WEATHER DATA IN AQUATIC 

EXPOSURE MODELING

US EPA ENVIRONMENTAL 

MODELING PUBLIC MEETING

APRIL 28, 2015

Meridith Fry, Gabe Rothman, and Dirk Young

US EPA Environmental Fate and Effects Division (EFED)
Motivation

• Weather contributes importantly to a 

chemical’s modeled fate and transport

• Representative weather is important for 

conducting pesticide exposure simulations

• EFED standard weather files 

• No longer maintained by NOAA National Climatic 

Data Center (NCDC)

• Only span 1961-1990

• Limited to 237 locations nationwide

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Outline

• Overview of existing source: SAMSON

• Development of new source: NOAA CPC 

and NCEP/NCAR gridded datasets

• Processing and new format

• Evaluation of processed dataset

• Maintenance and outlook

1 Solar and Meteorological Surface Observation Network (SAMSON) 

2 NOAA Climate Prediction Center (CPC) Unified Rain Gauge Analysis

3 NOAA National Center for Environmental Prediction (NCEP)/National 

Center for Atmospheric Research (NCAR) Reanalysis 3
Existing source: SAMSON

Weather Bureau Army Navy (WBAN) SAMSON

• 237 National Weather Service (NWS) locations

• 1/1/1961 to 12/31/1990

NCDC, 1983, 1992. Climatography of the United States No. 81. National 

Climatic Data Center, NOAA, Asheville, NC.

SCS, 1981. Land Resource Regions and Major Land Resource Areas of the 

United States. Agriculture Handbook 296. USDA SCS, Washington, DC.

http://www2.epa.gov/exposure-assessment-models/meteorologicaldata

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NOAA CPC and NCEP/NCAR datasets

• More expansive and recent (1/1/1948 – present)

• Daily weather variables available:

Precipitation, Potential Evapotranspiration (PET)*, Temperature, 

Wind speed, Solar radiation

• Uniform national coverage 

• High spatial resolution, especially precipitation * derived 

Improved source: NOAA Gridded Weather

SAMSON  NOAA Gridded Locations

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Source 1: NOAA CPC

Daily Total Precipitation

NOAA CPC Unified Rain Gauge Analysis (raw format: netCDF)

0.25 x 0.25 degree resolution – lower 48 states

Daily Precipitation

(Before 2007)

http://www.esrl.noaa.gov/psd/data/gridded/data.unified.daily.conus.html

(2007 – Present)

http://www.esrl.noaa.gov/psd/data/gridded/data.unified.daily.conus.rt.html

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Source 2: NOAA NCEP/NCAR 

Other variables

NOAA NCEP/NCAR Reanalysis (raw format: netCDF)

2.5 x 2.5 degree resolution – global

Daily Mean Temperature, Wind Speed, Solar Radiation, 

Potential Evapotranspiration (derived)

http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.surfaceflux.

html

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Hargreaves-Samani method (1985) used for 

its simplicity and comparable performance to 

Penman-Monteith and other recognized 

techniques: 

PETdaily = (0.0023*Sntat,b) (Tb +17.8) (Trange

0.5)/10

Where PETdaily = daily potential evapotranspiration (cm/day)

Sntat,b = downward solar radiation flux at nominal top of the 

atmosphere (mm/day)

Tb = daily mean air temperature (◦

C)

Trange = daily maximum minus daily minimum temperature (◦

C)

[Droogers and Allen, 2002; Lu et al., 2005; Tuya et al., 2006; Alexandris et al., 

2008; Kra, 2012; Weiland et al., 2012]

Potential Evapotranspiration: derived

Available from 

NOAA 

NCEP/NCAR

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Weather data processing

Harvested 2.5x2.5-deg. 

NOAA NCEP/NCAR Weather

(1961 – 2014)

Harvested 0.25x0.25-deg.

