U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2009–1066

In cooperation with the Cuyahoga County Board of Health; Northeast Ohio 

Regional Sewer District; Ohio Lake Erie Office; and U.S. Environmental 

Protection Agency, Advanced Monitoring Initiative 

Testing and Refining the Ohio Nowcast at Two 

Lake Erie Beaches—2008
Cover image: View of Edgewater Beach, Cleveland, Ohio. (Photograph by Donna S. Francy, 

U.S. Geological Survey.)
Testing and Refining the Ohio Nowcast at 

Two Lake Erie Beaches—2008

By Donna S. Francy, Erin E. Bertke, and Robert A. Darner

In cooperation with the Cuyahoga County Board of Health; Northeast Ohio Regional 

Sewer District; Ohio Lake Erie Office; and U.S. Environmental Protection Agency, 

Advanced Monitoring Initiative

Open-File Report 2009–1066

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Francy, D.S., Bertke, E.E., and Darner, R.A., 2009, Testing and refining the Ohio Nowcast at two Lake Erie

beaches—2008: U.S. Geological Survey Open-File Report 2009–1066, 19 p.
iii

Contents

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

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

Methods ..........................................................................................................................................................2

Data Collection and Operation of the Nowcast...............................................................................3

Variables for Predictive Models.........................................................................................................3

Testing and Refinement of Predictive Models .................................................................................6

Results .............................................................................................................................................................7

Performance of the Models in 2008...................................................................................................7

Exploratory Data Analysis ...................................................................................................................9

Model Refinement and Selection for 2009......................................................................................13

Edgewater...................................................................................................................................14

Huntington...................................................................................................................................14

Next Steps.....................................................................................................................................................16

Acknowledgments.......................................................................................................................................16

References Cited..........................................................................................................................................17

Appendix 1. Candidate models used to predict Escherichia coli concentrations at two 

Lake Erie beaches..........................................................................................................................19

Figures

1. Location of study area. ................................................................................................................2

2. Locations of sampling sites and instrumentation....................................................................4

3. Boxplots showing comparison of lake levels in subseason 2 for 2004–7 and 2008 

at Edgewater, Cleveland, Ohio....................................................................................................7

4. Wave heights measured by use of the wave rod (manual) or wave buoy 

(automated), Edgewater, Cleveland, Ohio, 2008 ....................................................................12

5. Turbidity measured by use of a grab sample and turbidimeter (manual) or probe 

attached to a nearshore buoy (automated), Edgewater, Cleveland, Ohio, 2008 ..............12

6. Wave heights measured by use of the wave rod (manual) or wave buoy 

(automated), Huntington, Bay Village, Ohio, 2008 .................................................................13

7. Comparison of morning and afternoon Escherichia coli concentrations at 

Huntington, Bay Village, Ohio, 2008 .........................................................................................14
iv

Tables

1. Models developed for the Ohio Nowcast in 2008....................................................................5

2. Nowcast model responses as compared to using previous day’s Escherichia coli

concentration (persistence model) in 2008 ..............................................................................8

 3. Pearson’s r correlations between log10 Escherichia coli (E. coli) concentrations 

and explanatory variables for subseason 1 (May 19−June 17), subseason 2 

(June 18−Aug. 13), subseason 3 (Aug 14−Sept. 11), and the recreational year at 

Edgewater, Cleveland, Ohio, 2008 ............................................................................................10

4. Pearson’s r correlations between log10 Escherichia coli (E. coli) concentrations 

and explanatory variables for subseason 1 (May 19−July 23), subseason 2 

(July 24−Sept. 5), the recreational year, and for afternoon samples at Huntington, 

Bay Village, Ohio, 2008...............................................................................................................11

5. Examination of candidate Edgewater 2005–8 models..........................................................15

6. Examination of candidate Huntington 2005−8 models..........................................................16

Conversion Factors

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

yard (yd) 0.9144 meter (m)

mile (mi) 1.609 kilometer (km)

kilometer (km) 0.6214 mile (mi) 

milliliter (mL) 0.06102 cubic inch (in3

) 

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Concentrations of Escherichia coli are given in colony-forming units per 100 milliliters 

(CFU/100 mL).

Turbidities are given in nephelometric turbidity units (NTU).
Testing and Refining the Ohio Nowcast at Two Lake Erie 

Beaches—2008

By Donna S. Francy, Erin E. Bertke, and Robert A. Darner

Abstract 

The Ohio Nowcast has been providing real-time beach 

advisories to the public on the basis of predictive models since 

2006. In support of the nowcast, data were collected during 

the recreational season of 2008 to validate and refine predictive models at two Lake Erie beaches. Predictive models yield 

data on the probability that the single-sample bathing-water 

standard for E. coli will be exceeded. Field personnel collected 

or compiled data on Escherichia coli (E. coli) concentrations 

as well as variables expected to affect these concentrations, 

including manual and automated measurements of turbidity, 

wave height, and water temperature; lake level; and radar and 

airport rainfall amounts. Two new variables were measured 

during 2008—photosynthetically-active radiation at Huntington (Bay Village) and foreshore head at Edgewater (Cleveland).(The foreshore is a strip of land along a body of water 

between low and high water marks.) 

The performance of the nowcast was monitored during 

2008. The Huntington nowcast yielded a greater percentage 

of correct responses (84.9 percent) than did the previous day’s 

E. coli concentration (75.2 percent). In contrast, at Edgewater, 

the nowcast yielded a slightly higher percentage of correct 

responses (61.0 percent) as compared to the previous day’s E. 

coli concentration (56.5 percent), but both percentages were 

relatively low. Lake levels in 2008 were significantly higher 

than levels in the data used to develop the Edgewater models (2004−7), confounding their abilities to provide correct 

responses. At Edgewater during 2008, the strongest relation 

(as measured by Pearson’s correlation) was between E. coli

concentrations and the difference in foreshore head over the 

past 24 hours (r=0.48), a variable not included in the models. 

At Huntington, photosynthetically-active radiation on the 

previous day showed a significant negative relation to E. coli

concentrations (r=-0.33) during 2008.

Refined models were developed for Huntington and 

Edgewater using data collected from 2005−8. The refined 

models included the variables wave height, log turbidity, radar 

or airport rainfall, and day of the year in various combinations 

for different dated segments of the recreational season. Waterresource managers will determine which models to apply to 

the Ohio Nowcast for issuing water-quality advisories in 2009. 

Introduction 

In Ohio, concentrations of the fecal-indicator bacterium 

Escherichia coli (E. coli) are used as the basis for issuing 

water-quality advisories at beaches. If the E. coli concentration exceeds the single-sample bathing water standard of 235 

colony-forming units per 100 milliliters (CFU/100 mL), an 

advisory is issued for the beach (Ohio Department of Health, 

2007). The widely acknowledged shortcoming of using this 

approach is that standard culture methods for E. coli take at 

least 18−24 hours before results are available. The beach is 

posted with an advisory or is determined to be acceptable for 

swimming on the basis of the previous day’s concentration of 

E. coli. Sanitary conditions may change overnight and even 

throughout the day (Boehm and others, 2002), so beach managers may issue water-quality advisories based on outdated 

information of current public-health risk. As an alternative, 

some water-resource managers are using predictive models to 

provide near-real-time estimates of recreational water quality 

to better protect public health. Predictive models use waterquality and environmental variables, such as rainfall and 

turbidity, to provide these estimates. 

The Ohio Nowcast has been providing near-real-time 

beach advisories to the public on the basis of predictive 

models since May 2006 (http://ohionowcast.info). The Ohio 

Nowcast is an Internet-based system that displays data on 

the probability that the E. coli concentration at a beach will 

exceed safe levels—similar to a weather forecast, except for 

a shorter timeframe. During the recreational season (May−

Aug.) of 2007 at Huntington Reservation (Bay Village), the 

bathing-water standard was exceeded on 14 percent of the 

days sampled (15 out of 104 days). The nowcast for Huntington resulted in a greater percentage of correct responses (82.7 

percent) than did the use of the previous day’s E. coli concentrations (80.2 percent) (the method used for many advisories 

in the United States, often referred to as the “persistence 

model”). Although this difference was smaller than expected, 

the nowcast model better predicted exceedance of the standard (53 percent correct) than did the persistence model (31 

percent correct) (Francy and Darner, 2007). Data from 2007 

were added to the existing dataset (2000−2006), and two new 

models were developed for Huntington for use in the nowcast 
2 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

in 2008. Work was also done to develop predictive models for 

a second beach, Edgewater State Park (Cleveland) (Francy and 

Darner, 2007), and Edgewater was added to the Ohio Nowcast 

in 2008. During 2007 at Edgewater, the bathing-water standard was exceeded on 28 percent of the days sampled (22 out 

of 78 days).

