U.S. Department of the Interior

U.S. Geological Survey

Basic Requirements for Collecting, 

Documenting, and Reporting 

Precipitation and Stormwater-Flow 

Measurements

Open-File Report 99-255

A Contribution to the 

National Highway Runoff Data and Methodology Synthesis
U.S. Department of the Interior

U.S. Geological Survey

Basic Requirements for Collecting, 

Documenting, and Reporting 

Precipitation and Stormwater-Flow 

Measurements

By PETER E. CHURCH, GREGORY E. GRANATO, and DAVID W. OWENS

Open-File Report 99-255

A Contribution to the 

National Highway Runoff Data and Methodology Synthesis

Northborough, Massachusetts

1999
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director 

The use of trade or product names in this report is for identification 

purposes only and does not constitute endorsement by the 

U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased 

District Chief U.S. Geological Survey

Massachussetts–Rhode Island District Information Services

U.S. Geological Survey Box 25286, Denver Federal Center

10 Bearfoot Rd. Denver, CO 80225-0286

Northborough, MA 01532
PREFACE

Preface 

Knowledge of the characteristics of highway runoff (concentrations and loads of 

constituents and the physical and chemical processes that produce this runoff) is 

important for decision makers, planners, and highway engineers to assess and 

mitigate possible adverse impacts of highway runoff on the Nation’s receiving 

waters. In October, 1996, the Federal Highway Administration and the U.S. 

Geological Survey began the National Highway Runoff Data and Methodology 

Synthesis to provide a catalog of the pertinent information available; to define 

the necessary documentation to determine if data are valid (useful for intended 

purposes), current, and technically supportable; and to evaluate available 

sources in terms of current and foreseeable information needs. This paper is 

one contribution to the National Highway Runoff Data and Methodology 

Synthesis and is being made available as a U.S. Geological Survey Open-File 

Report pending its inclusion in a volume or series to be published by the Federal 

Highway Administration. More information about this project is available on the 

World Wide Web at http://ma.water.usgs.gov/fhwa/runwater.htm

Fred G. Bank

Team Leader

Water and Ecosystems Team

Office of Natural Environment

Federal Highway Administration

Patricia A. Cazenas

Highway Engineer

Water and Ecosystems Team

Office of Natural Environment

Federal Highway Administration

Gregory E. Granato

Hydrologist 

U.S. Geological Survey

Con

CONTENTS

Abstract .............................................................................................................................................................................

Introduction .......................................................................................................................................................................

Problem ...................................................................................................................................................................

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

Precipitation Data..............................................................................................................................................................

Site Selection...........................................................................................................................................................

Frequency and Duration of Precipitation Measurements........................................................................................

Methods for Measuring Precipitation......................................................................................................................

Stormwater-Flow Measurements.......................................................................................................................................

Site Selection...........................................................................................................................................................

Frequency and Duration of Stormwater-Flow Measurements ................................................................................

Methods for Measuring Stormwater Flow ..............................................................................................................

Primary Devices/Methods .............................................................................................................................

Channel Friction Coefficient Method..................................................................................................

Index Velocity Method ........................................................................................................................

Weirs....................................................................................................................................................

Flumes .................................................................................................................................................

Differential Pressure Method ..............................................................................................................

Acoustic and Electromagnetic Methods..............................................................................................

Dilution Methods.................................................................................................................................

Secondary Devices/Methods .........................................................................................................................

Floats ...................................................................................................................................................

Pneumatic sensors ...............................................................................................................................

Electronic sensors................................................................................................................................

Acoustic sensors..................................................................................................................................

Comparison of Flow Measurement Methods ................................................................................................

Quality Assurance/Quality Control ...................................................................................................................................

Conclusion.........................................................................................................................................................................

References .........................................................................................................................................................................

FIGURES

1,2. Box plots showing:

1. Population statistics from stormwater data recorded at ten historical highway-runoff-monitoring 

sites with available total precipitation, storm duration, and stormwater-flow data from individual 

events .............................................................................................................................................................

2. Seasonal population statistics of stormwater-flow data, including nine highway-runoff-monitoring 

sites with available total precipitation, storm duration, and stormwater-flow data from individual events ..

3. Graph showing stage and specific conductance monitored at various frequencies in response to changes of 

stage and specific conductance...............................................................................................................................

4-6. Diagrams showing:

4. Example of stream channel cross section showing current-meter method where flow velocities in 

subsectional areas are used to measure stream discharge..............................................................................

5. Highway-drainage-monitoring station with Palmer-Bowlus flume installed in trunkline drainpipe of a 

highway-drainage system ..............................................................................................................................

6. Instrumentation and equipment for measuring stormwater flow in a highway-drainage-monitoring 

station.............................................................................................................................................................

7. Box plots showing comparison of results from stormwater-flow measuring methods, 1998, 

Madison, Wisconsin ...............................................................................................................................................
VI Contents SI* (MODERN METRIC) CONVERSION FACTORS

* SI is the symbol for the International System of Units. Appropriaterounding should be made to comply with Section 4 of ASTM E380.
Ab

Basic Requirements for Collecting, 

Documenting, and Reporting 

Precipitation and Stormwater-Flow 

Measurements

By Peter E. Church, Gregory E. Granato, and David W. Owens

Abstract

Accurate and representative precipitation 

and stormwater-flow data are crucial for use of 

highway- or urban-runoff study results, either individually or in a regional or national synthesis of 

stormwater-runoff data. Equally important is 

information on the level of accuracy and representativeness of this precipitation and stormwaterflow data. Accurate and representative measurements of precipitation and stormwater flow, however, are difficult to obtain because of the rapidly 

changing spatial and temporal distribution of precipitation and flows during a storm. Many hydrologic and hydraulic factors must be considered in 

performing the following: selecting sites for measuring precipitation and stormwater flow that will 

provide data that adequately meet the objectives 

and goals of the study, determining frequencies 

and durations of data collection to fully characterize the storm and the rapidly changing stormwater 

flows, and selecting methods that will yield accurate data over the full range of both rainfall intensities and stormwater flows.

 To ensure that the accuracy and repre

tiveness of precipitation and stormwater-flo

can be evaluated, decisions as to (1) where 

drainage system precipitation and stormwat

flows are measured, (2) how frequently pre

tion and stormwater flows are measured, (3

methods are used to measure precipitation a

stormwater flows, and (4) on what basis are 

decisions made, must all be documented an

municated in an accessible format, such as 

project description report, a data report or a

appendix to a technical report, and (or) arch

a State or national records center.

A quality assurance/quality control pr

must be established to ensure that this infor

is documented and reported, and that decisi

made in the design phase of a study are cont

reviewed, internally and externally, through

study. Without the supporting data needed t

uate the accuracy and representativeness of 

precipitation and stormwater-flow measurem

the data collected and interpretations made 

have little meaning.
2 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

INTRODUCTION

Accurate and representative precipitation and 

stormwater-flow data are crucial for valid, current, and 

technically defensible interpretations of highway- or 

urban-runoff study results. Additionally, results from a 

number of accurate and representative studies are necessary for developing a regional or national synthesis 

of stormwater-runoff data. Obtaining such data is not a 

trivial matter because the stormwater-monitoring environment is complex. Varying rainfall patterns result in 

runoff flows, constituent concentrations, and constituent loads that vary considerably within and between 

storm events (Harrison and Wilson, 1985; Hoffmann 

and others, 1985; Irish and others, 1996). Different 

antecedent conditions, different storm volumes and 

durations, and different patterns of precipitation intensity make each storm a unique event. These differences 

can cause large variations in event-mean concentrations 

(EMCs) and total constituent loads measured for each 

storm (Driscoll and other, 1990a; Irish and others, 

1996). Models describing highway- and urban-runoff 

constituent loads will not be quantitative without 

detailed characterization of these complex physical and 

hydrochemical processes that govern constituent accumulation and release (Spangberg and Niemczynowicz, 

1992).

Knowledge of variations in the intensity and 

duration of precipitation and the resultant effects on 

stormwater flows, pollutant concentrations, and pollutant loads is necessary to characterize stormwater runoff 

from highways, urban areas, and other areas contributing nonpoint-source pollution to receiving waters. The 

amount of energy available to mobilize and transport 

dissolved and suspended constituents is a function of 

rainfall intensity. A tenfold increase in intensity will 

increase the kinetic energy of rainfall impact by about 

15 times (Smith, 1993). Average storm intensity and 

total flow per unit area were the most statistically significant predictors for all common highway-runoff constituents in a recent study of highway runoff that 

included a rainfall simulator and natural storms (Irish 

and others, 1996). Accurate measurement of the intensity and duration of each precipitation event and resultant total storm discharge is important to quantify the 

pollutant mass balance and effects upon a receiving 

water body (Thoman and Mueller, 1987; Irish and 

others, 1996). Characterization of storm intensity 

and duration are also important to the monitoring process because the accuracy of both time-based and flowweighted compositing schemes depends on accurate 

flow measurements (U.S. Environmental Protec

Agency, 1992). Also, because automatic sample

monly used in stormwater studies have a fixed v

for sample collection, it is difficult to match the 

quency and duration of the sampling period to v

tions in the intensity and duration of monitored 

for the optimization of sampling schemes. Data 

pretation is also dependent upon knowledge of 

intensity and duration of precipitation and resul

runoff because calculation of loads and EMCs (

lated from discrete samples, or composited man

or automatically) all depend upon the accuracy 

precipitation and (or) flow measurements. 

Problem

Accurate and representative measuremen

precipitation and stormwater flow are difficult to 

because of the rapidly changing spatial and tem

distribution of precipitation in the drainage syst

and the rapidly changing flows during a storm. 

quality of precipitation and stormwater-flow me

ments found in the literature is difficult to asses

out the supporting data needed to evaluate their 

accuracy and representativeness. Accurate meas

ments of precipitation are confounded by difficu

finding a representative site, the ability of instru

to record data accurately over a wide range of r

intensity, concerns with spatial and temporal va

ity, the reliability of measuring and recording in

ments, and problems with freezing conditions a

where commercial power is not available. Physi

logistical complications also affect the quality o

stormwater-flow measurements.

Stormwater-flow rates can range over sev

order of magnitudes in a short period. Flow reg

(steady or unsteady flow, subcritical or supercri

flow) can change in response to the varying flow 

Flow durations and intervening dry periods also 

within drainage systems. The resolution of field 

surements and commercially available measurin

equipment is relatively coarse for measuring flo

small streams, pipes, swales, and sheet flow ove

pavements and soils. The introduction of a flow

measurement device in a small channel or pipe 

turb the flow being measured. Because storm dr

systems often have little, if any, base flow, errat

surements can result when measuring instrumen

dry; instruments may not be able to accurately m

flow until stormwater flows reach a minimum w
Precipitatio

level. Also, equipment and instrumentation required for 

accurate flow measurements may be costly. Selecting a 

site where flows are consistent with the data objectives, 

the appropriate frequency and duration of flow measurements to fully characterize the stormwater-flow 

event can be obtained, and a method for measuring the 

full range and types of flow in a natural or controlled 

channel with minimal disturbance is not a trivial task, 

but is critical to ensure accurate and representative 

stormwater-flow measurements. 

Documentation of the steps followed and the 

uncertainty involved in the selection of sites for measuring precipitation and stormwater flow, the frequency 

and duration of monitoring, and the methods, equipment, and instruments used to monitor precipitation 

and stormwater flow are needed for evaluation of the 

accuracy and representativeness of the data collected. 

This evaluation is important for assessing the validity 

of the data collected because errors in precipitation or 

flow data result in inaccurate relations between rainfall 

and runoff, and errors in flow and (or) pollutant concentration result in erroneous calculations of pollutant 

loads, event-mean concentrations, and total mean daily 

loads. Validation of the accuracy and representativeness of flow and constituent concentrations data in 

highway and urban runoff also are important because 

these data form the baseline on which models developed for prediction of stormwater loads and eventmean concentrations are calibrated (Guerard and 

Weiss, 1995; U.S. Environmental Protection Agency, 

1997; Zarriello, 1998), and from which best management practices are developed. Without the supporting 

data needed to evaluate the accuracy and representativeness of the precipitation and stormwater-flow measurements, the data collected and interpretations made 

may have little meaning.

