Development of Conceptual Models for 

Estimating Aquatic Exposure from the Use of 

Pesticides on Rice using the Pesticide 

Flooded Application Model

By: Melanie Biscoe, Marietta Echeverria, Meridith Fry, Jim 

Hetrick, Greg Orrick, Chuck Peck, Mohammed Ruhmann, 

Andrew Shelby, Nelson Thurman, Katrina White, and 

Dirk Young

Presentation to EMPM, March 24, 2014

U.S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division
Outline

• The physical model (PFAM)

• Site selection

• California Site

– Inputs for conceptual model

– Spray drift

– Spread of applications and flood releases

• Arkansas/ Missouri site

• Monitoring analysis

2
Pesticide in Flooded Application Model 

(PFAM)

Potential 

Overflow

Water 

Input

Pesticide 

Application Volatilization

degradation

degradation

mass 

transfer

Evaporation/

Precipitation

Adjustable

Weir

Leaching 

Receiving 

Water Body or 

Mixing Cell

3

PFAM provides pesticide concentration in paddy water and 

receiving water body or mixing cell (daily average value)
Site Selection

• Main rice growing area

• Surface water drinking water intake influenced 

by rice

• Monitoring data to validate conceptual model 

development approach

4
Where is rice grown?

5
2007 AgCensus – Rice Acres Harvested

State

Acres 

Harvested

% of Acres 

in U.S.

AR 1,326,377 48%

CA 531,075 19%

LA 377,115 14%

MS 185,076 7%

MO 179,300 7%

TX 145,006 5%

FL 11,376 0.4%

TN 2,317 0.1%

6
Areas of Interest

• Arkansas and California are the major rice 

growing areas

• Both have at least one surface water intake 

that could be influenced by rice

• Monitoring data are available for the 

Sacramento River in California to validate the 

conceptual model

• Some monitoring data available in Arkansas

7
• Drinking water intake of 

interest: near Sacramento, 

CA on Sacramento River

• 511,824 acres of rice in 

watershed

• 14.4 million total acres in 

watershed

• Rice positioned in lower 

part of watershed

8
Conceptual Model for Drinking Water

Flow from 

surrounding 

watershed 

and river

Flow from rice paddy 

water released over 

paddy draining 

duration

Rice Field

Drinking Water Intake

Spray Drift of 

pesticide

Direct 

Application of 

Pesticide

9
Inputs to Define for Conceptual Model

• Receiving mixing cell

– Width, depth, length

– Base flow 

• Area of watershed

• Area of rice in watershed 

– Amount of pesticide applied

– Volume of paddy water

• Spray drift fraction

• Dates of applications 

• Dates of flood release

• Depth of paddy

• Agronomic practices

10
Inputs to Define for Conceptual Model

• Receiving mixing cell

– Width, depth, length

– Base flow 

• Area of watershed

• Area of rice in watershed 

– Amount of pesticide applied

– Volume of paddy water

• Spray drift

• Dates of applications 

• Dates of flood release

• Depth of paddy

• Agronomic practices

11
Summary of flow at USGS Site 11425500 

Sacramento River at Verona, CA

Parameter

Discharge (cubic feet per second)

2011 2012 1946-2012

Annual Mean 24,490 14,970 19,750

Highest Daily Mean 67,700 (Mar 26) 46,100 (Mar 18) 95,600 (Jan 3, 1997)

Lowest Daily Mean 9,870 (Nov 11) 7,610 (May 18) 3,590 (June 24, 1992)

Annual 7-day 

minimum 10,000 (Nov 5) 8,020 (May 15) 3,960 (June 22, 1992)

Maximum peak flow -- 46,900 (Mar 18) 102,000 (Jan 2, 1997)

Data were calculated by the USGS and reported in Water-Data Report 2012 for the site.

Flow from paddies is potentially double-counted. 

Lowest flows occur during rice growing season.

This impact is minimized by using a low flow estimate.

12
PFAM Conceptual Model: CA

• Mixing Cell:

– Width, depth, length (194m × 5.1m × 30m)

– Base flow – Based on USGS data

– Residence time of water in mixing cell (seconds to 

minutes)

• Area of Watershed: 14.4 million acres

• Area of Rice: 511,000 acres

• Sacramento, CA weather file and latitude

13
Usage and Monitoring: Molinate in the Sacramento River

14

0

1

2

3

4

1-Apr8-Apr

15-Apr22-Apr29-Apr

6-May

13-May20-May27-May

3-Jun

10-Jun

17-Jun

24-Jun

1-Jul

8-Jul

15-Jul

22-Jul

29-Jul

Sampling Date

Molinate Concentration 

in ppb (Site 34_12)
Water Management Practices for Rice in CA

• Emergency releases are rare

– 1 in 2009; 33 in 2002, a flood year1

• Seepage from the berm is also rare

– 97% of inspected paddies had no seepage in 20091

– 3% of inspected paddies had seepage of <5 

gallons/day1

• Flood depth 4 inches (range is typically 3-5 

inches)