The U.S. Geological Survey (USGS), in cooperation with 

the Cuyahoga County Board of Health (CCBH), Northeast 

Ohio Regional Sewer District (NEORSD), Ohio Lake Erie 

Office, and U.S. Environmental Protection Agency, Advanced 

Monitoring Initiative, continued to operate and evaluate the 

Ohio Nowcast during 2008 at Huntington and Edgewater. This 

project involved the daily operation of the nowcast and 

the validation and refinement of predictive models. Equipment 

was installed to measure and transmit near-real-time-data on 

wave height at Huntington and Edgewater and to measure 

turbidity and foreshore head at Edgewater. A device to measure photosynthetically active radiation (PAR) was installed at 

Huntington to determine whether predictive models could be 

improved by adding PAR as an explanatory variable. Afternoon sampling was added to daily morning sampling at Huntington to determine whether this additional sampling would 

result in more accurate nowcast advisories. Finally, the data 

from 2008 were compiled and new models for Huntington and 

Edgewater were developed for 2009.

Methods 

Data were collected for the Ohio Nowcast during the 

recreational season of 2008 (May−August) at two Lake Erie 

beaches in northeast Ohio—Huntington Reservation (“Huntington”) and Edgewater State Park (“Edgewater”) (fig. 1). 

Huntington is in a suburb of Cleveland (Bay Village) and is 

operated by the Cleveland Metroparks. Edgewater, in Cleveland, is an urban beach operated by the Ohio Department of 

Figure 1. Location of study area.
Methods 3

Natural Resources. Both beaches are popular bathing beaches, 

used extensively during the recreational season. The NEORSD 

and CCBH are responsible for monitoring Edgewater and 

Huntington, respectively; these agencies updated the Ohio 

Nowcast Web site daily by 9:30 a.m. with water-quality 

advisories based on the models and other information (http://

ohionowcast.info). Nowcast results were also listed by the 

Ohio Department of Health through their “Beach Monitoring” 

Web site (Ohio Department of Health, 2007).

Data Collection and Operation of the Nowcast

Lake-water samples were collected by CCBH and 

NEORSD for the nowcast between 7:30 and 9:30 a.m., 7 

days per week, at a depth of 2−3 ft in two designated areas 

of each beach (fig. 2) by means of a grab-sampling technique 

(Myers and others, 2007). Additional samples were collected 

at Huntington between 1:00 and 3:00 p.m., Monday through 

Thursday; these sample results were not used in the nowcast 

during 2008 but may be used in future modeling efforts. Water 

samples were immediately placed on ice while the field crew 

completed field measurements and observations. 

After the daily water-sample collection, equipment for 

measuring turbidity and a field laptop computer were set up 

on site. Rainfall from the nearest airport and lake-level data 

were obtained via the World Wide Web. Radar rainfall data 

and wave heights measured by use of a site-specific buoy (see 

“Variables for Predictive Models” section of this report) were 

received through an automated email from the USGS. Field 

personnel entered water-quality and environmental data into a 

computer program. The program provided results for the nowcast as the probability that today’s water sample would exceed 

235 CFU/100 mL. On the basis of a threshold probability 

established for each model from historical data (Francy and 

Darner, 2007), the nowcast was updated with current waterquality and advisory information (http://ohionowcast.info). 

Threshold probabilities were determined before the swimming 

season by taking the dataset used to develop the model and 

finding the probability that was a reasonable balance between 

achieving a high number of correct responses and a low number of false negative responses; this procedure is described in 

detail by Francy and Darner (2006, example 10). Computed 

probabilities less than the threshold indicated that bacterial water quality was most likely acceptable for swimming. 

Computed probabilities equal to or greater than the threshold 

probability indicated that the water quality was most likely not 

acceptable, and an advisory was issued. 

The models used in 2008 were developed from data 

collected during 2000−2007 for Huntington and 2004−7 for 

Edgewater. Two models for Huntington and three models for 

Edgewater were used in the 2008 nowcast, based on specific 

time periods and denoted as subseason 1, subseason 2, or 

subseason 3 models (table 1). Threshold probabilities for the 

models ranged from 27 to 34 percent. Different models were 

used for each subseason because, in the data used to develop 

the models, it was noted that the percentages of false negatives 

were high in the beginning of the season and the percentages 

of false positive were high late in the season (Francy and 

Darner, 2007). To minimize the number of incorrect model 

responses, the 2000−2007 data for Huntington and 2004−7 

data for Edgewater were, therefore, split into segments based 

on the patterns of false negatives and positives that would 

have occurred in responses from a model. In addition, because 

of poor performance of the Edgewater nowcast during subseason 2 of 2008, two modified models were developed for use in 

the later part of subseason 2 and in subseason 3. 

Samples were collected and analyzed for E. coli concentrations within 3 hours. Morning samples were analyzed by 

use of the modified mTEC membrane-filtration method (U.S. 

Environmental Protection Agency, 2006). Because of the 

required 2-hour resuscitation for the modified mTEC method, 

afternoon samples were analyzed by use of the Colilert 

Quanti-Tray/2000 (IDEXX Laboratories, Inc., Westbrook, 

Maine), a commercially available enzyme-substrate liquidbroth medium that can be processed in less than 5 minutes. 

For Edgewater, composite samples were analyzed. To form a 

composite sample, 100-mL aliquots from each of the east and 

west sampling points (fig. 2A) were combined. For Huntington, the central and west sampling point samples (fig. 2B) 

were analyzed separately, and average concentrations were 

used to compute daily E. coli concentrations. 

Quality-assurance and quality-control (QA/QC) procedures 

were implemented to ensure the collection of high-quality data 

for the nowcast. The USGS did several onsite QA/QC checks 

of field and laboratory operations. Duplicates, field blanks, and 

positive-control reference cultures for E. coli were analyzed as 

described in Francy and others (2008). Duplicate measurements 

of each water sample for turbidity were made. The turbidity 

measurements that did not agree within 1 NTU for values <10 

NTU or 10 percent for values >10 NTU were repeated. Turbidity reference standards were sent to participating agencies. 

Results from QA/QC procedures were documented and retests 

were done or corrective measures taken when needed. 

Variables for Predictive Models

Field personnel collected or compiled daily data for 

environmental and water-quality variables expected to affect 

E. coli concentrations. All these data were used in exploratory 

data analysis done after the 2008 recreational season, whereas 

some data were collected specifically for the 2008 nowcast. 

Wave height and wave period. Wave heights were 

measured by automated and manual techniques. For Edgewater, where automated data were collected and tested in the 

previous year (2007), automated wave-height data measured 

at 7:30 a.m. were used in the nowcast. If automated data were 

not available because of equipment or transmission problems, 

manual measurements were used. For Huntington, because 

this was the first year for automated data collection, only manually measured wave-height data were used in the nowcast. 
4 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Figure 2. Locations of sampling sites and instrumentation. A, Edgewater, Cleveland, Ohio. B, Huntington Reservation, 

Bay Village, Ohio.
Methods 5

For automated data collection, wave heights and wave 

periods were compiled every 30 minutes from buoys placed 

outside the swimming areas at Huntington and Edgewater in 

approximately 6 ft of water. These data are available seasonally in near real time through the USGS National Water 

Information System Web-interface (NWISWeb), for Lake 

Erie at Edgewater Beach (USGS site ID 412923081442600) 

and Lake Erie at Huntington Reservation (USGS site ID 

412928081560220). The buoys were custom made by an 

environmental equipment manufacturer for use in nearshore 

shallow waters. Buoys were equipped with pressure transducers to measure wave heights and wave periods; a datalogger to 

store the data; a radio, amplifier, and antenna to transmit the 

data; and a battery and solar panel for power. 

