum or 

minimum, if at least 12 hours of values were available for the day, and if values were present adjacent to the 

extreme for the day.

312 days of record out of 365 days (85%) will be published.

Attachment 7. Example of a station analysis for specific conductance on the Yampa River near Maybell, Colorado.

7–1
CALIBRATIONS.—Calibration checks were conducted six times from October 2001 through October 2002. 

Standard solutions from the USGS Water-Quality Supply Unit at Ocala, Florida, were used. All standards that 

were used bracketed the recorded values. 

The largest error in any standard-solution calibration was less than 4%. This error was observed in a low 

standard and had no effect on the recorded values that were significantly higher for the data period. The mean 

error for 18 standards was 1.1%. The instrument was recalibrated once in October 2002 to maintain calibration 

error at a minimum.

CROSS-SECTION DATA.—The August cross-section variation was significant. Specific conductance ranged from 

1,246 to 1,430 μS/cm. This survey was conducted at an extreme low flow of 3.6 ft3/s. Poor communication 

between the sonde pool and the active stream was noted.

COMPUTATIONS.—The National Water Information System (NWIS), Automated Data Processing System 

(ADAPS) version 4.2 was used to compute the record. Calibration checks and field meter readings were used to 

verify the logged record. Field meter and “live” value comparisons indicate that the recorded values are within 2.5% 

of the field-measured values. The field meter readings were not used in computation of any portion of the record; 

they were for general comparison only. The largest fouling error was a minus 2.3%. There was no consistent trend 

in fouling or calibration errors. Errors for fouling and calibration drift with the same arithmetic sign were less than 

3%. No individual fouling or calibration drift error was greater than 3%. No corrections were applied to the record. 

The greatest uncertainty of the record occurred when the sonde pool became isolated. It is estimated that it occurred 

when flow was less than 7 ft3/s. 

RECOMMENDATIONS.—Photos were taken but additional photos are needed to document conditions upstream 

from the sonde location. 

REMARKS.— It is uncertain at what flow the sonde pool lost communication with the main flow. It is estimated 

that when flows approached 30 ft3/s the communication was sufficient. Record at or above this flow is 

considered excellent.

A new maximum for the period of record was established. It occurred on July 21 at a peak of 60 ft3/s. Data for 

other periods when the flow was about the same look reasonable; the sonde pool appears to be representative 

(see July 1–3 and 29–31). Values of conductance can be high at this site during the first flush of an event.

J.R. Dungan collected field data. Cross-section survey results can be retrieved from NWIS, Database 02. The 

Excel “Field Measurement Summary” spreadsheet for this site and water year summarizes the number of site 

visits, calibration results, and calculations of the magnitude of fouling and calibration drift and are available 

electronically in the Subdistrict–Western Slope data directory, work/gj5work/ws_pub/qw.monitors.

 

 Computed: R.W. Boulger, March 19, 2003

Checked: J. R. Dungan, March 22, 2003

Reviewed: T.A. Solberg, March 26, 2003

7–2
[°C, degrees Celsius; ±, plus or minus; ≤, less than or equal to value shown; >, greater than; %, percent; μS/cm, microsiemens per centimenter at 25 degrees Celsius; mg/L, milligram per liter; 

pH unit, standard pH unit]

Ratings of accuracy (based on combined fouling and drift corrections applied to record) 

Measurement Calibration criteria Data-correction criteria Maximum allowable limits Excellent Good Fair Poor

Temperature, 

in oC 

+0.2 

±0.2 

±2.0 

<

±0.2 

>

±0.2–0.5 

>

±0.5–0.8 

>

±0.8 

Specific conductance, in 

μS/cm 

+5 or 

+3% of 

the measured value, whichever is greater

±5 or ±3% of 

the measured value, whichever is greater

±30% 

<

±3%

>

±3–10%

>

±10 – 15% 

>

±15% 

Dissolved oxygen, in mg/L 

+0.3 

±0.3 

±2.0 or 20%, 

whichever is greater 

<

±0.3 or 

<

±5%, whichever is greater 

>

±0.3–0.5 or 

>

±5–10%, 

whichever is greater

>

±0.5 – 0.8 or 

>

±10 – 15%, 

whichever is greater

>

±0.8 or 

>

±15%, whichever is greater 

pH, in pH unit

+0.2 

±0.2 ± 2 

<

±0.2 

>

±0.2–0.5 

>

±0.5–0.8 

>

±0.8 

Turbidity, in 

turbidity units

+0.5 or 

+5% of 

the measured value, whichever is greater

±0.5 or ±5% of 

the measured value, whichever is greater

±3.0 or ±30%, 

whichever is greater

<

±0.5 or 

<

±5%, whichever is greater

>

±0.5–1.0 or 

>

±5–10%, 

whichever is greater

>

±1.0 – 1.5 or 

>

±10 –15%, 

whichever is greater

>

±1.5 or 

>

±15%, whichever is greater

Attachment 8. Calibration criteria, data-correction criteria, maximum allowable limits, and ratings of accuracy.

8–1

Prepared by:

U.S. Geological Survey

Enterprise Publishing Network

North Carolina Water Science Center

3916 Sunset Ridge Road

Raleigh, NC 27607
Printed on recycled paper

Techniques and Methods 1–D3 Wagner and others—Guidelines and Standard Procedures for Continuous Water-Quality Monitors:

 Station Operation, Record Computation, and Data Reporting