Additionally, it is important that precipitation 

and flow measurements fulfill a particular need or 

objective and that this objective and the acceptable 

uncertainty be clearly stated. Collection of data without 

a clear data-quality objective may result in collection of 

marginal or useless data (Whitfield, 1988). The accuracy and representativeness of data collected can be 

evaluated quantitatively only if information is available 

about (1) where in the drainage system the flows were 

measured, (2) how frequently the flows were measured, 

(3) what methods were used to measure flows, and 

(4) on what basis these decisions were made. All of 

this information should be documented in terms of 

project data-quality objectives. Furthermore, the 

Intergovernmental Task Force on Monitoring WaterQuality has recommended that flow measureme

component of water-quality studies and that dat

monitoring programs be collected, documented

reported in a consistent manner (Intergovernme

Task Force on Monitoring Water-Quality, 1995a

Purpose and Scope

The purpose of this report is to present th

requirements for collection of accurate and repr

tive precipitation and stormwater-flow measure

and the supporting data that must be documente

reported to ensure that these data can be indepe

validated. Data requirements for determination 

rate and representative precipitation and stormw

flow measurements are evaluated within the con

building a quantitative national data base that w

used to record and predict highway-runoff pollu

(Granato and others, 1998).The methods availab

measuring precipitation and stormwater flow ar

widely reported in the literature, so they are des

only briefly here. The information that needs to 

documented and reported to allow for independ

evaluation of the accuracy and representativene

precipitation and stormwater-flow measuremen

however, is less well described, and therefore, i

emphasized in this report. References that prov

more detailed guidance for collection of accura

representative precipitation and stormwater-flow 

measurements are provided.

PRECIPITATION DATA

Precipitation is the driving force of the st

ter runoff process and its accurate monitoring is 

sary to characterize the rainfall-runoff process. R

can be highly variable in space and in time (All

1977). Precipitation intensity and duration are m

factors determining removal of runoff constitue

during a storm. Varying rainfall patterns result in 

flows and contaminant washoff rates that vary c

erably within and between storm events (Harris

Wilson, 1985; Hoffmann and others, 1985; Irish 

others, 1996). A positive correlation between th

cal and chemical characteristics of rainfall and r

expected and well documented (Driscoll and ot

1990a; Irish and others, 1996). Higher intensity 

wash more dissolved and suspended constituen

watershed surfaces than equivalent volumes from 

intensity events (Athayde and others, 1983; Iris

others, 1996).
4 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

Theoretically, uncertainty in precipitation measurements should be lower than uncertainty in stormwater-flow measurements because precipitation 

measurements are direct, whereas many stormwater 

flow “measurements” are calculated from a stage measurement and a discharge rating. If predictive models 

are implemented by using regression techniques that do 

not account for possible uncertainties in the independent variables, then rainfall may be considered a better 

regressor for water-quality variables than runoff for a 

given site because of these lower uncertainties in precipitation monitoring (Irish and others, 1996). It is necessary, however, to measure or derive accurate 

stormflow volumes for the collection and interpretation 

of runoff-quality data because sample compositing 

methods and constituent load calculations depend on 

the availability of runoff-flow volumes. 

Examination of precipitation and runoff data 

from the Federal Highway Administration (FHWA) 

highway-stormwater-runoff data base (Driscoll and 

others, 1990b) indicates that precipitation data is a useful, but not a direct, surrogate for measured stormflows. 

Data including date, total precipitation, storm duration, 

and total runoff from 264 storms at 9 highway sites and 

at 1 grassy plot, each having the required data for at 

least 10 storms, were selected (Driscoll and others, 

1990b). Precipitation intensities were calculated as the 

quotient of total precipitation and storm duration for 

each storm from these data. Runoff coefficients were 

calculated as the quotient of total runoff and total precipitation volume for each storm. Boxplots of the data 

for each of these ten sites are shown in order of increasing imperviousness, and increasing precipitation when 

percent impervious is the same (fig. 1). Although the 

average annual precipitation among these 10 sites 

varies from about 15 to about 84 inches per year, total 

storm precipitation, intensity, and runoff coefficients 

from the storms monitored are comparable. The boxplot graph of the runoff-coefficient populations is artificially truncated at 1.0 (the point where runoff equals 

precipitation) because, logically, the total runoff from a 

storm should not exceed the measured precipitation. 

Values above a runoff coefficient of 1 may reflect 

uncertainties in the data, between-storm storage within 

the highway catchment, base flow from ground water, 

and (or) contributions from additional drainage areas 

during some storms. Examination of figure 1 indicates 

that for these data, there is no single runoff coefficient 

that can be accurately used to predict total runoff from 

total precipitation at any given site. For example, the 

uncertainty in predictions of total stormflow based on 

measured precipitation would be about plus or m

50 percent at the Route 384 site in Florida, whi

the least variation in runoff coefficients among t

sites from the FHWA data set (Driscoll and oth

1990b).The population distributions for differen

in this figure do not indicate a simple relation b

the median runoff coefficient and increasing im

ous area. Common sense would suggest that ca

ments with a very high proportion of imperviou

would have less variability in the runoff coeffic

because runoff from impervious pavement wou

be affected by antecedent moisture. The popula

distributions for runoff coefficients in figure 1, 

ever, do not demonstrate lower variabilities at h

impervious sites. 

Differences in rainfall-runoff relations fro

season to season caused by effects of temperatu

cipitation characteristics, and the length of the a

ent dry period may obscure meaningful relation

figure 1. To explore the feasibility of establishin

sonal runoff coefficients that would be character

highways nationwide, the data from the 9 paved 

way sites were combined and are shown in figu

Each of the studies selected from the FHWA da

report (Driscoll and others, 1990b) had a durati

about 1 year, but the studies were done in differ

years and many studies did not sample a substa

number of storms in each month. In these boxp

total precipitation for each storm seems to be sl

more variable in the winter months than in the r

the year (except for January, because only four 

were sampled in this month). Also, the populati

intensities seems to be more variable in the sum

(possibly due to the occurrence of convective st

the warmer months). The populations featured i

figure 2, however, do not indicate a characterist

runoff coefficient for highway sites even when 

effects of seasonality are examined. The data re

by Driscoll and others (1990b) is a compilation 

distinct phases of the early FHWA water-quality 

research and similar studies conducted by trans

tion departments in several States. Differences i

ods, equipment, and measurement installations 

between these monitoring programs at different 

throughout the Nation may introduce bias and c

ute to the variability apparent in figure 1 and fig

Therefore, precipitation measurements can prov

valuable information for interpretation of result

may not be a direct surrogate for measured runo

flows, even in small catchments.
Precipitatio

420

STORM

PRECIPITATION,

IN INCHES

EXPLANATION

Outlier data value more than 3 times theinterquartile range outside the quartileOutlier data value less than or equal to 3and more than 1.5 times theinterquartile range outside the quartileData value less than or equal to 1.5 times theinterquartile range outside the quartile75th percentileMedian25th percentile

210

STORM

INTENSITY, IN

INCHES PER HOUR

0

1.0

0.5

RUNOFF

COEFFICIENT,

UNITLESS

2.7

27.6

0

12

18.537.7

2327

106

27.6

2931

62

58.3

4536

35.314.8

1637

45

55.6

3137

48.7

1590

1.5

18

100

0.25

25

100

2.1

27.6

33

84

100

0.28

35

WI

(Grass),

Rt-45 I-81,

PA WI

Rt-45,

FL

Rt384, I-25, CO I-40, TN I-30, AR Rt- 270,

WA

WI

I-794,

WA

Rt-12,

Number of EventsPercent ImperviousArea (Acres)Average Annual Precipitation (In)

STUDY SITE

INFORMATION
6 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

420

STORM

PRECIPITATION,

IN INCHES

EXPLANATION

Outlier data value more than 3 times theinterquartile range outside the quartileOutlier data value less than or equal to 3and more than 1.5 times theinterquartile range outside the quartileData value less than or equal to 1.5 times theinterquartile range outside the quartile75th percentileMedian25th percentile

210

STORM

INTENSITY, IN

INCHES PER HOUR

0

1.0

0.5

RUNOFF

COEFFICIENT,

UNITLESS

4 17 26 20 20 47 27 27 31 18 17 10

January

February

March

April

May

June

July

August

September

October

November

December

Number of Events

Figure 2. Seasonal population statistics of stormwater data including none highway-runoff sites with available total precipitation, storm duration, and stormwaterflow data from individual events (data from Driscol and others, 1990b).
Precipitatio

Many runoff models have been designed and 

implemented to compensate for the inaccuracies inherent in simple runoff coefficient methods used to predict 

runoff (Alley, 1977). Results of a recent comparative 

study, however, indicate that even complex rainfallrunoff models may not deliver high levels of predictive 

accuracy (Zarriello, 1998). When nine well documented stormwater-runoff models were used to predict 

stormflow volumes from precipitation data from two 

small watersheds (by experienced modelers using 

detailed precipitation and land-use data), the average 

root mean square model error was about 55 percent and 

simulated storm volumes differed from observed storm 

volumes by as much as 240 percent. 

Despite recognized limitations in accuracy and 

representativeness, precipitation data are necessary to 

document study results in a way that is valid and technically defensible. Although the relations in the existing FHWA data set are not quantitative, it is necessary 

to establish relations between precipitation characteristics, measured flows, and observed contaminant loads 

so that results from lengthy and expensive datacollection efforts can be applied to ungaged sites. Also, 

the ratio of measured runoff to rainfall provides verification data that can be used to identify problems with 

measurement conditions, changes in stage-discharge 

relations, storage between storms, variations in the 

contributing area under different conditions, and 

other possible problems in the data-collection efforts. 

Precipitation data are necessary to define each storm 

and each study period in terms of long-term cycles in 

precipitation. For example, a 1-year study during a 

long-term period of drought may not accurately represent concentrations, flows, and loads for more typical 

wetter years. 

Precipitation measurements also serve several 

useful functions that are not provided by runoff-flow 

monitoring. A recording rain gage provides detailed 

information about the intensity and timing of precipitation. Knowing exactly when precipitation starts and 

stops in relation to the beginning and end of measured 

flows indicates the time of concentration and the time 

of travel in the drainage basin. Precipitation gages will 

record light precipitation events, which may not cause 

a rise in stage sufficient to activate the stormwaterflow-measurement equipment (in which case the stage 

threshold for equipment activation may be reduced for 

subsequent events). Also, if heated gages are used, precipitation gages will record winter events that may not 

result in immediate runoff. 

To collect accurate and representative pre

tion data, a number of technical factors must be 

ered. These factors include the proper siting for 

measuring equipment, the selection of appropri

measurement intervals, the collection of enough 

characterize conditions at a site, and the selectio

methods that will meet data-collection objective

study design. A study may produce a detailed re

precipitation in a study area, but bias introduced 

problems in the study design may limit the qual

usefulness of data collected on site.

Site Selection

Proper siting is necessary for the collectio

accurate and representative precipitation data. T

small drainage areas and large proportion of im

ous areas characteristic of highway catchments 

large variations in measured flow within a few m

of variations in precipitation (Harned, 1988). Th

fore, the placement and density of gages in a stu

are critical factors for interpretation of precipita

data in highway- and urban-runoff studies. Indi

placement and precipitation-gage-network dens

are the two main factors to consider when siting 

for a given study. Proper gage placement will h

ensure that accurate and representative precipita

data may be collected at individual gage sites, a

sufficient gage density within a network will he

ensure the accuracy and representativeness of d

for estimates of precipitation in a given area.

The magnitude of errors for each gage is 

tion of wind speed, siting characteristics, and th

of precipitation (Smith, 1993). High winds are r

nized as the greatest source of error for rain-gag

integrity, so some type of wind shielding is nec

(Alley, 1977; Smith, 1993; U.S. Environmental 

Protection Agency, 1992). Effects of wind creat

vehicles travelling at highway speeds, therefore

factor to consider when siting precipitation gag

highway-runoff studies. Precipitation gages sho

located near the land surface, not on buildings o

elevated structures because mean wind velocitie

increase with height above local land surface (A

1977; Smith, 1993). Although buildings and tre

vide necessary wind shielding, gages should no

placed nearer than the height of the obstacle so 

they do not interfere with the path of falling pre

tion (Alley, 1977; U.S. Environmental Protectio
8 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

Agency, 1992). Poorly exposed gages can underestimate measured precipitation by 5 to 80 percent (Alley, 

1977). It is also important to locate gages on relatively 

level surfaces to prevent bias from poor exposure. In 

small catchments, a precipitation gage should be placed 

near the runoff flow gage to ensure close correlation 

between measurements because variations in measured 

runoff at the surface-water-flow gage are most sensitive 

to variations in precipitation near the measuring point 

(Alley, 1977).