For manual wave-height measurements, a graduated 

rod was placed at the sampling location and minimum and 

maximum water heights, in inches, were noted for 1 minute. 

Wave-height measurements were obtained in approximately 

3 ft of water. Minimum was subtracted from maximum water 

height to obtain the manual wave height. 

Water temperature. Water temperature was measured by 

field personnel at the sampling location at the time of sample 

collection by use of a digital thermometer. Although these data 

were entered into the nowcast Web site, they were not used in 

the predictive models. Additionally, water temperature, measured by use of a temperature probe attached to the Edgewater 

buoy, was recorded hourly and available on NWISWeb.

Turbidity. For the nowcast, duplicate manual measurements of turbidity were made with portable turbidimeters 

at Edgewater (Orion AQUAfast AQ4500) and Huntington 

(Orbeco-Hellige Model 966, Orbeco Analytical Systems, Inc., 

Famingdale, N.Y.). Additionally, at Edgewater, a turbidity 

probe (DTS-12 Digital Turbidity Sensor, FTS Forest Technology Systems Ltd., Victoria, British Columbia, Canada) 

attached to the buoy recorded hourly turbidity measurements. 

Turbidity data from the buoy were available on NWISWeb; 

however, the automated turbidity data were not used in the 

nowcast. 

Table 1. Models developed for the Ohio Nowcast in 2008.

[Threshold probability is based on meeting or exceeding the single-sample bathing standard for E. coli (Francy and Darner, 2007)]

Model Range of dates used Variables Threshold 

probability

Huntington

Subseason 1 May 19−July 23 Wave height, Rw48a

, log turbidity, antecedent dry daysb

, day of the year 28

Subseason 2 July 24−Sept. 1 Wave height, Rw48a

, log turbidity, day of 

the year 27

Edgewater

Subseason 1 May 13−June 17 Wave height, Radar2cell-w48c

, log turbidity 27

Subseason 2a June 18−July 18 Wave height, Radar2cell-w48c

, log turbidity, lake level 30

Subseason 2b (modified) July 19−Aug. 13 Wave height, Radar2cell-w48c

, log turbidity, day of the year 34

Subseason 3 Not used Wave height, log turbidity, lake level 32

Subseason 3 modified Aug. 14−Sept. 11 Wave height, Radar2cell-w48c

, log turbidity 32

a

 Rw48 was the amount, in inches, at Hopkins International Airport, Cleveland, Ohio, in the 48-hour period before sampling, with the 

most recent rainfall receiving the most weight.

b Antecedent dry days were the number of days with zero precipitation preceding the current day.

c Radar2cell-w48 was the maximum amount of rainfall in the previous 48-hour period that fell in one of two 4-km grids, as determined 

from radar data, with the most recent rainfall receiving the most weight.
6 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Foreshore head. At Edgewater, a temporary piezometer 

(shallow water well) was installed 20 ft inland from the edge 

of water at a depth of 3.5 ft (designated as “PZ-2”) (fig. 2A). 

(The foreshore is defined as the strip of land along a body 

of water between low and high water marks.) The 2-in.-

internal-diameter piezometer with a 0.5-ft-long screen was 

driven to the desired depth by hand. The relative elevation 

of the piezometer was determined by conventional surveying technique. The piezometer was equipped with a pressure 

transducer and data logger to measure and record water levels 

at 15-minute intervals. Water-level data at 8:30 a.m. from 

the piezometer were used in exploratory data analysis and 

are referred to as “foreshore head.” The foreshore head is 

not equal to an actual change in water level; rather, it reflects 

the increased pressure from waves on water levels (data not 

shown). The head measured in PZ-2 can best be characterized 

as a combination of the pressures changes due to the local 

water table, fluctuations in barometric pressure, the water level 

in the lake, and the pressure variance due to wave action.

Lake level. Lake-level data were obtained from a station 

in Cleveland, approximately 400 yards offshore (National 

Oceanic and Atmospheric Administration [n.d.], station 

9063063). These data were used in the Edgewater nowcast. 

Lake levels were also measured by the USGS by use of a 

pressure transducer placed on a pier in the harbor to the east 

of Edgewater (fig. 2A). The data from the pressure transducer 

were postprocessed to correct for barometric-pressure fluctuations. The elevation of the pressure transducer was surveyed with differential GPS to tie it to the same datum as the 

piezometer.

Photosynthetically active radiation. A PAR sensor 

(LI-COR Biosciences, Lincoln, Nebr.) and data logger were 

mounted on top of a 50-ft water tower at Huntington to record 

PAR data every 10 minutes (fig 2B). For exploratory data 

analysis, PAR data from midnight to midnight were summed 

to obtain daily PAR values. 

Rainfall. Rainfall data were obtained from the nearest 

airport, from radar, or from a site-specific USGS-installed 

gage near Edgewater. 

Airport rainfall data were obtained for Cleveland Hopkins International Airport (National Oceanic and Atmospheric 

Administration, 2002) (fig. 1) for Huntington and Edgewater. 

The total rainfall, in inches, for the 24-hour period preceding 

the morning sampling (8:51 to 7:51) was used in calculations for the nowcast (Rd-1). Similarly, the total rainfall in the 

24-hour period preceding the afternoon sampling (14:51 to 

13:51) or since the morning sampling (8:51 to 13:51) were 

used for data analysis of afternoon data. 

Radar rainfall data were compiled in order to obtain 

information from a more widespread area. Hourly radar 

rainfall data were provided for 4-km grids (“cells”) by the 

National Weather Service. For Edgewater and Huntington, 

previous analysis identified the radar rainfall variable that 

had the strongest relation to E. coli concentrations (Francy 

and Darner, 2007). For Edgewater, this variable was identified as the maximum value from two squares for the previous 

48 hours and weighted as shown below (radar2cell-w48) 

(Edgewater is near the center of two 4-km squares that are 

bounded by a box with the coordinates lat 41°29′07.7′′, 

long -081°48′29.4′′; lat 41°31′13.8′′, long -081°47′16.7′′; lat 

41°29′25.4′′, long -081°41′39.9′′; and lat 41°27′19.3′′, long 

-081°42′52.7′′.) For Huntington, radar rainfall data from the 

previous 24 hours from six squares were summed (radar6cell-24) and used in the model (Huntington is near the center 

of six 4-km squares that are bounded by a box given the coordinates lat 41°28′30.5′′, long -082°02′08.0′′; lat 41°32′42.2′′, 

long -081°59′42.8′′; lat 41°30′01.1′′, long -081°51′17.8′′; and 

lat 41°25′49.5′′, long -081°53′43.5′′.) 

Weighted rainfall variables were calculated from airport 

or radar rainfall data. Rainfall weighted 48 hours (Rw48) was 

48 hours of cumulative rainfall, giving more weight to the 

most recent rainfall amount as follows:

Rw48 = (2*Rd-1) + Rd-2

where Rd-1 is the amount of rain, in inches, that fell in the 

24-hour period preceding the morning sampling and Rd-2 is the 

amount of rain that fell in the 24-hour period 2 days preceding 

the morning sampling.

A rain gage was installed near Edgewater (fig. 2A). The 

raingage was installed on top of a nearby building at a location of lat 41°29′22′′, long -081° 44′ 17.5′′ and consisted of 

a tipping bucket calibrated for 0.01 in. per tip and electronic 

datalogger that recorded the number of tips in each 10-minute 

period. 

Antecedent dry days. Antecedent dry days were calculated by counting the number of consecutive days without 

measurable rainfall up to and including 7:51 a.m. on the date 

of sampling.

Swimmers. At the time of the afternoon sampling, the 

number of swimmers was counted and recorded by field personnel at Huntington.

Day of the year. Day of the year was the number representing the date beginning with 1 for January 1 and 365 or 366 

for December 31 (the latter being a leap year). 