NOAA CPC Precipitation 

(1961 – 2014)

Process and Merge Interpolate to 0.25x0.25-deg. 

resolution

ALL Weather on 0.25x0.25-

deg. grid

(1961 – 2014)

Interpolate and merge datasets with NCL:

https://www.ncl.ucar.edu/Applications/regrid.shtml

Same interpolation technique used by NOAA NCDC 

to combine NCEP/NCAR Reanalysis with CPC:

http://www.ncdc.noaa.gov/societal-impacts/airstagnation/overview

Weather at 237 SAMSON 

Locations for SWCC 

(1961 – 2014)

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Updated weather data format: 

.DVF → .WEA

Main differences

• 4-digit year replaces 2-digit year

• Comma-separated format

mmddyy precip pevap temp wind srad

010161 0.00 0.30 9.5 501.6 240.3 

010261 0.00 0.21 6.3 368.0 244.3

010361 0.00 0.28 3.5 488.3 303.0 

010461 0.00 0.34 4.7 448.3 316.6

mm,dd,yyyy,precip,pevap,temp,wind,srad

01, 01, 1961, 0.00, 0.30, 9.5, 501.6, 240.3

01, 02, 1961, 0.00, 0.21, 6.3, 368.0, 244.3

01, 03, 1961, 0.00, 0.28, 3.5, 488.3, 303.0

01, 04, 1961, 0.00, 0.34, 4.7, 448.3, 316.6

.dvf

.wea

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Evaluation of processed dataset

Quality assurance and quality control

Data quality objectives:

• From reliable, peer-reviewed source

• Processing is transparent, well-documented

• Format is standardized and compatible with OPP 

aquatic exposure models

• Model simulations using NOAA weather are 

reasonable compared to simulations using SAMSON

• Maintenance plan 

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Quality of dataset

• Comparison of spatial and temporal distributions to 

other datasets (e.g., precipitation, PET)

• Comparison of simulated aquatic concentrations with 

NOAA weather vs. SAMSON 

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Precipitation and PET distributions

[Fry et al.]

1991-2014

Precipitation, Annual Average of Monthly Sums: 

1961-1990

1991-2014

PETH, Annual Sum of Long-term Daily Mean: 

1961-1990

1991-2014

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CPC vs. Other Precipitation Datasets

1991-2014

Precipitation, Annual Average of Monthly Sums: 

1961-1990

1991-2014 1991-2014

[PRISM Climate Group, Oregon State, 2015]

30-year Annual Average of monthly sums: 

1981-2010

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Derived PET vs. Other PET Datasets

1991-2014

1991-2014

PETH, Annual Sum of Long-term daily mean: 

1961-1990

1991-2014

[Droogers and Allen, 2002]

Annual sum of long-term daily mean: 1970-2000

International Water Management Institute

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Quality of dataset

Pearson’s Correlation Coefficient, r

Quantify strength of linear relationship

rr =

∑(xx − xx̅)(yy − yy�)

∑(xx − xx̅)2(yy − yy�)2

r = 1 Perfectly positive linear relationship

r = 0 No linear relationship

r = -1 Perfectly negative linear relationship

• Comparison of spatial and temporal distributions to 

other datasets (e.g., precipitation, PET). 

• Comparison of Surface Water Concentration 

Calculator (SWCC) outputs generated using NOAA 

dataset to those using SAMSON

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SWCC comparison: Standard Pond

Results from 7 simulations – each run once with NOAA weather and 

once with SAMSON weather

[Fry et al.]

SWCC Standard Pond

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SWCC Standard Pond

SWCC comparison: Standard Pond

Results from 7 simulations – each run once with NOAA weather and 

once with SAMSON weather

[Fry et al.]

Zoom in of 0 – 40 ppb

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SWCC comparison: Index Reservoir

[Fry et al.]

SWCC Index reservoir

Results from 7 simulations – each run once with NOAA weather and 

once with SAMSON weather 19
SWCC comparisons: Summary

Pearson’s correlation coefficient 

for SWCC simulations 

(1961-1990)

Standard 

pond

Index 

reservoir

Peak (1-in-10 year) 0.996 0.995

4-day Average (1-in-10 year) 0.996 0.995

21-day Average (1-in-10 year) 0.998 0.998

60-day Average (1-in-10 year) 0.999 0.998

365-day Average (1-in-10 year) 0.988 0.986

Entire Simulation Mean 0.988 0.979

[Fry et al.]

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Maintenance plan

• On annual basis, OPP EFED will process prior year

• Methodology will remain consistent

• Processing is automated

• Continue expanding dataset to most recent year

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• QA/QC as part of Spatial Aquatic Model Quality 

Assurance Project Plan

• Peer-review of methodology 

• Scientific Advisory Panel – Sept 2015

Outlook

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Questions?

Meridith Fry, Ph.D.

Environmental Engineer, EISB

Water Quality Tech Team Co-chair

Environmental Fate & Effects Division

Office of Pesticide Programs

U.S. Environmental Protection Agency

Email: Fry.Meridith@epa.gov

Phone: 703.347.0128

Thank you

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