Good precipitation gage locations near highways 

and in urban areas can be hard to find. Highway structures, slopes, buildings, and trees can interfere with 

precipitation. Ground-level gages are prone to vandalism and tampering. Electricity for a heated gage may 

not be available in the highway right-of-way, and water 

formed as a by-product of combustion in fuel-heated 

gages can bias results. Winds and spray from moving 

vehicles can be substantial near the roadway (Irish and 

others, 1996) and cause bias in measured precipitation 

near the roadway.

Precipitation is recognized to be highly variable 

in both space and time. For example, Fontaine (1990) 

indicated that errors in estimates of basin average precipitation from national network data were often 

greater than plus or minus 20 percent and that supplemental study-site gages were necessary to increase network density for urban-runoff studies. During the last 

major FHWA field study in the early 1980's, differences in timing, intensity, and magnitude of precipitation were visible in data records among three stations 

within a few miles of each other (Harned, 1988). The 

need for multiple rain gages in studies of areal extent is 

generally recognized (Alley, 1977). Precipitation-gage 

density is defined as the number of gages per catchment area. The placement of rain gages in a study network should represent catchment topography, and 

ideally should tie in with historical stations in a larger 

network, such as the network operated by the National 

Oceanographic and Atmospheric Administration 

(NOAA) (Alley, 1977). National networks typically 

have density of about 1 gage per 230 square miles. For 

larger watersheds (greater than 100 square miles), gage 

density is more important than the design of gage distribution in the network to estimate basin average 

precipitation (Fontaine, 1990).

Thorough documentation of precipitation

monitoring sites and network design is necessar

the validation of precipitation-monitoring data. 

pertinent to gage siting, such as wind speed and 

tion of prevailing winds, site slope, proximity to 

cles, and location relative to surface-water-flow

measurement stations, must be considered. The 

tion should be specified to the extent that the sit

be reinstrumented for future studies that may la

examine source or land-use changes at a given 

area. Therefore, a detailed site map is warranted 

should have land features, a scale, and at least t

erence points with latitude and longitude to the 

second. The location of precipitation-monitorin

stations with respect to the location of long-term 

monitoring networks is important to help establ

relation between precipitation-monitoring recor

during the study and historical records that wou

cate the comparability of precipitation measure

study period to long-term climatic characteristi

Frequency and Duration of

Precipitation Measurements

The frequency and duration of precipitati

measurement operations is dependent upon the 

scales of the processes under study. Requiremen

sufficient data are defined by data-analysis tech

quality of data needed, program objectives and 

straints, and the representativeness and variabil

the storm events that are gaged and sampled (A

1977). For stormwater-quality studies, the recor

frequency must be sufficient to characterize and 

pret physical (hydraulic) and chemical processe

terms of duration, monitoring equipment needs 

able to record an entire event (at least up to a sp

design storm) and to be durable enough to oper

ably between scheduled maintenance visits. Th

tion of the monitoring program must be designe

to be able to put data into historical perspective

ically, measurement frequency has been control

the sampling budget and the program duration h

controlled by both budget and time constraints. 

Although these will always be real issues, conti

improvements in automatic-monitoring instrum

tion and equipment can improve upon data avai

from manual measurements.
Precipitatio

High-frequency-monitoring capabilities available from state-of-the-art data logger-controlledmonitoring systems have the potential to improve 

the understanding of physical and chemical rainfallrunoff processes. In terms of the monitoring frequency, 

the apparent randomness in stormwater processes 

from storm to storm and from site to site may be 

related to lack of adequate data, especially related to 

the time scales of measurement (Spangberg and 

Niemczynowicz, 1992). The maximum recording interval for individual precipitation measurements depends 

upon catchment size and can range from less than 1 

minute for very small paved catchments to a maximum 

of about 15 minutes for larger catchments (Alley, 1977; 

Spangberg and Niemczynowicz, 1992). In theory, the 

recording interval should not be longer than one-fifth to 

one-tenth of the time it takes for water from the furthest 

point in the catchment to reach the flow-gaging station 

during times of most rapid flows (Alley, 1977). Harned 

(1988) found that in one highway-runoff study, runoff 

in the smallest basin (with an area of 0.0032 square 

miles, including a highway and a rest area) responded 

within minutes to changes in rainfall intensity, and the 

maximum discharge coincided with periods of intensive rain. Stormflow recession was brief in this small 

catchment that had a high proportion of impervious 

cover and an engineered drainage system (Harned, 

1988). Chemical response time for the catchment 

should also be considered in stormwater-quality 

studies. In the field studies sponsored by the FHWA 

that were designed to characterize highway-runoff 

quality, precipitation data were recorded on a time 

scale of about 5 minutes (Shelley and Gaboury, 1986). 

When Spangberg and Niemczynowicz (1992) examined relations between measured precipitation, turbidity, pH, specific conductance, and flow rate (measured 

on a 10-second time interval on a 0.0001-square-mile 

paved parking lot), cross-correlation analysis indicated 

that changes in water quality occurred with changes in 

precipitation intensity and flow rate on a time scale of 

less than 1 minute. Although the high costs for collection and analysis of water samples are a limiting factor 

for many projects, costs for installation and operation 

of automatic precipitation, flow, and water-quality 

instruments do not vary with monitoring frequency. 

Relatively high-monitoring frequencies provide substantially more detail and insight, but do not necessarily require substantially more labor and resources for 

data collection, storage, processing, and interpr

The main drawback to high-monitoring frequen

the possibility of the loss of data by exceeding t

age capacity of the data recording device—can 

avoided by use of a regular station maintenance 

ule coupled with available technology for remo

retrieval by telephone, cellular telephone, radio

satellite link.

The required station maintenance schedu

precipitation-monitoring studies is defined by th

age capacity of the data recording device. Auto

monitoring stations can be programmed to mini

measurement activity during dry periods and to 

mize data collection frequencies during periods 

stormwater runoff (Church and others, 1996). A

tionally, many precipitation gages only record d

when activated by measured precipitation. The 

quency and duration of expected events in a giv

are important factors in these determinations. It 

important to characterize even small events bec

when the frequency distribution of storms of di

size and duration are grouped, the proportion of 

precipitation is about equal for the different sto

classes (Brown and others, 1995). A compromi

between high-resolution monitoring and duratio

be achieved using programming that measures 

high frequency but only records measurements 

frequencies during storm events when flows and 

quality measurements are changing rapidly (Ch

and others, 1996).

On a longer time scale, the duration of pr

tion-monitoring studies is limited by the duratio

project. Studies have shown that decades of rain

streamflow data are necessary to generate desig

statistics in a catchment, but it is also recognize

monitoring durations are impractical for most s

water projects (Alley, 1977). Theoretically, ove

periods of time, the random variation of storm p

in time and space in an area will be equal to ref

stations and, therefore, population statistics wil

similar. There are several standard methods for 

extension when data from one site can be corre

a monitoring site with a long period of record (

and Hirsch, 1992). Long-term monitoring data 

record extension are available from a national w

monitoring network maintained by the NOAA (

1977). Long-term precipitation records may als

available from municipal governments, water an
10 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

wastewater treatment plants, universities, airports, 

news organizations, and other sources. Daily precipitation values, however, are often based on a sampling day 

(for example, 9:00 a.m. one calendar day to 9:00 a.m. 

the next calendar day), so direct day-to-day correlation 

may be difficult if daily data is not synchronized 

among data sources.

Thorough documentation of the frequency and 

duration of data from precipitation-monitoring stations 

is necessary to ensure the validity and usefulness of 

data collected. Comparison of the characteristics of 

measured precipitation during the study period is necessary for immediate and future users of the data in 

order to put observations made during the study period 

into a long-term perspective that will improve the interpretive/decision-making process. Supporting data or 

the source of published data (such as the NOAA 

records from a given monitoring station) and the comparative analysis should be documented in published 

reports for future use.

Methods for Measuring

Precipitation

Methods that allow accurate monitoring of 

precipitation intensity and total accumulated precipitation are necessary for planning, design, collection, 

and interpretation of results for stormwater-quality 

studies. Historically, a 0.01-inch (0.25-mm) precision 

level has been considered to be comparable with 

distortions in precipitation catch encountered in urban 

areas, the areal variability of precipitation, and the precision level of other stormwater-monitoring instruments (Alley, 1977). At least one recording gage is 

necessary to provide the detailed precipitation information needed at each study site, but data from nonrecording gages can supplement this information, and (or) be 

used to build correlations among established 

precipitation-monitoring sites.

Nonrecording precipitation gages (manual measurements) are generally sufficient for measuring total 

precipitation during the measurement period. These 

gages do not directly provide information about the 

actual timing, duration, or intensity of precipitation that 

occurs during the measurement period. Any open container with an established rating between precipitation 

catch and either weight or depth of precipitation 

collected can be used as a nonrecording gage (U

Environmental Protection Agency, 1992). Nonr

ing precipitation gages can provide excellent ve

tion (Quality Assurance and Quality Control) d

because they are easily constructed and (or) ine

sive to obtain. One or more nonrecording gage(

be used in conjunction with a recording gage to 

substitute information in case of equipment fail

number of these devices can be emplaced to sup

ment recording gages and used to examine assum

about the areal distribution of total precipitation 

around a study area. Data from these gages can 

biased by evaporation or by overflow condition

time between manual measurements is substant

Results from visual gages can be biased by para

and water displacement, or absorption upon ins

of a measuring stick. When using nonrecording 

records for snow events must be derived from m

ments of snow depth and water content (Alley, 

Representative snow measurements from nonre

ing gages in highway rights-of-way may be diffi

because of variations caused by natural and veh

induced winds, as well as by snow removal/deic

operations. 

Recording precipitation gages (automatic 

surements) have several advantages over nonrec

gages. Recording gages can record the timing, d

tion, and intensity of precipitation that occurs d

the measurement period, as well as indicate the 

precipitation for each storm. Depending upon th

design of the gage, evaporation is either not an i

evaporation between events can be determined 

data records. Also, automatic gages are general

designed to prevent or reduce errors from overfl

Most rain gages, however, have a tendency to u

record when rain is greater than 3.0 inches per h

(Alley, 1977). Studies in areas with large variat

in precipitation intensities may require more tha

one gage, each with different resolutions, at eac

monitoring site (Spangberg and Niemczynowicz 

Weighing, float, and tipping-bucket gages 

three main types of recording precipitation gage

are widely accepted and readily available (Alley

FHWA, 1985; U.S. Environmental Protection A

1992). Weighing gages measure and record the 

of water in the collector at each time interval. F

gages measure accumulated rain by recording t

tion of a float in a collector. Float gages can be e
Stormwater-Flow Measurem

by a siphon tube or by an automatic pump when full. 

Tipping-bucket gages measure precipitation by recording the actuation of a small seesaw each time the receptacle (the bucket) at an end of the pivot fills, tips, and 

empties. Tipping-bucket gages have a long record of 

proven ability, commercial availability, and are the 

most widely used (Alley, 1977). 

Snow is more difficult to measure than rain. 

Weighing gages are generally better for snow than 

other gages. Float gages and tipping-bucket gages are 

not suitable for measuring snow unless they are heated. 

Requirements for heating gages raise logistical and 

interpretive complications due to the necessity for fuel 

or power for heating and accounting for the precipitation lost to condensation as a result of this heating.

Improvements in collection and interpretation of 

weather radar and satellite data over the last 10 years 

should be considered to provide information about 

local precipitation characteristics when planning a 

study or verifying data collected. Radar has high temporal (as small as 5 minutes) and spatial (as small as 

0.386 square mile) resolution and range over a range of 

up to 130 miles (Smith, 1993). Radar measurements 

are subject to a number of sources of uncertainty, and 

so may not be sufficient as a primary precipitationmonitoring system, but they may be obtained from the 

National Weather Service, news organizations, and airports. Many of these organizations post these data to 

the internet. Precipitation estimates from satellite measurements are based upon infrared imagery of cloudtop characteristics. Although these estimates are not 

precise, this information may be used to estimate rainfall in areas not covered by data networks using more 

precise methods (Smith, 1993).