Testing and Refinement of Predictive Models

The models previously developed for the Huntington and 

Edgewater nowcast were validated during 2008 by monitoring 

the correct predictions and sensitivies and specifities of the 

model responses compared to the use of the previous day’s E. 

coli concentrations (persistence model). Looking at these data 

in hindsight, a correct response is based on the actual E. coli

concentration. The sensitivity is the percentage of exceedances 

of the bathing-water standard that were correctly predicted 

by the nowcast or the persistence model. The specificity is 

the percentage of nonexceedances where the nowcast or the 

persistence model correctly predicted nonexceedance of the 

standard. To further evaluate performance of the models in 

2008, Pearson’s r correlations (Helsel and Hirsch, 2002) were 
Results 7

calculated to determine the strength of linear relations between 

E. coli concentrations and the nowcast variables or other 

variables. 

After validation of the models in 2008, the 2008 data 

were added to the existing dataset, and refined models were 

developed for 2009. Steps to developing new models involved 

the use of multiple linear regression techniques, described in 

detail in Francy and Darner (2006), and consist of (1) exploratory data analysis and (2) model development, diagnostics, 

and selection. 

Results

Performance of the Models in 2008

The Huntington nowcast models (table 1) yielded a 

greater percentage of correct responses (84.9 percent) than 

did the persistence model (75.2 percent) for the recreational 

season of 2008 (table 2). The nowcast models had a greater 

sensitivity than the persistence model, but specificity differences were small. The improved sensitivity of the nowcast 

models over the persistence model is especially important; for 

the 14 days the bathing-water standard was exceeded in 2008, 

the nowcast predicted 8 of them (57.1 percent) whereas the 

persistence model predicted only 1 (7.1 percent). 

Although the Edgewater nowcast models yielded a 

slightly higher percentage of correct responses (61.0 percent) as compared to the persistence model (56.5 percent), 

both percentages were relatively low (table 2). Specificity 

was improved by using the nowcast models, but the nowcast 

models did not improve sensitivity (31.7 percent) over the 

persistence model (37.5 percent). The subseason 2a nowcast 

model was unable to predict any exceedance events correctly. 

The subseason 2a model included lake level as an explanatory variable. A t-test (Helsel and Hirsch, 2002) was used 

to evaluate the difference between lake levels in 2008 for 

subseason 2 and the lake levels used to develop the subseason 

2a model (2004−7) for the same dates. Lake levels in 2008 for 

subseason 2 were significantly higher than those in 2004−7 

(p<0.0001, fig. 3). Because of the negative relation between 

E. coli concentrations and lake levels for 2004−7 (Francy and 

Darner, 2007), the subseason 2a model was underpredicting 

the probability that the standard was exceeded during 2008.

Consequently, a modified subseason 2 model was developed 

without lake level (subseason 2b model) and was adopted 

for use in the nowcast on July 19. The subseason 2b model 

improved the sensitivity of the nowcast over the subseason 

2a model (54.5 and 0 percent, respectively) but resulted in 

lower specificity than the subseason 2a model (60.0 and 90.5 

percent). 

570.5

571.0

571.5

572.0

572.5

LAKE LEVEL, IN FEET

2008

(57)

A B

2004–7

Smallest value

less than 1.5 times IQR

Largest value

less than 

1.5 times IQR 

Outside value

(>1.5 × IQR)

75th percentile

25th percentile

IQR, interquartile range;

distance between 75th and 

25th percentiles 

Median

EXPLANATION

(228) Number of values in dataset 

(228)

Figure 3. Boxplots showing comparison of lake levels in subseason 2 for 2004–7 and 2008 at Edgewater, Cleveland, Ohio
8 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Table 2. Nowcast model responses as compared to using previous day’s Escherichia coli concentration 

(persistence model) in 2008. 

Model Subseason Sample 

size

No. Days 

>235a

Percentage of responses 

Correct Sensitivityb Specificityc

Huntington 

[Subseason 1 is May 19−July 23, subseason 2 is July 24−Sept. 5]

Nowcast 1 66 8 81.8 50.0 86.2

Persistence 1 65 8 76.9 0.0 87.7

Nowcast 2 40 6 90.0 66.7 94.1

Persistence 2 40 6 72.5 16.7 82.3

TOTAL NOWCAST 1 and 2 106 14 84.9 57.1 89.1

TOTAL PERSISTENCE 1 and 2 105 14 75.2 7.1 85.7

Edgewater 

[Subseason 1 is May 19−June 17, subseason 2a is June 18−July 18, subseason 2b is July 19−Aug. 13, 

and subseason 3 is Aug. 14−Sept. 11]

Nowcast 1 34 13 64.7 30.8 85.7

Persistence 1 32 12 59.4 41.7 70.0

Nowcast 2a 31 10 61.3 0.0 90.5

Persistence 2a 31 10 45.2 20.0 57.1

Nowcast modified 2b 26 11 57.7 54.5 60.0

Persistence 2b 26 11 61.5 54.5 66.7

Nowcast 3 27 7 59.2 42.9 65.0

Persistence 3 26 7 61.5 28.6 73.7

TOTAL NOWCAST 1,2, and 3 118 41 61.0 31.7 76.6

TOTAL PERSISTENCE 1,2, and 3 115 40 56.5 37.5 66.7

a

 235 colony-forming units per 100 milliliters is the single-sample bathing-water standard.

b

 Sensitivity was the proportion of exceedance responses that were correctly predicted as as an exceedance.

c

 Specificity was the proportion of nonexceedance responses that were correctly predicted as as an nonexceedance.
Results 9

Exploratory Data Analysis

Exploratory data analysis was done to better understand 

the performance of the models in 2008 and identify the most 

promising variables for inclusion in refined models. Comparisons of data on automated and manual measurements 

were made to determine whether they could be used interchangeably in the nowcast models. In addition, afternoon and 

morning E. coli data were examined in terms of meeting or 

exceeding the standard and for their relations to explanatory 

variables to determine whether afternoon data can provide 

more accurate estimates of changing water-quality conditions. 

E. coli and explanatory variables. The correlations 

between E. coli and explanatory variables are shown for data 

collected in 2008 at Edgewater (table 3) and Huntington (table 

4). Correlations significant at p<0.05 are bolded, and the relations for the variables used in the nowcast are shaded. 

At Edgewater, none of the relations examined were 

significant during subseason 1, including the variables used in 

the nowcast; however, wave height measured with the wave 

rod was nearly significant (p=0.0545). For the subseason 2 

nowcast variables, two were significantly related to E. coli

concentrations (radar2cell-w48 and wave height)and log turbidity was nearly significant, but lake level and day of the year 

were not significantly related to E. coli. For subseason 3, only 

one of the nowcast variables—wave height—was significantly 

related to E. coli. For the recreational year (subseasons 1, 2, 

and 3 combined), the strongest relation was between E. coli

concentrations and the difference in foreshore head in PZ-2 

over the past 24 hours, a new variable first measured during 2008. The Edgewater rain gage for the previous 24 hours 

showed a slightly higher correlation to E. coli concentrations 

(r=0.34) than the comparable radar (r=0.32) or Hopkins rainfall (r=0.29).

At Huntington, relations were significant between E. coli

concentrations and the nowcast variables for subseasons 1 and 

2, except for day of the year. For the recreational year (subseasons 1 and 2 combined), day of the year was nearly significant 

with respect to E. coli concentrations (p=0.0626). The radar 

data resulted in a slightly higher correlation than the airport 

data for subseason 1 and the recreational year, but not for 

subseason 2. For example, for subseason 1, Radar6cell-w48 

had a higher correlation coefficient (r=0.45) than the comparable Rw48 (r=0.33). The sum of the PAR on the previous day 

showed a significant negative relation to E. coli concentrations 

(r=-0.33); this was a new variable introduced during 2008.

Manual and automated measurements. The relations 

between E. coli concentrations and wave height or turbidity 

measured manually (using the wave rod or turbidimeter) and 

automatically (using the buoys) were compared by means of 

datasets that included both measurements. These datasets were 

slightly smaller than those in tables 3 and 4 because some 

automated measurements were missing. It should be noted that 

there were differences in the ways these data were collected. 