• Post-application hold times specified on pesticide 

label

1 California Regional Water Quality Control Board Central Valley Region. Resolution No. R5-2010-9001. 

Rice Pesticides Program – Control of Rice Pesticides. 15
Inputs to Define for Conceptual Model

• Receiving mixing cell

– Width, depth, length

– Base flow 

• Area of watershed

• Area of rice in watershed 

– Amount of pesticide applied

– Volume of paddy water

• Spray drift fraction

• Dates of applications 

• Dates of flood release

• Depth of paddy

• Agronomic practices

16
Spray Drift to Canals

• 23 rice paddies selected from the Colusa 

Basin to represent the typical canal and 

paddy orientation

• AgDRIFT was used to determine the spray 

drift fraction for a 10, 50, 100, 200, and 

300 meter zone from edge-of-paddy

• NHD+ was intersected with each drift zone 

to determine the area of water in each 

zone, assuming a 3m canal width

• Area of water in buffer zone x spray drift 

faction = area of water receiving full 

application rate for that buffer zone

• Sum the areas and scale to total paddies

• Area of water receiving full app rate ÷ area 

of rice = spray drift fraction

17
18

Rice Paddy receiving 

full application

Drift Zone 

(10 m)

Area of water X 0.29 = y

Area of water x 0.15 = z

Drift Zone 50 m

Area water with 100% application rate= z+y

Canals 

flowing 

through 

drift zones
Spray Drift Fraction for Canals

Canal contribution Aerial Ground – High Boom

Very Fine 

to Fine

Fine to 

Medium Fine/Very Fine

Summed area of water in each 

zone adjusted by each buffer 

spray drift value (m2)

87950.79 47152.41 88922.94

Spray drift value (fraction) 0.0308 0.0201 0.0312

19
Spray Drift Fraction for Rivers

River and Stream contribution

Aerial Ground – high boom

Very Fine 

to Fine

Fine to 

Medium Fine/Very Fine

Weighted spray drift on typical 

river

(fraction)

0.11

0.046

0.050

Spray drift value (fraction) 0.0036 0.0015 0.0016

Canal contribution Aerial Ground – High Boom

Very Fine 

to Fine

Fine to 

Medium Fine/Very Fine

Summed area of water in each 

zone adjusted by each buffer 

spray drift value (m2)

87950.79 47152.41 88922.94

Spray drift value (fraction) 0.0308 0.0201 0.0312

Total Fraction of amount applied that is applied directly to the 

receiving water body flow: 0.015 to 0.034.

With 500,000 acres receiving applications, even this small fraction can have a big impact 

on EDWC. 20
Spread of Applications and Flood 

Releases

21
Arkansas/ Missouri Conceptual Model

Only one 

surface water 

drinking 

water intake 

of interest

USGS station 

near DWI: 

Black River, 

Pocahontas, 

Arkansas

22
PFAM Conceptual Model: AR/MO

• Mixing Cell:

– Width, depth, length (98m × 2.3m × 30m)

– Base flow – Based on USGS data

• Area of Watershed: ~3 million acres

• Area of Rice: 102,345 acres

• Little Rock, Arkansas weather file and 36oN

23
Monitoring Data for CA

• Monitoring data available on Sacramento 

River and Calusa Basin Drain

– CADPR

– NAWQA

• Will analyze both acute and annual average 

results

• Will examine results with and without the use 

of a bias factor

24
Monitoring Data for Arkansas/Missouri

• Little data available on Black River

• Data are available on nearby streams and 

rivers

• Data are also available for Mississippi River 

near Baton Rouge that have shown to be 

influenced by rice

25
PFAM Conceptual Model Team

• Management Lead: Jim Cowles

• Model Developer: Dirk Young

• Senior Science Advisor: Nelson Thurman

• Team Lead: Katrina White

• Other Team Members: Melanie Biscoe

Meridith Fry

Jim Hetrick

Greg Orrick

Chuck Peck 

Mohammed Ruhmann

Andrew Shelby

• Acknowledgement: Jim Breithaupt, Jim Carleton

26

Questions
27

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

Monthly Mean Data 1946-2012

Cubic feet per second

Mean

Max

Min

Flow in Sacramento River
Black River at Pocahontas, Arkansas

Parameter

Discharge (cubic feet per second)

2011 2012 1936-2012

Annual Mean 8,734 4,520 5,638

Highest Daily 

Mean 83,700 (Apr 28) 15,800 (Dec 7) 83,700 (Apr 28, 2011)

Lowest Daily 

Mean 1,820 (Jan 31) 1,200 (Jul 6) 1,080 (Oct 16, 1956)

Annual 7-day 

minimum 1,840 (Jan 25) 1,250 (Jul 2) 1,090 (Oct 15, 1956)

Maximum peak 

flow -- 16,000 (Dec 7) 86,600 (Apr 28, 2011)

28

* Data were calculated by the USGS and reported in Water-Data Report 2012 (USGS, 2012a, 2012b). 
Flow in Black River

29
USGS gage station 

near West 

Sacramento DWI

30

USGS Gage on 

Sacramento River at 

Verona