Thorough documentation of precipitationmonitoring methods and measurement equipment 

used is necessary for the validation of precipitationmonitoring data. Factors pertinent to manual and electronic recording device, such as calibration and maintenance records, the maintenance schedule, the 

measurement interval, and equipment malfunctions, 

should be documented and archived in project records. 

Details about equipment construction and operation of 

gages (including equipment specifications) should also 

be documented and archived in project records. Precipitation records in published reports should include the 

measurement interval and equipment specifications 

that are relevant to interpretation and calibration of the 

data. Simply recording the make and model of a 

will not be sufficient if specifications change or 

detailed information may not be available from 

manufacturer.

STORMWATER-FLOW 

MEASUREMENTS

The accuracy and representativeness of 

stormwater-flow measurements for computation 

pollutant loads and event-mean concentrations, 

whether from a natural stream channel, an engi

channel, a highway or urban drainpipe, sheetflo

a parking lot, or overland flow from a grassy sw

based on many common factors that all contribu

the uncertainty of the data set. These factor incl

• The representativeness of the site selected 

relation to the contributing area of conce

• The ability to obtain accurate flow measure

at the selected site,

• The timing, frequency, and duration of flow 

surements, relative to the timing, intensit

duration of the storm, to fully characteriz

flow event, and

• The ability of the flow-measurement metho

accurately measure the full range of flow

frequency required to fully characterize t

event.

Selecting representatives sites, ensuring t

suitability for accurate flow measurement, deter

ing appropriate measurement frequencies, and s

ing the best method for measuring flows may re

significant effort, but are critical for the measur

of accurate and representative flows. For examp

when receiving waters are also monitored, the s

of the stream channel bed and banks up and dow

dient of the proposed site must be assessed befo

site can be assumed to consistently yield accura

streamflow data. Selecting a representative sect

pipe for measuring flow requires analysis of the 

network above the site to identify all contributin

areas, and analyses of the pipe network below t

to identify potential for backwater flow. The flo

regime (steady- or unsteady-state flow, subcritic

supercritical, or pressure flow) and changes in t

regime with stage need to be evaluated for selec

the appropriate method for measuring the flow. 
12 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

many stormwater-flow measurements are made for the 

determination of pollutant loads, factors that may affect 

water-quality properties and constituents, and collection of water samples also must be considered in selecting a site and in determining frequency of flow 

measurements. Although this report is focused on 

stormwater flow in small streams and in highway- and 

urban-drainage systems, many of the principles upon 

which accurate and representative flow measurements 

are obtained in large streams are applicable to flow 

measurements in small streams and drainpipes, and are 

therefore included in this report.

Documentation and reporting of the supporting 

data from which decisions were made as to where 

along a stream channel or within a highway- or urbandrainage network flow will be measured, how 

frequently flow will be measured, and what method 

will be used to measure the flow are required for internal and external evaluation of the accuracy and representativeness of the flow data. Important questions that 

must be addressed in the selection of a representative 

site where accurate and complete flow measurements 

can be obtained are listed in the following sections. 

Although the time and effort expended to address these 

questions to ensure accurate and complete flow measurements at a representative site may be considerable, 

documentation and reporting of this effort should be a 

rather simple task if each step in the process is 

described in detailed field notes during the selection 

process. To ensure that the accuracy and representativeness flow measurements can be evaluated, the supporting data and information used to make the final 

decisions must be documented and communicated in 

an accessible format, such as a project description 

report, a data report or an appendix to a technical 

report, and (or) archived in a State or national records 

center.

Site Selection

Selecting a location for obtaining flow measurements within the drainage network requires evaluation 

of the representativeness of the site in yielding flow 

data that are consistent with the objectives of the investigation, and the hydraulic and physical suitability of 

the site where accurate flow measurements can be 

expected to be obtained. The importance of proper site 

selection cannot be overstated. No matter how a

the flow data, if the site does not provide inform

meet project objectives, the data have little mea

(Whitfield, 1988). Ideal sites rarely exist, howev

a compromise between many factors must be m

selecting the best site. The basic questions that 

be addressed in selection of the best, or most re

tative, site are:

• Will flow measured at this site represent th

tribution from the area of study?

• How are the flow velocities distributed?

• How stable is the flow regime?

• Can a stage-discharge relation be develope

• How steady would this stage-discharge rel

remain over time?

• Is access to the site acceptable?

• Can equipment be installed?

• Can manual measurements of flow be mad

• Is floating debris manageable?

• Is the site safe for personnel and equipmen

Consideration of the above questions in se

a site may require a significant amount of office 

fieldwork. The time and effort expended, howev

ensure that the site selected, from among other 

tial sites, will yield stormwater-flow data most r

sentative for the project objectives. The risk of 

selected a poor or non representative site is sign

cantly reduced by this initial investment. Additi

the information obtained during the site-selectio

process must be clearly documented and includ

data report or in another accessible format to al

independent evaluation of the selected site, and 

potential use of the site for future investigations

Guidelines for site selection of gaging stations 

streams are provided by Carter and Davidian (1

Rantz (1982a), the Federal Highway Administr

(1985), and the Natural Resources Conservation 

vice (1996).

The initial site selection (whether along a 

channel, within a highway- or urban-drainage n

or from a paved surface or grassy swale) and alte

site selections should include review of reports 

other documents concerning the hydrology of th

drainage area, examination of maps or highway

urban-drainage network plans, and personnel co

nication with State and town transportation age

and residents living near the proposed site. Dra

basin area, relief, slope, elevation, stream-netw
Stormwater-Flow Measurem

pattern, and locations of tributary streams can be determined from topographic maps or readily available geographic information system (GIS) data bases. Land use 

may be inferred from these maps as well. Drainpipe 

network, pipe slope, locations of catch basins and manholes, pervious and impervious areas, slopes of paved 

areas and grassy swales with drainage catchments, and 

other physical structures can be determined from asbuilt, or pre-built site plans, although in older built 

areas this information is sometimes difficult to find. 

This initial information in site selection is necessary 

because it provides a general understanding of the flow 

system, identification of location within the drainage 

system where the most representative data can be collected, and an initial evaluation of upgradient and 

downgradient factors that may unduly influence flow 

measurements at the selected location.

Field inspection is required to ensure that the site 

is hydraulically and physically suitable for accurate 

measurements of flow and that the site can be accessed 

and data collected safely. The basic hydraulic considerations are the distribution of velocities within the flow 

and potential changes in flow regime with stage. A uniform velocity distribution in the flowing water throughout the full range of flow, with no change in flow 

regime, would provide for the ideal conditions whereby 

the flow rate could be determined from one measurement of water depth. The velocities in most flows are 

not distributed uniformly, however, and the distribution 

of velocities and flow regimes may change over the 

range of flows. To account for this non-uniform 

velocity distribution and potential changing flow 

regime in streamflow measurements from moderate to 

large streams, flow rates are measured in many thin 

vertical sections along a line perpendicular to the 

stream channel (Buchanan and Sommers, 1969; Rantz, 

1982a). In small streams and in highway- and urbanstormwater drains, however, multiple measurements 

are typically restricted by space and time. The small 

number of flow measurements attainable due to the 

narrow widths, and sometimes shallow depths, are 

insufficient for accurate flow measurements, and due to 

the rapidly changing flow, each individual measurement could represent part of a different flow rate and 

velocity distribution. In these types of flows, flowcontrol devices, such as weirs and flumes, are commonly used (Buchanan and Sommers, 1969; Marsalek, 

1973; Alley, 1977; Kilpatrick and Schneider, 1983; 

Kilpatrick and others, 1985; Federal Highway 

Administration, 1985; Natural Resources Conse

Service, 1996). Flow measurements from these 

typically require only one measurement of stag

unit time because they produce a consistent distr

of velocities throughout the nearly full range of 

Physical considerations are generally rela

selecting a site where the distribution of velocit

the flowing water is minimally disturbed, and is 

expected to remain so over the period of investi

Although multiple flow measurements are used 

stream-discharge measurement and flow-contro

devices are used for flow measurements in high

and urban-drainage systems to account for the n

uniform distribution of velocities, evaluation of 

tribution of velocities remains an important par

site selection process. Therefore, field inspectio

includes an upstream and downstream evaluatio

flow characteristics and factors that may affect t

in space and time, such as the stability and unif

of the stream-channel-bed and bank sediment, t

bility of the channel bank and adjacent flood-pl

vegetation, the straightness of the channel, later

tion of the channel within the flood plain, flow p

within channel, variations in channel width and 

and the proximity of small tributaries, rivulets, 

and physical structures that are not shown on th

of the area, and the presence of floating or subm

debris. Visual inspection of land use and its pos

effect on flow and flow measurements should b

For highway- and urban-drainage systems, the l

tions and elevation of catch basins, manholes, p

intersections, and outfalls should be checked w

plans, and corrected on the plans if needed. Alth

validation of location of underground pipes in h

way- and urban-drainage networks may be diffic

flow routes can usually be determined by visual 

tion of the elevation and direction of pipes, and 

material composition, diameter, and number, fr

which flow enters and exits catch basins and ma

Field inspection should also include an estimate 

relative amount of pervious and impervious area 

the catchment area. Thorough field inspection w

ensure that a site of minimal-flow turbulence is 

selected, or can be constructed, for measuremen

flow representative of the expected sources of r
14 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

As the measurements of stormwater flow in 

highway-runoff studies are used primarily for determination of pollutant loads, factors affecting measurements of water-quality properties and constituents and 

sample collection should also be considered in the siteselection process. For example, sufficient flow depth 

for complete submergence of water-quality probes is 

necessary, and factors such as backwater from downstream controls that may affect the temporal representativeness of samples and water-quality measurements 

need to be evaluated. If project objectives allow, select 

a site where data may be applicable to more than one 

investigation, or where data collected in the future can 

be used to evaluate trends.

Maps, tables, and written descriptions of the 

hydrologic features of the stream or drainpipe network 

are necessary to evaluate the quality of flow data 

with respect to the appropriate location of the flowmeasurement features. A report should clearly indicate 

the position of the flow-measurement station with 

respect to the catchment area, local and surrounding 

land uses, and the relative amount of pervious and 

impervious areas contributing. It is important to document the location and characteristics of the natural or 

constructed flow-control features. It is also important to 

document the slope of the stream/pipe/swale to help 

establish the flow regime. Where overland flow is measured, detailed information about the surface characteristics and flow-concentration structures are necessary.

This careful and thorough review of maps or 

construction plans and field inspection will help ensure 

that reasonably accurate and representative flow measurements can be obtained. Documentation of the initial site evaluation and the field inspection will ensure 

that the site located for collection of flow measurements can be validated. It would be unusual if an ideal 

site was found. But by documenting the information 

obtained during the site-selection process, archiving 

the documentation, and including pertinent information 

in a published project description or data report, or in 

an appendix to an interpretive report, a level of certainty of the data collected and interpretations made 

may be evaluated.

Frequency and Duration of

Stormwater-Flow Measurements

The timing, frequency, and duration of flo

surements are critical factors in monitoring acc

flows in small streams and highway- and urban

because of the rapid response to stormwater run

the wide ranges of flow over short periods of tim

with precipitation, frequency and duration of flo

surements are dependent upon the time scales o

process under study. Additionally, flows in resp

stormwater runoff typically rise more quickly th

fall, and pollutant concentrations have been sho

rise and fall more quickly than the flow in whic

are transported (Vanderborght and Wollast, 199

Spangberg and Niemczynowicz, 1992; Barrett a

ers, 1993). This phenomenon, referred to as the 

flush or initial wash off, is especially prominent 

highway- and urban-drains. Irish and others (19

found that most of the constituents in highway 

are attached to fine-grained sediments that tend 

accumulate within 3 feet of the curb during dry 

ods. This proximity to the curb allows for the se

and chemical constituents to be entrained in the 

ment runoff and curb flow, and discharged into 

drainpipes in the early part of a storm. The mag

and extent of this first flush also can be affected 

nature and solubilities of the constituents being 

ported in the water (Hvitved-Jacobsen and Yous

1991). Although pollutant concentrations may b

siderably less in the latter part of the runoff even

in the first part, pollutants may continue to be d

charged, necessitating flow measurements throu

the entire duration of the event (Barrett and oth

1993). Stormwater flows respond differently to 

ent types of storms and may respond differently 

same type of storm in different seasons of the y

Therefore, it is critical that measurements of flo

at the beginning of the storm, continue through 

duration of the event, and are measured at a freq

corresponding to the rate of change of flow and 

uent concentrations to ensure the accuracy and 

sentativeness of the resultant flow and pollutant 

Collection of water-quality data should be sync

nized with the timing of flow measurements so 

concentrations can be directly applied to measu

flows.
Stormwater-Flow Measurem

A general understanding of the rainfall-runoff 

relation in the region (area) is needed to evaluate 

the timing, frequency, and duration of flow measurements that will ensure accurate and representative 

stormwater-flow data. The basic questions that need to 

be addressed in selecting the timing, frequency, and 

duration of flow measurements include:

• What is the time of concentration of flow and 

pollutants in relation to storm intensity?