Automated measurements were made in approximately 6 ft 

of water and consistently recorded at 7:00 (turbidity) or 7:30 

(waves) each morning. Manual measurements were made in 

approximately 3 ft of water and made at the time and location 

of sampling, which varied between 7:30 and 9:30 a.m. 

A comparison of manual and automated wave heights at 

Edgewater is shown in fig. 4. The relation to E. coli concentrations was slightly stronger for the manual measurement (fig. 

4A, r=0.43) than for the automated measurement (fig. 4B, 

r=0.33). The slopes and y-intercepts that resulted from the 

regression relations between E. coli concentrations and wave 

heights were similar for manual and automated measurements. 

Even though these relations were similar and the manual and 

automated measurements were highly correlated (r=0.80), the 

plot of manual and automated measurements shows considerable scatter around the 1:1 and also shows that manual measurements were often higher than automated measurements 

(fig. 4C). 

At Edgewater, relations were not significant between E. 

coli concentrations and either manual turbidity (r=0.24) or 

automated turbidity (r=0.05) (fig. 5A). Closer examination of 

two outliers measured by the buoy on July 24 and August 10 

revealed that turbidities were fluctuating on both days, wave 

heights were elevated, and several data values were missing 

on August 10. Strong waves had moved the buoy closer to 

shore on July 24, and the buoy had to be placed back to the 

original nearshore location on July 25. After removing the 

data for July 24 and August 10, the correlation between E. coli 

concentrations and automated turbidity was still low (r = 0.05, 

p=0.6130). Fig. 5C shows that automated values were biased 

high as compared to manual values for turbidity. 

A comparison of manual and automated wave heights 

at Huntington is shown in figure 6. The relation to E. coli 

concentrations was slightly stronger for the automated measurement (fig. 6B, r=0.55) than for the manual measurement 

(fig. 6A, r=0.47). The slope that resulted from the regression 

relation between E. coli concentrations and wave heights was 

steeper for the automated measurement than manual measurement. The plot of manual and automated paired measurements 

(fig. 6C) further illustrates the differences between these two 

measurements. Although the relation between the two measurements was significantly correlated (r=0.75, p<0.001), the 

placement of data around the 1:1 line shows that the manual 

measurements were often the higher. 

Comparisons can also be made between wave heights 

measured by the Huntington and Edgewater buoys. The 

automated wave heights measured at Edgewater were significantly related to E. coli concentrations at Huntington (table 

4, r=0.59). Similarly, the automated wave heights measured 

at Huntington were significantly related to E. coli concentrations at Edgewater (table 3, r=0.45). The correlation between 

wave heights measured at Huntington and Edgewater by each 

respective buoy was very high (r=0.92, p<0.0001).
10 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Table 3. Pearson’s r correlations between log10 Escherichia coli (E. coli) concentrations and explanatory variables for 

subseason 1 (May 19−June 17), subseason 2 (June 18−Aug. 13), subseason 3 (Aug. 14−Sept. 11), and the recreational year at 

Edgewater, Cleveland, Ohio, 2008.

[The p values are in parenthesis; relations that were significant at p < 0.05 are in bold; variables used in the Ohio nowcast are shaded; ND is not determined; 

PAR is photosynthetically active radiation; n indicates number of samples and those different from heading numbers are specified]

Variable Subseason 1

n=34

Subseason 2

n=57

Subseason 3

n=27

Recreational year

n=118

Radar2cell-24a 0.20 (0.2669) 0.42 (0.0013) 0.42 (0.0279) 0.32 (0.0005)

Radar2cell-48a 0.08 (0.6686) 0.10 (0.4805) -0.14 (0.4921) 0.06 (0.5554)

Radar2cell-w48b 0.20 (0.2683) 0.42 (0.0012) 0.28 (0.1569) 0.29 (0.0017)

Rd-1

c 0.24 (0.1653) 0.29 (0.0274) 0.58 (0.0014) 0.29 (0.0013)

Rainfall, total, Edgewater rain gage, previous 

24 hrs ND ND ND 0.34 (0.0050)

n=66

Lake level at 8:30 a.m., measured by NOAA 0.15 (0.4040) 0.13 (0.3265) 0.13 (0.5217) 0.09 (0.3232)

Lake level at 8:30 a.m., measured by USGS 0.05 (0.6913)

r=67

Foreshore head measured in Piezometer 2 at 

8:30 a.m.

0.52 (0.1023)

n=13 0.29 (0.0280) 0.97 (0.0068)n=5

0.31 (0.0084)

n=73

Difference in foreshore head in Piezometer 2, 

24 hrs ND ND ND 0.48 (<0.0001)

n=72

Turbidity, manual 0.25 (0.1508) 0.20 (0.1294) 0.20 (0.3055) 0.21 (0.0208)

Turbidity, automated, measured by buoy at 7:00 

a.m.

0.06 (0.7942)

n=20 0.06 (0.6859) -0.01 (0.9537)n=22

0.05 (0.6094)

n=98

Log Turbidity, manual 0.23 (0.1905) 0.26 (0.0543) 0.22 (0.2711) 0.24 (0.0096)

Log Turbidity, automated, measured by buoy at 

7:00 a.m.

0.04 (0.8532)

n=20 0.07 (0.6079) -0.04 (0.8567)n=22

0.05 (0.6484)

n=98

Wave height, manual, measured with wave rod 0.33 (0.0545) 0.53 (<0.0001) 0.39 (0.0460) 0.44 (<0.0001)

Wave height, automated, measured by Edgewater buoy at 7:30 a.m.0.22 (0.3422)

n=20 0.37 (0.0042) 0.33 (0.1295)n=22

0.33 (0.0009)

n=99

Wave height, automated, measured by Huntington buoy at 7:30 a.m. ND ND ND 0.45 (0.0015)

n=46

PAR measured at Huntington, sum of previous 

day ND ND ND -0.16 (0.1682)

Day of the year -0.01 (0.9594) 0.11 (0.4106) 0.04 (0.8583) 0.04 (0.6465)

a

 Radar2cell-24 and radar2cell-48 were the maximum amounts of rainfall that fell in one of two 4-km grids, as determined from radar data in the previous 

24- and 48-hour periods, respectively.

b

 Radar2cell-w48 was the maximum amount of rainfall that fell in one of two, 4-km grids as determined from radar data in the previous 48-hour period, 

with the most recent rainfall receiving the most weight.

c

 Rd-1 was the amount of rainfall that fell, in inches, at Hopkins International Airport Cleveland, Ohio in the 24-hour period preceding sampling.
Results 11

Table 4. Pearson’s r correlations between log10 Escherichia coli (E. coli) concentrations and explanatory variables for 

subseason 1 (May 19−July 23), subseason 2 (July 24−Sept. 5), the recreational year, and for afternoon samples at Huntington, 

Bay Village, Ohio, 2008.

[The p values are in parenthesis; relations that were significant at p < 0.05 are in bold; variables used in the Ohio nowcast are shaded; ND is not determined; 

PAR is photosynthetically active radiation, n indicates number of samples and those different from heading numbers are specified]

Variable

Morning 2008 Afternoon 2008 

recreational year 

n=46

 Subseason 1 

n=66

Subseason 2 

n=40

Recreational year 

n=106

Radar6cell-24a 0.42 (0.0006) 0.38 (0.0159) 0.37 (0.0001) ND

Radar6cell-48a 0.16 (0.2044) 0.27 (0.0883) 0.16 (0.1120) ND

Radar6cell-w48b 0.45 (0.0002) 0.45 (0.0034) 0.39 (<0.0001) ND

Rd-1

c 0.32 (0.0080) 0.45 (0.0035) 0.29 (0.0027) 0.19 (0.1992)

Rd-2

c 0.09 (0.4604) 0.22 (0.1796) 0.07 (0.4534) -0.04 (0.7935)

Rw48d 0.33 (0.0075) 0.48 (0.0015) 0.28 (0.0035) 0.15 (0.3107)

Number of antecedent dry days since last rainfall (Rd-1) -0.45 (0.0002) -0.55 (0.0002) -0.39 (<0.0001) -0.25 (0.1012)

Day of the year 0.20 (0.1111) -0.02 (0.8793) 0.18 (0.0626) 0.02 (0.9108)

PAR, day of sampling (midnight to midnight) ND ND 0.02 (0.8788) -0.01 (0.9421)

PAR, previous day (midnight to midnight) ND ND -0.33 (0.0029) -0.26 (0.1137)

log PAR, previous day (midnight to midnight) ND ND -0.29 (0.0111) ND

PAR, day of sampling (8:50 to 13:50) ND ND ND -0.03 (0.8682)

Turbidity, manual 0.37 (0.0022) 0.60 (<0.0001) 0.46 (<0.0001) 0.52 (0.0002)

Log Turbidity, manual 0.42 (0.0004) 0.64 (<0.0001) 0.50 (<0.0001) 0.51 (0.0003)

Wave height, manual, measured by wave rod 0.38 (0.0017) 0.68 (<0.0001) 0.49 (<0.0001) 0.58 (<0.0001) 

Wave height, automated, measured by Huntington buoy at 7:30 (AM samples) or 13:30 (PM 

sample)

0.48 (0.0029) 

n = 37

0.85 (0.0035) 

n = 9

0.55 (<0.0001) 

n = 46

0.49 (0.0077) 

n = 28

Wave height, automated, measured by Edgewater buoy at 7:30 AM ND ND 0.59 (<0.0001)

n=96 ND

Number of swimmers ND ND ND -0.13 (0.3955)

a

 Radar6cell-24 and radar6cell-48 were the summed amounts of rainfall that fell in six 4-km grids, as determined from radar data in the previous 24- and 

48-hour periods, respectively.

b

 Radar6cell-w48 was the summed amount of rainfall that fell in six 4-km grids, as determined from radar data in the previous 48-hour period, with the 

most recent rainfall receiving the most weight.

c

 Rd-1 was the amount of rainfall that fell, in inches, at Hopkins International Airport Cleveland, Ohio in the 24-hour period preceding sampling; Rd-2 was 

the amount 2 days before sampling.

d

 Rw48 was the amount of rainfall that fell, in inches, at Hopkings International Airport, Cleveland, Ohio in the previous 48-hour period, with the most 

recent rainfall receiving the most weight.
12 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Figure 4. Wave heights measured by use of the wave rod (manual) or wave buoy (automated), Edgewater, Cleveland, Ohio, 2008. 

Relations between: A, Manual wave heights and E. coli concentrations. B, Automated wave heights and E. coli concentrations. C, 

Automated and manual wave heights. (r is the Pearson’s correlation coefficient, p is the significance of the relation.)

Figure 5. Turbidity measured by use of a grab sample and turbidimeter (manual) or probe attached to a nearshore buoy (automated), 

Edgewater, Cleveland, Ohio, 2008. Relations between: A, Manual turbidity and E. coli concentrations. B, Automated turbidity and E. coli

concentrations. C, Automated and manual turbidity. (r is the Pearson’s correlation coefficient, p is the significance of the relation.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

MANUAL WAVE HEIGHT, IN FEET

0.0

1.0

2.0

3.0

4.0

LOG10 E. COLI

y = 0.32x + 1.99

r = 0.43, p < 0.0001

0.0 0.5 1.0 1.5 2.0 2.5 3.0

AUTOMATED WAVE HEIGHT, IN FEET

0.0

1.0

2.0

3.0

4.0

y = 0.33x + 2.02

r = 0.33, p = 0.0009

A B

0.0 0.5 1.0 1.5 2.0 2.5 3.0

AUTOMATED WAVE HEIGHT, IN FEET

0.0

0.5

1.0

1.5

2.0

2.5

3.0

MANUAL WAVE HEIGHT, IN FEET

1:1

r = 0.80, p < 0.0001

C

0.0 0.5 1.0 1.5 2.0

MANUAL LOG10 TURBIDITY

0.0

1.0

2.0

3.0

4.0

LOG10 E. COLI

y = 0.42x + 1.84

r = 0.24, p = 0.184

0.0 0.5 1.0 1.5 2.0

AUTOMATED LOG 10 TURBIDITY

0.0

1.0

2.0

3.0

4.0

y = 0.08x + 2.13

r = 0.05, p = 0.6484

0.0 0.5 1.0 1.5 2.0

0.0

0.5

1.0

1.5

2.0 1:1

r = 0.82, p < 0.0001

A

C

B

8/10

7/24

7/24

8/10

8/10

7/24

AUTOMATED LOG 10 TURBIDITY

MANUAL LOG10 TURBIDITY
Results 13

Afternoon sampling. Morning and afternoon E. coli concentrations for 46 paired samples are shown in figure 7 with 

four quadrant designations (A, B, C, and D) based on meeting 

and exceeding the single-sample bathing-water standard. For 

samples below the bathing-water standard in the morning (A 

and B), results for morning and afternoon samples were evenly 

spaced around the 1:1 line, except for two samples collected in 

the afternoon that exceeded the standard. In contrast, for samples above the standard in the morning (C and D), morning 

E. coli concentrations were generally higher than afternoon 

concentrations. Among the 10 samples above the standard in 

the morning or afternoon, 3 were in agreement in terms of 

exceeding the standard (C) and 7 were not (A and D); 5 out 

of 7 had exceedances in the morning, but not in the afternoon 

(D). On many of the seven days not in agreement, there were 

differences in turbidity or wave heights measured for morning 

and afternoon samples (data not shown). 

Model Refinement and Selection for 2009

Explanatory variables consistently related to E. coli during 2008 and previous years were used for model refinement. 

These variables were the same for Huntington and Edgewater 

and included radar and Hopkins rainfall, manual wave heights, 

log turbidity, and day of the year. Automated wave height data 

were not used because the datasets were too small. Similarly, 

only one year of PAR and afternoon data were available at 

Huntington and only year of foreshore head data were available at Edgewater. For both beaches, data from 2005−8 were 

used to develop refined models because data collected before 

2005 included categorical wave-height measurements (Francy 

and Darner, 2007), which were not as accurate as manual 

measurements collected with a wave rod. The best model 

for each subseason and for the recreational year combined 

(Appendix 1) was identified by multiple linear regression and 

model diagnostic techniques, and threshold probabilities were 

established as described by Francy and Darner (2006). Data 

segments for each subseason were defined by day of the year, 

and the cutoff date varied slightly from year to year because 

of leap years. Datasets with Hopkins rainfall data had slightly 

higher numbers than datasets with radar data because of missing values for the latter. 

Figure 6. Wave heights measured by use of the wave rod (manual) or wave buoy (automated), Huntington, Bay Village, Ohio, 2008. 

Relations between: A, Manual wave heights and E. coli concentrations. B, Automated wave heights and E. coli concentrations. C, 

Automated and manual wave heights. (r is the Pearson’s correlation coefficient, p is the significance of the relation.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

MANUAL WAVE HEIGHT, IN FEET

0.0

1.0

2.0

3.0

4.0

LOG10 E. COLI

y = 0.41x + 1.49

r = 0.47, p = 0.0009

0.0 0.5 1.0 1.5 2.0 2.5 3.0

AUTOMATED WAVE HEIGHT, IN FEET

0.0

1.0

2.0

3.0

4.0

y = 0.67x + 1.42

r = 0.55, p < 0.0001

0.0 0.5 1.0 1.5 2.0 2.5 3.0

AUTOMATED WAVE HEIGHT, IN FEET

0.0

0.5

1.0

1.5

2.0

2.5

3.0

MANUAL WAVE HEIGHT, IN FEET

1:1

r = 0.75, p < 0.0001

A B

C
14 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

1 01 001 000,1

MORNING E. COLI, IN COLONY-FORMING UNITS PER 100 MILLILITERS

1

10

100

1,000

Bathing-water standard

1:1 line

(A)

(B)

(C)

(D)

AFTERNOON E. COLI, IN COLONY-FORMING UNITS PER 100 MILLILITERS

Figure 7. Comparison of morning and afternoon Escherichia coli concentrations at Huntington, Bay 

Village, Ohio, 2008. (Quadrant designations are based on meeting and exceeding the single-sample 

bathing-water standard of 235 colony-forming units per 100 milliliters.)