• What is the rate of change of flow?

• What is the range of flows?

• How do rates of changes of flow and ranges of 

flow differ between storm types and seasons?

• Can flow measurements be synchronized with 

collection of pollutant samples?

Guidance in the initial selection of frequency of 

flow measurements, whether measuring flow in a highway drainpipe or in a stream channel, can be obtained 

by examining historical precipitation and hydrologic 

data from near the proposed site, or from other similar 

sites within the same type of physiographic region. For 

highway- and urban-drains, data should be available 

from the engineering firm that designed the drainage 

networks, or from the State or municipal agency 

responsible for maintaining the drainage system. Pipe 

diameters were likely designed for a maximum openchannel-flow depth for a specific storm intensity, duration, and recurrence interval. Field observations of flow 

during and after storm events can be very useful. Additionally, a numerical method for approximating the 

minimum frequency of flow and concentration measurements for meeting a desired accuracy is available 

(Nesmerak, 1986). However, a more accurate method 

for selecting frequencies of flow measurements to 

ensure the accuracy and representativeness of flow volumes and pollutant loads measured in stormwater 

runoff is use of continuous electronically measured and 

recorded-stage and water-quality measurements, such 

as specific conductance and (or) turbidity, in response 

to storm events. The times of concentration of flow and 

constituent concentrations can be interpreted from the 

electronically recorded data, and the frequencies of 

measurements needed at various stages and times 

throughout the event can be determined. Continuous 

measurements and recording of stage and water quality 

at different times of the year, during different types of 

storms, or under different antecedent conditions, will 

provide data needed to optimize recording and 

pling frequency under a variety of conditions an

similar sites.

In a study in which the constituents of ro

in highway runoff were measured in the trunkli

drainpipes of a six-lane highway (Church and F

1993; Church and others, 1996), stage and spec

ductance measurements in the approach section

Palmer-Bowlus flumes were electronically mea

every minute, but only recorded every hour at ti

minimal-to-zero flow. To account for the initial 

flush of the road-salt constituents during runoff

recording and water-quality-sampling frequenci

automatically increased to a minimum of 10 mi

and a maximum of 1 minute in response to chan

stage and specific conductance (fig. 3).

Data used to establish the timing, frequen

duration of flow measurements need to be docu

and reported to help ensure that the flow data ca

validated. These data include the type of draina

system (i.e., stream, highway, or urban drainage

drainage area, stream channel or pipe slope, per

impervious area, climatic and meteorological da

if pollutant concentrations and loads are to be m

sured, the source, amount, distance to monitorin

station, and when pollutants are released.

Methods for Measuring

Stormwater Flow

Methods have been developed for measur

flow in many types of conduits (flow in natural 

nels, engineered channels, pipes, sheetflow, and 

land flow) under various flow regimes (steady- 

unsteady-state flow, subcritical, supercritical, or 

sure flow). Most of these methods have two part

mary device that directly interacts with or contr

flowing water, and a secondary device for meas

water depth or pressure (Marsalek, 1973; Alley, 

Selection of the most appropriate method for co

of accurate flow data that are representative of t

ticular site requires knowledge of the flow regim

range of flow and flow depths, rapidity of chang

flow, channel geometry, and the capabilities and 

accuracies of the methods available for measurin
16 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

0

20,000

5,000

10,000

15,000

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER CENTIMETER

2400 0400 0800 1200 1600 2000 2400 04

24 25

February 1994

0

3.5

0.5

1.0

1.5

2.0

2.5

3.0

FLUME STAGE, IN INCHES

ABOVE THE THROAT

TIME:

DAY:

MONTH:

Figure 3. Stage and specific conductance monitored at various frequencies in response to changes of stage and 

specific conductance (Church and others, 1996).
Stormwater-Flow Measurem

Assuming that the flow and channel characteristics 

were assessed in the site-selection process, the important questions that remain are:

• Is the flow measuring method applicable to the 

flow and channel characteristics at the site?

• Is the flow measuring method capable of measuring the full range of flows?

• Is the flow measuring method capable of measuring flow at frequencies required to fully characterize the event?

• Will the flow measurements be of sufficient accuracy to meet the objectives of the study?

• Can the accuracy of the flow measurement method 

be verified with another method?

Data that need to be documented and reported 

for validation of a selected flow measurement method, 

and to ensure that the flow measurement method can be 

independently validated, include the hydraulic characteristics of the flow and the capabilities and limitation 

of the method. A report should include the observed 

type of flow and changes in flow type during storm 

events, ranges of types of flows measured, and the 

method used to measure the flow. If a flow-control 

device was used, details as to the construction, installation, depth/flow relation, calibration, and maintenance 

should be reported. Equipment and instrumentation 

(primary and secondary devices), and their resolution, 

tolerance, and design limits, as defined by the manufacturer, should be documented as well as calibration and 

maintenance records. Modifications to the method and 

the resultant resolution, tolerance, and design limits 

also should be documented. With this information, 

the appropriateness of the selected method, the placement and use of a flow-control structure, and the accuracy and precision of the flow data measured, can be 

evaluated.

The most common types of flow measurement 

methods and their applications are described below. 

Further guidance in the selection of an appropriate 

method of flow measurement can be found in Marsalek 

(1973), Shelley and Kirkpatrick (1975), Alley (1977), 

Federal Highway Administration (1985), and Natural 

Resources Conservation Service (1996).

Primary Devices/Methods

Channel Friction Coefficient Method

This method is best described by Mannin

equation, in which flow is related to the hydrau

radius of the flow cross section, slope of the wa

face, and an estimated friction of the channel, r

to as a roughness coefficient. The accuracy of fl

determined by Manning’s equation are depende

steady, uniform flow in straight channels or pip

uniform shape, slope, and roughness. Manning’

tion is useful for estimating flow in ungaged op

channels and pipes. The accuracy of the flow de

mined, however, varies widely. Brown and othe

(1995) reported of errors up to 15 percent in flo

surement in short, straight channels by use of th

method. Others studies have shown that the accu

this method, at best, is about 15 to 20 percent (A

1977). Marsalek (1973) stated that under condit

unsteady, non-uniform flow in drainpipes, typic

flows in highway- and urban-drainpipes, Manni

equation will underestimate flows in the rising 

and overestimate flows in the falling stage. As p

concentrations have been shown to peak before 

flow in which they are transported (Barret and o

1993; Spangberg and Niemczynowicz, 1992), e

pollutant loads are likely to be greater than 20 p

if the flow was determined by Manning’s equat

Index Velocity Method (Current-Meter Metho

In most streams, where flows change slow

time in comparison to flows in highway- and ur

runoff-drainage systems, point velocities are me

in multiple vertical sections along a cross sectio

stream channel by use of a velocity meter or cu

meter (Buchanan and Somers, 1969; Rantz, 198

Rantz, 1982b). The velocity in each vertical sec

measured at a depth that theoretically, and field 

fied, represents the mean velocity in that section

depth of the flow is sufficient, velocity is measu

two or more depths, the average representing m

velocity. Mean velocities are multiplied by the c

sectional area they represent, and are then summ

obtain the total flow, or discharge, at that stream 

section. Many velocity/area measurements are t

along the stream cross section to reduce the infl

of irregularities in the stream channel and non-u

distribution of velocities at the stream cross sec

the total flow measurement. Rantz (1982a) state
18 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

total discharge at a streamflow section is usually represented by the sum of discharges from 25–30 subsections (fig. 4). Relations between stage (the height of the 

water surface relative to a stable reference point) and 

discharge are developed and refined as more discharge 

measurements are made. Secondary devices, such as 

floats or pneumatic bubbler systems and mechanical or 

electronic data recorders, are used for continuous monitoring and recording of stage (Buchanan and Somers, 

1968; Marsalek, 1973). Continuous records of stage are 

applied to the stage-discharge relations to generate 

continuous records of discharge.

The accuracy of a subsection discharge measurement is a function of the accuracies of the measured 

cross-sectional area of each vertical section, the velocity measurements, and whether the measured velocities 

represent mean velocities which are based on assumed 

velocity profiles (Alley, 1977). Errors in velocity measurements arise primarily from poorly calibrated and 

poorly maintained meters, and from velocity measurements obtained at inappropriate depths or under turbulent flow conditions. The accuracy of the stage 

measurements is dependent upon the resolution of the 

equipment and instrumentation, use within their 

designed ranges, and proper calibration and maintenance. Sauer and Meyer (1992) stated that the error of 

most discharge measurements using the current-meter 

method (with vertical axis, cup-type current meters 

(Buchanan and Somers, 1969)) ranges from 3 to 6 percent. Under ideal conditions, an error as low as about 2 

percent can be achieved, but under poor conditions the 

error may be greater than 20 percent.

Documentation of the tolerance and resolution of 

the equipment and instrumentation is needed along 

with calibration data and service records to ensure that 

stage measurements can be validated. The importance 

of accurate and verifiable stage measurements cannot 

be understated because stage is used as a surrogate for 

flow in most reported flow measurements. The accuracy of the stage-discharge relation is dependent upon 

the accuracy of the individual stage and discharge measurements that define the relation, which in turn are 

dependent upon the accuracy of the many velocity/area 

measurements that constitute a single discharge measurement. The extent to which the stage-discharge 

relation can be confidently applied is related to the 

range of flows measured. Therefore, collection 

flow measurements that represent the full range 

discharges is desirable.

The current-meter method is used primar

flow measurements in moderate to large streams

small streams with moderate to low slopes. Oth

methods, such as weirs, flumes, and dye dilutio

more accurate flow measurements in small strea

with high slope, and in highway- and urban-dra

pipes (Katz and Fisher, 1983).

Weirs

Weirs are overflow-control structures inst

a small stream channel or culvert that produce a 

tion between the depth of water behind the weir 

flow (Marsalek, 1973; Alley, 1977). Weirs are ty

made of thin rectangular metal plates set vertica

across the channel. Weirs referred to as “broad c

are constructed with concrete. Flow is forced ov

top edge of the metal plate or concrete weir. To 

sure low flows more accurately, the middle of th

edge of the metal plate is cut to form a V-shaped

zoidal, or rectangular notch. Stage is measured 

floats or pneumatic bubbler systems and record

mechanical or electronic data recorders. Contin

records of stage are applied to the stage-flow rel

generate continuous records of flow. However, t

stage-flow relation breaks downs under the cond

submergence or surcharge. Field calibration of 

stage-flow relation is necessary.

Weirs are useful for measuring flows in s

low-velocity stream channels where the index v

method may be inappropriate due to shallow de

and non uniform flow (Buchanan and Somers, 1

and at outfalls and in open channels (Alley, 197

Accuracies within 5 percent can be attained if t

is calibrated (Marsalek, 1973). Although weirs 

simple to construct and cost little compared to m

other methods, they are not recommended for u

within pipes because they restrict the flow and c

excessive backwater and debris accumulation, a

susceptible to submergence and surcharge (Alle

1977).
Stormwater-Flow Measurem

Water Surface

Streambed

Initial

Point 1

2

3

4

5

6

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

n

(n-1)

b1

b2

b3

b4

b5

b6

bn

b(n-1)

2 d

3 d

4 d

5 d

6 d

(n-1) d

n d

EXPLANATION

1, 2, 3 ................. n OBSERVATION VERTICALS

b1, b2, b3 ............ bn DISTANCE, FROM THE INITIAL POINT TO OBSERVATION VERTICAL

d1, d2, d3 ............ dn DEPTH OF WATER AT OBSERVATION VERTICAL

Figure 4. Example of stream channel cross section showing current-meter method where flow velocities in subsectional areas are used to 

measure stream discharge (modified from Rantz, 1982a).
20 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

Flumes

Flumes are flow-constriction structures that control the flow hydraulics such that flow is directly related 

to head (Marsalek, 1973; Alley, 1977; Kilpatrick and 

Schneider, 1983). Flow in a small stream or drainpipe 

passing through a flume is accelerated, resulting in 

decreased depth, by some combination of sidewall 

contractions, raised floor, or increased slope. 