Edgewater

For Edgewater, a best model for the recreational year 

dataset was first identified; this model included Hopkins rainfall (table 5). Because of poor performance of the Edgewater 

models in the 2008 nowcast, the output from the recreationalyear model was examined and split into three new segments, 

based on day of the year and occurrence of false negatives 

and positives (Francy and Darner, 2007): (1) Day of the year 

134−159, May 13 through June 8 (“subseason 1”), (2) day of 

the year 160−222, June 9 through August 10 (“subseason 2”), 

and (3) day of the year 223–258, August 11 through September 

15 (“subseason 3”). Candidate models were developed by use 

of radar or Hopkins data for subseasons 1, 2, and 3. Candidate 

models were evaluated by examining the numbers and percentages of correct responses, false positives, and false negatives 

for 2005−8 data (table 5). On the basis of percentage of correct 

responses, the best individual subseason models included radar 

rainfall for subseason 1 (81.0 percent) and Hopkins rainfall for 

subseasons 2 (81.5 percent) and 3 (76.9 percent). The summed 

responses from the best separate models for subseasons 1, 

2, and 3 resulted in a greater percentage of correct responses 

(80.1) than for the recreational-year model (78.2 percent). 

Sensitivities were 8.6 percent higher for the summed model 

as compared to the recreational-year model, and specificities 

were about the same. The summed model resulted in eight 

fewer false negatives and two more false positives than the 

recreational-year model for 2005−8. 

Huntington

Because of good performance of the Huntington models in the 2008 nowcast, the same data segments used in 

2008 were retained for model refinement: (1) Day of the 

year 140−205, May 19 through July 23 (“subseason 1”) and 

(2) day of the year 206−249, July 24 through September 6 

(“subseason 2”). Models were developed by use of radar or 

Hopkins rainfall for subseasons 1 and 2 and compared to the 
Results 15

Table 5. Examination of candidate Edgewater 2005–8 models.

[R2, fraction of the variation of E. coli concentrations that is explained by the model; threshold probability is based on meeting and exceeding the singlesample bathing standard for E. coli; subseason 1 is day of the year 134-159 (May 13−June 8), subseason 2 is day of the year 160-222 (June 9−August 10), 

and subseason 3 is day of the year 223-258 (August 11−September 15); calculated sums are shaded below] 

Model Variables R2

Threshold 

probability

Number Percentage

Correct False + False - Correct Sensitivity

Specificity

Subseason 1, 2, and 3 models with radar rainfall

Subseason 1 Wave height, radar2cell-w48a 0.41 22 47 7 4 81.0 69.2 84.4

Subseason 2

Wave height, log turbidity, 

radar2cell-w48, day of 

the year

0.41 27 154 25 15 79.4 68.8 82.9

Subseason 3 Wave height, radar2cell-w48, 

day of the year 0.29 32 74 17 8 74.7 74.2 75.0

Subseason 1, 2, and 3 models with Hopkins rainfall

Subseason 1 Wave height, Rw48b 0.27 23 48 8 5 78.7 64.3 83.0

Subseason 2 Wave height, log turbidity, 

Rw48, day of the year 0.40 26 167 23 15 81.5 68.8 85.4

Subseason 3 Wave height, Rw48, day of 

the year 0.33 33 80 17 7 76.9 78.1 76.4

Best models

Subseason 1 

(radar), and 

subseasons 2, 3 

(Hopkins)

294 47 26 80.1 72.0 82.8

Recreational year Wave height, log turbidity, 

Rw48b

, day of the year 0.37 30 283 45 34 78.2 63.4 83.3

a

Radar2cell-w48 was the maximum amount of rainfall in the 48-hour period preceding sampling that fell in one of two 4-km grids, as determined from 

radar data, with the most recent rainfall receiving the most weight.

b

Rw48 was the amount of rainfall, in inches, at Hopkins International Airport, Cleveland, Ohio, in the 48-hour period before sampling, with the most recent 

rainfall receiving the most weight.

best recreational-year model which included radar rainfall 

(table 6). Candidate models were further evaluated by examining the numbers and percentages of correct responses, false 

positives, and false negatives for 2005−8 data. The summed 

responses for separate models for subseasons 1 and 2 including radar or Hopkins rainfall resulted in a greater percentage 

of correct responses (86.1 and 87.1 percent, respectively) than 

for the recreational year model ( 82.4 percent). Sensitivities 

were at least 10 percent higher and specificities only slightly 

higher for the summed subseason 1 and 2 models as compared 

to the recreational-year model. In comparing the two summed 

models, the model with radar rainfall resulted in one more 

false negative, and the model with Hopkins provided one more 

false positive; the percentage correct for the summed Hopkins model was higher because the Hopkins rainfall dataset 

included 12 more days. 
16 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Table 6. Examination of candidate Huntington 2005−8 models.

[R2, fraction of the variation of E. coli concentrations that is explained by the model; threshold probability is based on meeting and exceeding the singlesample bathing standard for E. coli; subseason 1 is day of the year 140–205 (May 21–July 23) and subseason 2 is day of the year 206–249 (July 24–Sept 6); 

combined subseason models are shaded below] 

Model Variables R2

Threshold 

probability

Number Percentage

Correct False + False - Correct Sensitivity

Specificity

Subseason 1 and 2 models with radar rainfall

Subseason 1 Log turbidity, radar6cell-24a

, 

day of the year

0.38 23 176 17 12 85.9 62.5 90.2

Subseason 2 Wave height, radar6cellw48b0.50 26 115 11 7 86.5 74.1 89.6

Subseason 1 plus 

subseason 2 

291 28 19 86.1 67.8 90.0

Subseason 1 and 2 models with Hopkins rainfall

Subseason 1 Log turbidity, Rd-1

c

, day of 

the year

0.36 23 181 15 11 87.4 65.6 91.4

Subseason 2 Wave height, Rw48d 0.49 26 122 12 7 86.5 76.7 89.2

Subseason 1 plus 

subseason 2 

303 27 18 87.1 71.0 90.6

Best recreational-year model

Recreational year Radar6cell-w48, log turbidity, wave height, day of 

the year

0.42 26 277 34 25 82.4 57.6 87.7

a

Radar6cell-24 was the summed amount of rainfall in the previous 24-hour period that fell in six 4-km grids, as determined from radar data.

b

Radar6cell-w48 was the summed amount of rainfall in the previous 48-hour period that fell in six 4-km grids, as determined from radar data, with the 

most recent rainfall receiving the most weight.

c

Rd-1 was the amount of rainfall, in inches, at Hopkins International Airport, Cleveland, Ohio, in the 24-hour period before sampling.

d

Rw48 was the amount of rainfall, in inches, at Hopkins International Airport, Cleveland, Ohio, in the 48-hour period before sampling, with the most recent 

rainfall receiving the most weight.

Next Steps

Water-resource managers and public health officials at 

Huntington and Edgewater will determine which 2005−8 

model(s) to use to issue advisories for the nowcast during 

2009. As in previous years, daily performance of the nowcast 

will be monitored, and models will be validated and refined to 

improve predictions during subsequent years. 

Acknowledgments

The authors acknowledge the following individuals for 

their hard work in sampling and running the nowcast—Jill Lis 

and Alex Brown of the Cuyahoga County Board of Health, 

and Sara English and Tom Vasel of the Northeast Ohio 

Regional Sewer District. Thanks are also extended to collaborators Mark Citriglia, Eva Hatvani, and Lester Stumpe at the 

Northeast Ohio Regional Sewer District. 
References Cited 17

References Cited

Boehm, A.B., Grant, S.B., Kim, J.H., Mowbray, S.L., McGee, 

C.D., Clark, C.D., Foley, D.M., and Wellman, D.E., 2002, 

Decadal and shorter period variability of surf zone water 

quality at Huntington Beach, California: Environmental Science & Technology, v 36, no. 18, p. 3885–3892.