Flow exiting the flume decelerates when reentering the 

channel or pipe. Flumes in which subcritical flow in the 

approach section of the flume remains subcritical, but 

at a higher velocity in the contraction (throat), require 

head measurements at both the approach and throat 

sections of the flume to compute flow. A direct relation 

between head in the approach and flow in the throat 

exists in flumes where flow becomes supercritical in 

the throat. Due to the need for two head measurements 

in the subcritical flow flumes, they are seldom used 

today (Kilpatrick and Schneider, 1983). The most commonly used flumes are the Parshall flume and the 

Palmer-Bowlus flume, both of which produce supercritical flow in the throat.

The Parshall flume is used for measuring flow in 

small streams, at outfalls, and in open channels where 

the index velocity method may not be appropriate due 

to shallow depths, narrow widths, and non-uniform 

flow. Due to its rectangular shape, elongated structure, 

and requirement for a vertical drop through the flume, 

it is not very useful in measuring flow within drainpipes. The Palmer-Bowlus flume was designed for use 

in drainpipes, and is not very useful in any other flow 

conduit. A Palmer-Bowlus flume installed in the trunkline drainpipe of a six-lane highway-drainage system is 

shown in figure 5 (Church and others, 1996). Flumes 

should be installed at sites where the potential for surcharge, full-pipe pressurized flow, and backwater 

effects are expected to be negligible. Although the 

Palmer-Bowlus flume acts as a venturi meter under 

full-pipe pressurized flow, and flow rate can be calculated from head measurements in the approach and 

throat of the flume, the relation between head and flow 

breaks down in the transition zone from near-pipe-full 

to pipe-full flow (Kilpatrick and others, 1985). Advantages of flumes over weirs include less backwater and 

their self-cleaning abilities.

Theoretical depth/flow relations are deve

for weirs and flumes based on their geometry, a

the case of flumes, slope (Buchanan and Somm

1969; Kilpatrick and Schneider, 1983, Kilpatric

others, 1985). The accuracy of these flow measu

ments is dependent upon the accuracy of the co

tion and installation of the weirs and flumes in t

stream channel or drainpipe (i.e., level in a dire

perpendicular to stream channel or pipe, no def

tion during construction or installation, no leak

approach section), and the measured geometry, 

and friction of flume surfaces. A well-construct

calibrated, and maintained flume may yield flow

accuracies of 2-3 percent (Buchanan and Somm

1969; Marsalek, 1973); however, when factorin

error of the secondary device used for monitori

stage, the accuracy is about 5 percent (Marsalek

Alley, 1977).

Differential Pressure Method

The differential pressure method is used t

sure full-pipe pressurized flow and, as this flow 

tion is rare in highway- and urban-drainpipes, i

limited value in highway- and urban-runoff flow 

surements (Alley, 1977). Flow-constriction dev

however, have been developed that act as critica

flumes under open-channel-flow conditions (the 

in the approach section is directly related to flow 

throat section), and venturi meters under full-pi

pressurized conditions when the flow can be cal

from the difference in heads in the approach and 

sections of the flume. One is designed with a si

side wall constriction (Wenzel, 1975) and the o

with a U-shaped throat constriction (Smoot, 19

Flows at near-pipe-full, the transitional zone fro

open-channel to pipe-full flow, are difficult to m

because the flow pulsates from open-channel to 

surized flow (Kilpatrick and others, 1985). Kilp

and Kaehrle (1986) used a modified Palmer-Bo

flume to measure open-channel and pipe-full flo

an electromagnetic velocity meter to measure fl

the transitional zone from open-channel to pipe

flow. They reported that the electromagnetic ve

meter was not successful in measuring accurate 

in this transition zone.
Stormwater-Flow Measurem

EQUIPMENT SHELTER

CENTRAL

PALMERBOWLUSFLUME

PALMER-BOWLUS FLUME

MANHOLE

LAND SURFACE

C O N C R E T E V A U LT

APPROACH

SECTION

THROAT

SECTION

SPECIFIC CONDUCTANCEPROBE AND AUTOMATICWATER-SAMPLER INTAKE

ACCESS PIPE OPENING

DRAINPIPE

SECTIONAL VIEW

END VIEW

NOT TO SCALE

Figure 5. Highway-drainage-monitoring station with Palmer-Bowlus flume installed in trunkline drainpipe of a highway-drainage system (Church and others, 

1996).
22 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

Acoustic and Electromagnetic Methods

The acoustic transit-time flowmeters (Laenen, 

1985; Kilpatrick, and others, 1985; Kilpatrick and 

Kaehrle, 1986; Burch and Philips, 1994) and electromagnetic velocity meters (Kilpatrick, and others, 1985; 

Kilpatrick and Kaehrle, 1986) are designed to measure 

flow velocities under open-channel and full-pipe flow. 

Flow velocities are multiplied by cross-sectional areas 

of flow to determine flow rates. The acoustic flowmeter 

has been reported to obtain flow accuracies of 2–3 percent under open-channel flow, and 0.5–1.0 percent 

under pressurized flow (Burch and Philips, 1994). 

However, the acoustic flowmeter was not successful in 

measuring accurate flows in the transition zone 

between open-channel and full-pipe flow. As with other 

primary devices, the accuracy of flows measured with 

the acoustic and electromagnetic methods decrease 

when factoring in the error of the secondary stage 

measurement device (Marsalek, 1973; Alley, 1977).

Dilution Methods

Dilution methods involve injecting of tracer of 

known volume and concentration into a stream or flowing water in a pipe and measuring the concentration of 

the tracer in water collected at some short distance 

downstream (Alley, 1977; Kilpatrick and Cobb, 1985; 

Kilpatrick and Wilson, 1989). Dilution methods are 

unique because they do not require a secondary device 

for measuring stage. There are three types of dilution 

methods: instantaneous injection with steady-state 

flow; continuous, constant-rate injection with steadystate flow; and continuous, constant-rate injection with 

changing flow. The accuracy of the three methods 

depends upon complete mixing of the tracer by the 

time it reaches the point of collection and minimal dye 

loss during transport. The first two methods require 

multiple samples to provide one measurement of flow. 

In the third method, each sample of diluted tracer 

represents a specific flow.

Dye-dilution-discharge measurements have been 

used to verify stage-discharge relations in streams. 

Assuming steady-state flow and complete mixing, a 

streamflow measurement can be made by analyzing the 

concentration of dye in water samples collected at the 

measuring section from an upstream instantaneous 

injection of a known volume and concentration of dye. 

Flow is inversely proportional to the concentration of 

the tracer in the diluted sample. Due to the rapi

changes in flow in highway- and urban-drains, t

tinuous, constant-rate dye-injection dilution me

must be used (Duerk, 1983; Katz and Fisher, 19

Kilpatrick, and others, 1985). This method of d

tion has proven to be effective in calibrating the

stage-discharge relation from Parmer-Bowlus fl

(Katz and Fisher, 1983). Ellis and others (1984) 

dye-dilution methods to calibrate a velocity-flow 

Abrahams and others (1986) have used dye-dilu

techniques to measure overland flow. In a moni

study of road salt in highway runoff (Church an

Friesz, 1993; Church and others, 1996), theoret

ings for Parmer-Bowlus flumes were developed 

on flume geometry and pipe/flume slope. Bias i

ratings caused by small irregularities in the flum

walls and in the floors of the throat sections we

rected by use of constant-rate dye-injection dilu

measurements. 

Secondary Devices/Methods

Floats

Monitoring the level of floats connected b

cable or a pivot arm in flowing water is a well-k

method for monitoring water levels (Marselak, 

Alley, 1977). The vertical movement of the floa

water levels change is typically recorded on a c

record or recorded electronically. Floats should 

in stilling wells where water-surface oscillation

dampened and the float is protected from floatin

debris. Under these controlled conditions, accur

water-level measurements of 0.01 ft can be atta

(Marsalek, 1973). Highway- and urban-drainpip

however, generally do not provide space for a s

well.

Pneumatic Sensors

Water levels are measured with pneumati

sors as gas (air or nitrogen) is forced through a 

tube and slowly bubbles into the water. The pres

the gas is equal to the static pressure of the wate

the orifice of the tube (Marsalek, 1973; Buchan

Somers, 1968). Water levels are converted from 

sure to depth by manometers or pressure transd

Pneumatic sensors are well suited to measuring 

levels in highway- and urban-drainpipes becaus
Stormwater-Flow Measurem

can be easily installed, do not obstruct flow, and with 

the exception of the thin tube, all equipment for measuring stage can be installed above ground. A small 

drawdown occurs as flow velocities increase; however, 

this can be accounted for when the stage-discharge 

relation is verified. Figure 6 presents an example of 

how pneumatic sensors were used to measure stage in 

Palmer-Bowlus flumes (Church and others, 1996). A 

pressure transducer in an equipment shelter (fig. 5) 

converts the pressure of the nitrogen gas at the bubble 

orifice in the approach section of the flume to an electrical signal recorded by the data logger. Also, a calibrated standpipe is located in the equipment shelter for 

field calibration of the pressure transducer.

Pressure transducers used with the pneumatic 

sensors are of the non-submersible type. Submersible 

pressure transducers can be used to directly measure 

water depth, but, similar to floats, a stilling well may be 

needed to dampen water-surface oscillations and to 

protect the transducer from floating debris.

Electronic Sensors

The most commonly used electronic sensors to 

measure water levels are capacitance probes and dipper 

probes. The capacitance probe is immersed in the flowing water where the water is part of the electrical circuit. The measured capacitance varies in proportion to 

the depth of the water. This method has been use with 

weirs and with flumes in drainpipes. Accuracies of 

better than 1-percent have been achieved during 6-

month periods without failure (Marsalek, 1973). Frequent maintenance was required, however, because of 

the collection of floating debris. Therefore, the capacitance probe would work best in a stilling well or sidewall cavity of a flume, limiting its usefulness in sewer 

and storm drains. Also, the capacitance may be affected 

by the relative high variations of specific conductance 

expected in highway runoff where deicing chemicals 

are used.

The dipping probe, hanging from a cable controlled by a precision motor, completes an electrical 

circuit when in contact with the surface of the flowing 

water. The probe is retracted slightly by the motor after 

contact then is lowered again for the next measurement. The depth of flow is recorded as the amount of 

cable that is is paid out or retracted to meet the water 

surface. Marsalek (1973) reported that the dipper probe 

required a significant amount of maintenance an

that the time resolution on the chart record was 

sufficient for runoff studies.