Francy, D.S., and Darner, R.A., 2006, Procedures for developing models to predict exceedance of recreational water-quality standards at coastal beaches: U.S. Geological Survey 

Techniques and Methods 6–B5, 34 p.

Francy, D.S., and Darner, R.A., 2007, Nowcasting beach advisories at Ohio Lake Erie beaches: U.S. Geological Survey 

Open-File Report 2007–1427, 13 p.

Francy, D.S., Bushon, R.N., Bertke, E.E., Likirdopolus, C.A., 

Brady, A.M.G., Kephart, C.M., Stelzer, E.A., and Stoeckel, 

D.M., 2008, Quality-assurance/quality-control manual for 

the Ohio Water Microbiology Laboratory, accessed November 2008 at http://oh.water.usgs.gov/micro_qaqc.htm

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 3, chap. A3, accessed 

March 2006 at http://pubs.er.usgs.gov/pubs/twri/twri04A3

Ohio Department of Health, 2007, Bathing beach monitoring 

program, accessed November 2008 at http://www.odh.ohio.

gov/odhPrograms/eh/bbeach/beachmon.aspx

Myers, D.N., Stoeckel, D.M., Bushon, R.N., Francy, D.S., and 

Brady, A.M.G., 2007, Fecal indicator bacteria: U.S. Geological Survey Techniques of Water-Resources Investigations, 

book 9, chap. A7, section 7.1 (version 2.0), available from 

http://pubs.water.usgs.gov/twri9A/.

National Oceanic and Atmospheric Administration, Center 

for Operational Oceanic Products and Services [n.d.], NOS 

water level observation network Great Lakes, accessed 

November 2008 at http://glakesonline.nos.noaa.gov/

geographic.html

National Oceanic and Atmospheric Administration, National 

Weather Service, 2002, National Weather Service, Eastern 

Region Headquarters home page, accessed November 2008 

http://www.erh.noaa.gov/

U.S. Environmental Protection Agency, 2006, Method 1603—

Escherichia coli in water by membrane filtration using 

modified membrane-thermotolerant Escherichia coli agar: 

Washington, D.C., EPA 821–R–06–011, 42 p.
18 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

View of Huntington Beach, Bay Village, Ohio. (Photograph by Robert A. Darner, U.S. Geological Survey.)
Appendix 1. Candidate Models Used To Predict Escherichia coli Concentrations at Two Lake Erie Beaches 19

Appendix 1. Candidate Models Used To Predict 

Escherichia coli Concentrations at Two Lake Erie Beaches

[Water-resource managers will select from among these models to use the Ohio Nowcast during 2009]

Huntington Reservation (Bay Village, Ohio)

Subseason 1 model with radar rainfall

Log EC = -0.885 + 0.782(LOG TURBIDITY) + 0.171(RADAR6CELL-24) + 0.009(DAY OF THE YEAR)

or

EC = 0.130 + 6.061LOG TURBIDITY + 1.481RADAR6CELL-24 + 1.022DAY OF THE YEAR

Subseason 2 model with radar rainfall

Log EC = 1.449 + 0.307(WAVE HEIGHT) + 0.033(RADAR6CELL-W48) 

or

EC = 28.116 + 2.027WAVE HEIGHT + 1.080RADAR6CELL-48W

Subseason 1 model with Hopkins rainfall

Log EC = -0.866 + 0.806(RD-1) + 0.783(LOG TURBIDITY) + 0.010(DAY OF THE YEAR)

or

EC = 0.136 + 6.392RD-1 + 6.071LOG TURBIDITY + 1.022DAY OF THE YEAR

Susbseason 2 model with Hopkins rainfall Log EC = 1.449 + 0.311(WAVE HEIGHT) + 0.181(Rw48) 

or

EC = 28.149 + 2.046WAVE HEIGHT + 1.514Rw48

Recreational-year model 

Log EC = 0.465 + 0.167(WAVE HEIGHT) + 0.045(RADAR6CELL-W48) + 0.406(LOG TURBIDITY) + 0.003(DAY OF THE YEAR)

or

EC = 2.918 + 1.467 WAVE HEIGHT + 1.109RADAR6CELL48W + 2.550LOG TURBIDITY + 1.007DAY OF THE YEAR

Edgewater Park (Cleveland, Ohio)

Subseason 1 model with radar rainfall

Log EC = 1.253 + 0.494(WAVE HEIGHT) + 0.910(RADAR2CELL-W48)

or

EC = 17.909 + 3.118WAVE HEIGHT + 8.126 RADAR2CELL-W48

Subseason 2 model with radar rainfall

Log EC = 0.561 + 0.239(WAVE HEIGHT) + 0.215(RADAR2CELL-W48) + 0.335(LOG TURBIDITY) + 0.005(DAY OF THE YEAR)

or

EC = 3.641 + 1.734WAVE HEIGHT + 1.641 RADAR2CELL-W48 + 2.161LOG TURBIDITY + 1.011 DAY OF THE YEAR

Subseason 3 model with radar rainfall

Log EC = -0.799 + 0.294(WAVE HEIGHT) + 0.115(RADAR2CELL-W48) + 0.011(DAY OF THE YEAR)

or

EC = 0.159 + 1.966WAVE HEIGHT + 1.304 RADAR2CELL-W48 + 1.027 DAY OF THE YEAR
20 Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008

Edgewater Park (Cleveland, Ohio)—Continued

Subseason 1 model with Hopkins rainfall

Log EC = 1.381 + 0.613(WAVE HEIGHT) + 0.635(Rw48)

or

EC = 24.031 + 4.103WAVE HEIGHT + 4.316 Rw48

Subseason 2 model with Hopkins rainfall

Log EC = 0.615 + 0.274(WAVE HEIGHT) + 0.209(Rw48) + 0.278(LOG TURBIDITY) + 0.005(DAY OF THE YEAR)

or

EC = 4.123 + 1.878WAVE HEIGHT + 1.617 Rw48 + 1.898LOG TURBIDITY + 1.011 DAY OF THE YEAR

Subseason 3 model with Hopkins rainfall

Log EC = -0.828 + 0.311 (WAVE HEIGHT) + 0.089(Rw48) + 0.012(DAY OF THE YEAR)

or

EC = 0.148 + 2.046WAVE HEIGHT + 1.227 Rw48 + 1.027 DAY OF THE YEAR

Recreational-year model 

Log EC = 0.726 + 0.237(WAVE HEIGHT) + 0.158(Rw48) + 0.401(LOG TURBIDITY) + 0.004(DAY OF THE YEAR)

or

EC = 5.325 + 1.725WAVE HEIGHT + 1.440 Rw48 + 2.516LOG TURBIDITY + 1.009 DAY OF THE YEAR

In all these equations,

 EC is the Escherichia coli concentration, in colony-forming units per 100 milliliters.

 LOG TURBIDITY is defined as the log10 of the turbidity in Nephelometeric Turbidity Units.

 RADAR6CELL-24 is the summed amount of rainfall in the previous 24-hour period that fell in six 4-km grids, 

as determined from radar data.

 RADAR6CELL-W48 is the summed amount of rainfall in the previous 48-hour period that fell in six 4-km grids, 

as determined from radar data, with the most recent rainfall receiving the most weight.

 DAY OF THE YEAR is the number representing the date beginning with 1 for January 1 and 365 or 366 for 

December 31 (the latter being a leap year).

 WAVE HEIGHT is obtained by manual measurements at the time of sampling.

 

 RADAR2CELL-W48 is the maximum amount of rainfall in the 48-hour period preceding sampling that fell 

in one of two 4-km grids, as determined from radar data, with the most recent rainfall receiving the most 

weight.

 RD-1 is the total rainfall, in inches, at Hopkins International Airport, Cleveland, Ohio, in the 24-hour period 

before sampling.

 RW48 is the amount of rainfall, in inches, at Hopkins International Airport, Cleveland, Ohio, in the 48-hour 

period before sampling, with the most recent rainfall receiving the most weight.
Francy and others—Testing and Refining the Ohio Nowcast at Two Lake Erie Beaches—2008—Open-File Report 2009–1066