Acoustic Sensors

Acoustic sensors measure the time of trav

sound that is emitted and reflected back from th

surface. Depth of water is determined from the 

depth to the base of the pipe and the measured d

of the sensor to the water surface. The sensors h

contact and do not affect the flowing water. Ma

(1973) reported that the accuracy of an acoustic 

tested was ± 4 mm. For these sensors to be accu

however, the water surface must be smooth, so 

ing well may be needed. Additionally, false refl

may be encountered in pipes and narrow chann

particularly at low flows (Marselak, 1973; Alley

Comparison of Flow 

Measurement Methods

An experiment done by the USGS in coop

with the FHWA indicates that different method

measuring stormwater flow may not be compar

and that some methods may have considerable v

ity and (or) bias. Stormwater flows from about 

storms were measured in an urban stormwater d

pipe by several different methods at a site in M

Wisconsin during 1995. In this experiment, flow

measured using a Palmer-Bowlus flume, the fri

coefficient method, acoustic and electromagnet

velocity meters, and acoustic and electromagne

meters. Stage was measured by pneumatic bubb

acoustic, and electromagnetic methods. Flows f

these storms were measured concurrently by ea

method in a 54-inch-diameter, 200-foot continu

section of drainpipe. A reliable and accurate sta

discharge relation for this site had been previou

developed using a Palmer-Bowlus flume, stage 

surements from a nitrogen gas pneumatic bubbl

system in the approach section of the flume and 

sure transducer, and discharge measurements ca

using the constant-rate dye-injection dilution m

When different flow measurement methods are 

pared to this reference method, potential proble

associated with the bias and variability among m

is apparent (fig. 7). 
24 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements DATA LOGGER TIPPING BUCKET RAIN GAGE NITROGEN GAS SUPPLY PRESSURE TRANSDUCER MINI- MONITOR DATA STORAGE MODULE DYE- INJECTION PUMP AUTOMATIC WATER SAMPLER WATER SAMPLER INTAKE DYE- INJECTION PORT BUBBLE ORIFICE: FLUME BUBBLE ORIFICE: CALIBRATED STANDPIPE RELAY CONDUCTIVITY AND WATER TEMPERATURE PROBES AIR TEMPERATURE AND RELATIVE HUMIDITY PROBE MODEM CELLULAR PHONE INCREMENTAL SHAFT ENCODER PNEUMATIC BUBBLE REGULATOR Figure 6. Instrumentation and equipment for measuring stormwater flow in a highway-drainage-monitoring station (Church and others, 1996).
Stormwater-Flow Measurem

(47) (50) (50) (49) (50) (48) (48) (33) (42) (38) (50) (38) (50) (29) (43) (49) (50) (49)

BAT

AAR

AAT

BTR

BTT

ATR

ATT

FCB1

FCA1

FCA2

VM1B

VM1A

VM2B

VM2A

FM1

FM2

FM3

FM4

|------- PALMER-BOWLUS FLUME -------|--- FRICTION ---|--- VELOCITY-METER ---|---- FLOW-METER ----|

 METHOD COEFFICIENT METHOD METHOD

 METHOD 

-100

160

-20-40-60-80

0

80604020

140120100

PERCENT DIFFERENCE FROM DYE-DILUTION DISCHARGE RATING,

APPROACH SECTION OF PALMER-BOWLUS FLUME

Outlier data value more than 3 times theinterquartile range outside the quartileOutlier data value less than or equal to 3and more than 1.5 times theinterquartile range outside the quartileData value less than or equal to 1.5 times theinterquartile range outside the quartile75th percentileMedian25th percentile

(50) Number of observations

Bubbler stage, flume approach,theoretical ratingAcoustic stage, flume approach,dye-dilution ratingAcoustic stage, flume approach,theoretical ratingBubbler stage, flume throat,dye-dilution ratingBubbler stage, flume throat,theoretical ratingAcoustic stage, flume throatdye-dilution ratingAcoustic stage, flume throat theoretical ratingFriction coefficient method, bubblerstage, site 1 downstream of flumeFriction coefficient method, acousticstage, site 1 downstream of flumeFriction coefficient method, acousticstage, site 2 downstream of flumeVelocity meter I, electromagnetic,bubbler stageVelocity meter I, electromagnetic,acoustic stageVelocity meter II, acoustic,bubbler stageVelocity meter II, acoustic,acoustic stageFlow meter I, acousticFlow meter II, acousticFlow meter III, acousticFlow meter IV, electromagnetic

BAT

AAR

AAT

BTR

BTT

ATR

ATT

FCB1FCA1FCA2

VM1BVM1AVM2BVM2A

FM1FM2FM3FM4

EXPLANATION

Figure 7. Comparison of results from stormwater-flow measuring methods, 1998, Madison, Wisconsin.
26 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

The comparability of stormwater-flow measurements by different methods is poor, and the variability 

of individual methods ranges widely (fig. 7). The data 

clearly indicate the need to calibrate a stage-discharge 

measurement method using check measurements with 

an independent method, such as dye dilution. Although 

stage-discharge relations derived from these flow measurement methods can be adjusted (using verification 

data) to minimize bias, the very large variation of 

uncertainty in flow measurements exhibited by some of 

these flow measurement methods is still likely to exist. 

These variations would add uncertainty to the representativeness of event-mean concentrations derived from 

flow-proportional-sampling methods, and would also 

add uncertainty to loads calculated from measured 

flows. Therefore, this comparison of flow methods 

emphasizes the importance of (1) selecting a flow measurement method that will yield sufficiently accurate 

flow data to meet project objectives and goals, (2) verifying all stage-discharge relations with proven, documented methods, and (3) documenting and reporting 

detailed information about the flow measurement 

methods and verification methods used in a given study 

so that the accuracy of flow data collected can be 

evaluated.

QUALITY ASSURANCE/QUALITY 

CONTROL

Quality assurance and quality control programs 

(QA/QC) need to be established at the beginning of a 

project to ensure that precipitation and stormwater-flow 

measurements are accurate and representative of the 

flow system investigated. Clark and Whitfield (1993), 

Brown and others (1995), and Jones (1999) stress the 

importance of QA/QC programs for all phases of 

stormwater flow and water-quality investigations—

from project planning through report preparation—and 

demonstrate how comprehensive QA/QC programs can 

be developed and implemented.

Addressing, documenting, and reporting the 

hydrologic and hydraulic factors for selection of 

(1) monitoring site, (2) frequency and duration of monitoring, and (3) methods for measuring precipitation 

and stormwater-flow data constitute a QA/QC program 

for the design of stormwater runoff studies. A QA/QC 

program for data-collection activities is also necessary 

to document that the implementation of the stud

design was successful. An effective QA/QC pro

for stormwater-flow data-collection activities w

include:

• Frequent and routine site visits by trained/ 

experienced field personnel.

• Redundant methods for measuring precipit

and stormwater flow.

• Technical training for project personnel.

• Frequent review by project personnel of 

precipitation and stormwater-flow data 

collected.

• Quality audits, in the form of periodic inte

reviews.

• Quality audits, in the form of periodic exte

reviews.

Frequent and routine site visits by trained

enced field personnel cannot be over emphasize

equipment and instrumentation must be mainta

good working order to ensure the integrity of th

collected. The site must be inspected for debris 

mulation, natural corrosion of equipment, vand

and other potential problems, including the pres

rodents. Debris can affect measurements by inte

with the operation of measurement equipment. 

accumulation, whether upstream or downstream 

the measuring point, may affect the stage-disch

relation. Service and maintenance of equipmen

instrumentation and (or) the flow-control struct

necessary for consistency in the measurements 

the stage-discharge relation. Frequent calibratio

equipment and instrumentation is necessary bec

the difficult monitoring environment. Standard 

forms designed to record who visited the site, th

and time of each visit, site conditions, the statu

equipment and instrumentation, records of instr

calibration, and other information pertinent to t

ation of the station are necessary for data verific

These field forms should be archived with proje

records, and at the least, use of these forms sho

mentioned in the QA/QC documentation in pro

reports.

Redundant systems for measuring precip

and stormwater flow can be used to ensure that 

data collected are correct and complete. The diffi

in measuring and recording data in a stormwate

monitoring environment create a high probabili

an incomplete record, even when stations are w
Quality Assurance/Quality Co

maintained and the instruments are properly calibrated. 

Redundant systems provide for comparison of primary 

and backup data for detection of errors and backup data 

collection in the event of failure of the primary system. 

For example, Church and others (1996) used a datalogger-controlled pressure transducer as the primary 

stage measurement and recording system, and a floatarm assembly with a shaft encoder as the backup stage 

measurement system (fig. 6). The float-arm assembly 

was also connected to a mechanical strip-chart recorder 

to serve as the backup data recording system. In this 

study, relations between measured stage and discharge 

among co-located stations were also useful as a defacto 

backup system for data interpretation and verification. 

This investigation also collected precipitation measurements near each station, which were compared to 

long-term data from a NOAA network station.

Training is an essential part of QA/QC programs. 

Consistent and correct flow data are necessary to 

assemble a national and (or) regional highway-runoffquality data base. To ensure that data are valid, current, 

technically defensible, and comparable from project to 

project, a standard training program is necessary. A 

continued program of organized training for project 

chiefs and experienced field personnel is necessary to 

maintain state-of-the-art knowledge of advances in precipitation and stormwater-flow-monitoring technologies. A series of training sessions for less experienced 

field personnel is necessary to establish the knowledge 

base for quality in data collection and interpretation. 

For example, the Federal Highway Administration 

designed a comprehensive student workbook devoted 

to the study of highway-runoff water quality and runoff 

quantity (Federal Highway Administration, 1985).

 Data assessment is an important component of 

the QA/QC process (Jones, 1999). It is necessary for 

project personnel to frequently review the precipitation 

and stormwater-flow data collected throughout the 

course of each project. It is necessary to do at least a 

cursory data review to ensure that the system is operational, and that it is collecting internally consistent 

information each time the data record is collected 

during a station visit, and (or) when the data are downloaded from a remote location. Periodically, it is necessary to do a more detailed review using the entire data 

record, field notes, and other available information to 

detect errors or anomalous data. For example, a comparison between the precipitation and runoff volumes 

for a given storm could indicate a bias in one or 

other measurement system if the relation for thi

departed from normal values for the station in qu

Analysis of field records, including calibration r

adjustments to measured values, and other infor

when compared to the data record, may indicate 

tematic bias, long-term drift, or an abrupt chang

performance of the instrumentation.

Quality audits, in the form of periodic int

reviews, are necessary to monitor and impleme

project QA/QC program (Jones, 1999). Internal 

establish that the project has a QA/QC plan and 

is being implemented and documented. Also, pe

internal reviews serve as a method to provide te

feedback from subject matter experts to examin

address problems and (or) potential problems in 

data-collection program. Internal reviews shoul

ensure that trained/experienced personnel are av

for frequent and routine site visits, that appropri

robust monitoring systems are in place and coll

data, and that project personnel are examining a

interpreting data using appropriate methods on a 

basis. For example, within the USGS, each proj

typically reviewed by subject matter experts wi

individual organizational units at the proposal s

and then again when the project is about 10-, 40

70-percent complete, or at fixed intervals, such 

quarterly or semiannually.

Quality audits, in the form of periodic ex

reviews, are also necessary to monitor and impl

the project QA/QC program (Jones, 1999). Exte

audits should examine project plans, project dat

project records, and QA/QC documentation to e

that study objectives are being met, and to ensu

study objectives will meet the goals of the mon

project. External reviews should ensure that the 

information is properly documented and that th

mentation is accessible. Within the USGS, exte

quality audits include periodic reviews by techn

specialists at different levels in the chain of com

above the local organizational unit and by techn

specialists from discipline offices such as the O

Surface Water, the Office of Ground Water, the 

of Quality Water, and the Branch of Quality Sy
28 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

CONCLUSION

Information from stormwater studies may be 

used to address local issues and (or) may contribute to 

a regional or national synthesis of highway- or urbanrunoff studies. Accurate and representative precipitation and stormwater-flow data are crucial for valid, current, and technically defensible interpretations of 

highway- or urban-runoff study results. Equally important is knowledge of the degree of accuracy and representativeness of available precipitation and stormwaterflow data. Accurate and representative measurements 

of precipitation and stormwater flow, however, are difficult to obtain because of the rapidly changing spatial 

and temporal distribution of precipitation in the study 

area and the rapidly changing flows during a storm. 

Many hydrologic and hydraulic factors must be considered in selecting sites for measuring precipitation and 

stormwater flow that are representative of the objectives and goals of the study. Many hydrologic and 

hydraulic factors also must be considered in determining frequencies and durations of data collection to fully 

characterize the rapidly changing rainfall intensities 

and stormwater flows, and in selecting methods that 

will yield accurate data over the full range of rainfall 

intensities and the full range and changing flow 

regimes of stormwater flows.

Without the supporting data needed to evaluate 

the accuracy and representativeness of the precipitation 

and stormwater-flow measurements, the data collected 

and interpretations made may have little meaning. To 

ensure that the accuracy and representativeness of precipitation and stormwater-flow data can be evaluated, 

decisions as to (1) where in the drainage system precipitation was collected and stormwater flows were measured, (2) how frequently precipitation and stormwater 

flows were measured, (3) what methods were used to 

measure precipitation and stormwater flows, and (4) on 

what basis these decisions were made must be documented and communicated in an accessible format, 

such as a project description report, a data report, or an 

appendix to a technical report, and (or) archived in a 

State or national records center. Additionally, a quality 

assurance/quality control program must be established 

to ensure that this information is documented and 

reported, and that the decisions made in design phase 

are continually reviewed, internally and externally, 

throughout the duration of the study.

REFERENCES

Abrahams, A.D, Parsons, A.J., and Luk, Shiu-Hung, 

Field measurement of the velocity of overland 

using dye tracing: Earth Surface Processes and 

forms, v. 11, p. 653–657.

Alley, W.M., 1977, Guide for collection, analysis, an

urban stormwater data: A Conference Report, E

Md., November 28–December 3, 1976: Americ

Society of Civil Engineers, 115 p.

Athayde, D.N., Shelley, P.E., Driscoll, E.D., and Bo

1983, Results of the nationwide urban runoff p

volume I, final report: U.S. Environmental Prot

Agency, 186 p.

Barrett, M.E., Zuber, R.D., Collins, E.R., III, Malina

Jr., Charbeneau, R.J., and Ward, G.H., 1993, A 

and evaluation of literature pertaining to the qua

control of pollution from highway runoff and c

tion: Austin, Tex., Center for Research in Water 

Resources, Bureau of Engineering Research, T

University of Texas, Balcones Research Center

Technical Report CRWR 239, 26 p.

Brown, T., Burd, W., Lewis, J., and Chang, G., 1995

ods and procedures in stormwater data collectio

Torno, H. C., ed., Proceedings of the Engineeri

dation Conference, Stormwater NPDES related 

toring needs: Crested Butte, Colo., American So

Civil Engineers, August 7–12, 1994, p. 229–23

Buchanan, T.J., and Somers, W.P., 1968, Stage meas

ments at gaging stations: U.S. Geological Surv

Techniques of Water-Resources Investigations, 

chap. A7, 28 p. (Fifth printing, 1988).

Buchanan, T.J., and Somers, W.P., 1969, Discharge m

ments at gaging stations: U.S. Geological Surv

Techniques of Water-Resources Investigations, 

chap. A8, 65 p. (Fourth printing, 1984).

Burch, T.L., and Phillips, J.M., 1995, High-accuracy 

and stormwater flow monitoring, in Torno, H. C

Proceedings of the Engineering Foundation Co

ence, Stormwater NPDES related monitoring n

Crested Butte, Colo., American Society of Civi

neers, August 7–12, 1994, p. 609–616.

Carter, R.W., and Davidian, Jacob, 1968, General pr

for gaging streams: U.S. Geological Survey Tec

of Water-Resources Investigations, book 3, cha

13 p. (Third printing, 1977).

Church, P. E., and Friesz P.J., 1993, Effectiveness of 

way drainage systems in preventing road-salt 

contamination of groundwater: Preliminary find

Transportation Research Record, no. 1420, Hig

and Facility Design, National Research Counci

Transportation Research Board, p. 56–64.
Refere

Church, P.E., Armstrong, D.S., Granato, G.E., Stone, V.J., 

Smith, K.P., and Provencher, P.L., 1996, Effectiveness 

of highway-drainage systems in preventing contamination of ground water by road salt, Route 25, southeastern Massachusetts—Description of study area, data 

collection programs, and methodology: U.S. Geological 

Survey Open-File Report, 96-317, 72 p.

Clark, M.J.R., and Whitfield, P.H., 1993, A practical model 

integrating quality assurance into environmental 

monitoring: Water Resources Bulletin, v. 29, no. 1, 

p. 119–130.

Driscoll, E.D., Shelley, P.E., and Strecker, E.W., 1990a, Pollutant loadings and impacts from highway stormwater 

runoff, volume I, Design procedure: U.S. Federal 

Highway Administration Final Report, FHWA-RD-88-

006, 67 p.

Driscoll, E.D., Shelley, P.E., and Strecker, E.W., 1990b, Pollutant loadings and impacts from highway stormwater 

runoff, volume IV, Research report data appendix: U.S. 

Federal Highway Administration Final Report, FHWARD-88-009, 143 p.

Duerk, M.A., 1983, Automatic dilution gaging of rapidly 

varying flow: U.S. Geological Survey Water-Resources 

Investigations Report 83-4088, 17 p.

Ellis, S.R., Doerfer, J.T., Mustard, M.H., Blakely, S.R., and 

Gibbs, J.W., 1984, Analysis of urban storm-runoff data 

and the effects on the South Platte River, Denver metropolitan area, Colorado: U.S. Geological Survey WaterResources Investigations Report 84-4159, 66 p.

Federal Highway Administration, 1985, Highway runoff 

water-quality training course, Student handbook, Section 6, Water-quality monitoring and data collection, 

variously paged.

Fontaine, T.A., 1990, Raingage network characteristics and 

runoff model error, in Jennings, M.E., Proceedings of 

the Symposium on Urban Hydrology, Denver, Colo., 

November 4-8, 1990: American Water Resources 

Association, p.59–66.

Granato, G.E., Bank, F.G., and Cazenas, P.A., 1998, Data 

quality objectives and criteria for basic information, 

acceptable uncertainty, and quality-assurance and 

quality-control documentation: U.S. Geological Survey 

Open-File Report, 98-394, 17 p.

Guerard, Paul, and Weiss, W.B., 1995, Water quality of storm 

runoff and comparison of procedures for estimating 

storm-runoff loads, volume, event-mean concentrations, 

and the mean load for a storm for selected properties 

and constituents for Colorado Springs, southeastern 

Colorado, 1992: U.S. Geological Survey WaterResources Investigations Report 94-4194, 35 p.

Harned, D.A., 1988, Effects of highway runoff on st

flow and water quality in the Sevenmile Creek 

rural area in the Piedmont Province of North C

July 1981 to July 1982: U.S. Geological Survey 

Supply Paper 2329, 105 p.

Harrison, R.M., and Wilson, S.J., 1985, The chemic

position of highway drainage waters II—Chem

associations of metals in the suspended sedime

Science of the Total Environment, v. 43, p. 79–

Helsel, D.R., and Hirsch, R.M., 1992, Statistical me

water resources: New York, Elsevier, 522 p.

Hoffman, E.J., Latimer, J.S., Hunt, C.D., Mills, G.L

Quinn, J.G., 1985, Stormwater runoff from hig

Water, Air, and Soil Pollution, v. 25, no. 4, p. 3

Hvitved-Jacobsen, T. and Yousef, Y.A., 1991, Highw

runoff quality, environmental impacts and cont

Hamilton, R.S., and Harrison, R.M, eds., Highw

lution. Studies in Environmental Sciences 44: A

dam, Elsevier, chap. 5, p. 165–208.

Intergovernmental Task Force on Monitoring Water 

1995a, The strategy for improving water-qualit

monitoring in the United States: U.S. Geologic

Survey, 25 p.

Intergovernmental Task Force on Monitoring Water 

1995b, The strategy for improving water-qualit

monitoring in the United States, Technical App

U.S. Geological Survey, 117 p.

Irish, L.B., Lesso, W.G., Barrart, M.E., Malina, J.F., 

Charbeneau, R.J., and Ward, G.H., 1996, An ev

of the factors affecting the quality of highway r

the Austin, Texas area: U.S. Federal Highway A

tration, Texas Department of Transportation, In

Report, FHWA/TX-96/1943-5, 246 p.

Jones, B.E., 1999, Principles and practices for qualit

ance and quality control: U.S. Geological Surve

File Report 98-636, 17 p.

Katz, B.G., and Fisher, G.T., 1983, A comparison of 

methods for measuring flow rate in a circular st

sewer: Lexington, Ky., International Symposium 

Urban Hydrology, Hydraulics and Sediment Co

University of Kentucky, July 25–28, 1983, p. 3

Kilpatrick, F.A., and Schneider, V.R., 1983, Use of fl

in measuring discharge: U.S. Geological Surve

Techniques of Water-Resources Investigations, 

chap. A14, 46 p.

Kilpatrick, F.A., and Cobb, E.D., 1985, Measuremen

discharge using tracers: U.S. Geological Survey 

Techniques of Water-Resources Investigations, 

chap. A16, 52 p.

Kilpatrick, F.A., Kaehrle, W.R., Hardee, Jack, Corde

and Landers, M.N., 1985, Development and tes

highway storm-sewer flow measurement and re

system: U.S. Geological Survey Water-Resourc

Investigations Report 85-4111, 98 p.
30 Basic Requirements for Collecting, Documenting, and Reporting Precipitation and Stormwater-Flow Measurements

Kilpatrick, F.A., and Kaehrle, W.R., 1986, Storm-sewer flow 

measurement and recording system: Transportation 

Research Record, no. 1073, Hydraulics and Hydrology, 

National Research Council, Transportation Research 

Board, p. 1–9.

Kilpatrick, F.A., and Wilson, J.F., Jr., 1989, Measurement of 

time of travel in streams by dye tracing: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A9, 27 p.

Laenen, Antonius, 1985, Acoustic velocity meter systems: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A17, 38 p.

Marsalek, J., 1973, Instrumentation for field studies of urban 

runoff: Research Program for the Abatement of 

Municipal Pollution under the Provisions of the 

Canada-Ontario Agreement on Great Lakes Water 

Quality, Ontario Ministry of the Environment, Project 

73-3-12, 74 p.

Natural Resources Conservation Service, 1996, National 

Handbook of Water Quality Monitoring: U.S. Department of Agriculture, National Water Quality Handbook, 

Part 600, variously paged.

Nesmerak, I., 1986, Errors of determining material load from 

a watershed: Limnologica (Berlin), v. 17, no. 2, p. 251–

254.

Rantz, S.Z., 1982a, Measurement and computation of 

streamflow: Volume 1. Measurement of stage and discharge: U.S. Geological Survey Water-Supply Paper 

2175, p. 1–284.

Rantz, S.Z., 1982b, Measurement and computation of 

streamflow: Volume 2. Computation of discharge: U.S. 

Geological Survey Water-Supply Paper 2175, p. 285–

631.

Sauer, V.B., and Meyer, R.W., 1992, Determination of error 

in individual discharge measurements: U.S. Geological 

Survey Open-File Report, 92-144, 21 p.

Shelley, P.E., and Kirkpatrick, G.A., 1975, An assessment of 

automatic sewer flow samplers—1975: Environmental 

Protection Technology Series EPA-600/2-75-065, 

336 p.

Shelley P.E., and Gaboury, D.R., 1986, Estimation of pollution from highway runoff-—Initial results: in Urbonas, 

B., and Roesner, L.A., eds., Urban Runoff QualityImpact and Enhancement Technology: Henniker, N.H., 

Proceedings of an Engineering Foundation Con

June 23–27, 1986, American Society of Civil E

neers, p. 459–473.

Smith, J.A., 1993, Precipitation, chap. 3, in Maidmen

ed., Handbook of Hydrology: New York, McGr

Inc., p. 3.1–3.47.

Smoot, G.F., 1975, A rainfall-runoff quantity-quality 

collection system: in Proceedings of a Research 

ence on Urban Runoff Quantity and Quality, Au

1994: New York, American Society of Civil En

p. 178–183.

Spangberg, A., and Niemczynowicz, J., 1992, High 

tion measurements of pollution wash-off from 

asphalt surface: Nordic Hydrology, v. 23, no. 4

256.

Thoman, R.V., and Mueller, J.A., 1987, Principles of 

water quality modeling and control: New York, 

and Row, Publishers, 644 p.

U.S. Environmental Protection Agency, 1992, Natio

Pollutant Discharge Elimination System Storm 

Sampling Guidance Document: Washington, D

Environmental Protection Agency, Office of Wa

Report EPA 883-B-92-001, 177 p.

U.S. Environmental Protection Agency, 1997, Comp

of Tools for Watershed Assessment and TMDL 

opment: Washington, D.C., U.S. Environmenta

Protection Agency, Office of Water, Report EPA 

97-006, 224 p.

Vanderborght, J.P., and Wollast, P., 1990, Continuou

toring of wastewater composition in sewers and 

water overflows: Water Science Technology, v. 

no. 10/11, p. 271–275.

Wenzel, H.G., 1975, Meter for sewer flow measurem

Journal of the Hydraulics Division, American S

of Civil Engineers, v. 101, no. HY1, p. 115–13

Whitfield, P.H., 1988, Goals and data collection desi

water quality monitoring: Water Resources Bul

v. 24, no. 4, p. 775–780.

Zarriello, P.J., 1998, Comparison of nine uncalibrate

models to observe flows in two small urban wat

in Proceedings of the First Federal Interagency 

Hydrologic Modeling Conference, April 1998: 

Vegas, Nev., v. 2, p. 7/163–7/170.
District Chief,Massachusetts—Rhode Island DistrictU.S. Geological SurveyWater Resources Division10 Bearfoot RoadNorthborough, MA 01532