SUPERSEDED Water Models; Guidance for Selecting Input Parameters
SUPERSEDED Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides
Version II
February 28, 2002
U.S. Environmental Protection Agency
Office of Pesticide Programs
Environmental Fate and Effects Division
On this Page
1.0 Introduction
EPA's Office of Pesticide Programs (OPP) has developed this guidance document to help modelers select the appropriate input values for OPP's aquatic exposure models. Using this guidance document should improve the consistency in modeling the fate of pesticides in the environment and ultimately the overall quality of OPP's aquatic risk assessments. The primary sources of input fate parameters which are used in these models include pesticide product chemistry and labeling information as well as sorption coefficients, halflives, and rate constants from acceptable or supplemental metabolism studies conducted by pesticide manufacturers.
2.0 Summary Guidance Tables for Model Input Parameters
The guidance tables listed below contain the parameters, input values, sources of data, and additional explanatory information (notes) for the following aquatic exposure models: SCI GROW, GENEEC2, FIRST, PRZM, and EXAMS. More specific information about these aquatic models can be found at the following web site: https://www.epa.gov/oppefed1/models/water/index.htm
Table 2.1 Input Parameters for SCIGROW(Version 2.1, May 8, 2001
Parameter (units)  Input Value  Data Source  Notes 

Maximum application rate (pounds active ingredient/acre) 
Use the maximum single application allowed on the label.  Product label  
Maximum number of applications per year  Use the maximum number of applications allowed on the label.  Product label  
Partition coefficient normalized for organic carbon content 
K_{oc} (mL g_{o.c.}^{1} ) 
If Koc shows greater than a threefold variation, use the lowest value. If not, then use the median value.  Mobility Adsorption/Desorption study (Guideline 1631) 
SCIGROW was developed using K_{oc} values ranging from 32180 L kg_{o.c.}^{1} and halflives from 131000 days. Extrapolation beyond these values will increase the uncertainty of the ground water EEC. 
Aerobic soil metabolism halflife (days) 
Use the average value. If there are four or more values available, use the median value. If there is more than a fivefold difference, make note of the range.  Acceptable or supplemental aerobic soil metabolism data (Guideline 1621) 
Table 2.2 Input Parameters for GENEEC2 (Version 1.0 , August 1, 2001)
Parameter (units) 
Input Value [Specific Parameter Guidance] 
Data Source  Notes  

Application rate (pounds a.i./ acre) 
Use the maximum application rate specified on the label.  Product label  When this information is not available on the label, OPP may need to work with local extension agents (USDA), crop experts (landgrant universities, grower groups, etc), BEAD, and other sources to determine a reasonable estimate.  
Number of applications  Use the maximum number of applications specified on the label.  Product label  
Interval between applications (days)  Use the minimum application interval specified on the label.  Product label  
Partition Coefficient K_{d} or K_{oc} (mL g_{o.c.}^{1 }) 
Use the lowest nonsand Koc or lowest nonsand K_{d} [Nonsand soil textures are all soil textural classes except sand and loamy sand, i.e., ≥ 70% sand sized separates  0.05 to 2.00 diameter (USDA texture classification)]  Acceptable mobility data (Guideline 1631) 

Aerobic soil metabolism halflife
(in days) 
If only one halflife value is available, use the following formula:
t_{input} = 3xt_{1/2} If more than one halflife value is available, use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at If no data is available, the modeler should assume that the compound is stable. 
Acceptable or supplemental aerobic soil metabolism data (Guideline 1621)  This method calculates the upper confidence bound on the mean metabolism halflife, not the upper percentile of the distribution of halflife values.
Some relevant t values:


Wetted in?  No  Product label  In practice, "wetting in" is used to reduce the amount of pesticide available for runoff. With GENEEC2, though, selecting the "wetting in" option changes the timing of the storm from two days after last application to immediately after the last application. The result is that pesticide concentrations may be greater, not less, especially for pesticides with a short halflife.  
Depth of incorporation (inches)  Use the information specified on the label for depth of soil incorporation. If the pesticide is not incorporated, use zero (0).  Product label  
Method of application  Select aerial or ground application as applicable.  Product label  
Solubility in water (mg/L) 
Use the maximum available value at 20 25°C.  Acceptable product chemistry data (Guideline 638)  
Aerobic aquatic metabolism halflife (in days) 
Total system mass half life (water plus sediment). This value should include the effects of metabolism and hydrolysis. If only one halflife value is available, use the following formula: t_{input} = 3xt_{1/2} If more than one halflife value is available, use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at If no data is available, the modeler should assume that the compound is stable. If no aerobic aquatic metabolism data is available and the pesticide is hydrolytically stable, use 2x aerobic soil metabolism halflife input value. If no aerobic aquatic metabolism data is available and the pesticide shows significant hydrolysis, 
Acceptable or supplemental aerobic aquatic metabolism data (Guideline 1624)  This input parameter implies degradation by both metabolism and hydrolysis. If the modeler enters a value, he/she will NOT be prompted for an independent hydrolysis rate (see hydrolysis section). Example for the case when no aerobic aquatic data are available: For a single aerobic soil halflife of 30 days, the aerobic soil metabolism halflife input parameter is 90 days ( 30 days x 3); thus the estimated aerobic aquatic halflife is 90 days x 2 or 180 days. 

Hydrolysis halflife (in days)  Use the maximum value at pH 7. If no data are available or the compound is stable, use zero (0).  Acceptable abiotic hydrolysis data (Guideline 1611)  GENEEC2 prompts for the hydrolysis halflife only if a zero (0) is entered for the aerobic aquatic halflife (see above explanation for aerobic aquatic metabolism halflife).  
Aquatic photolysis halflife(days)  Use a maximum darkcontrol corrected value. If no data is available or if the compound is stable, use zero (0).  Acceptable aqueous photolysis data (Guideline 1612) 
Table 2.3 Input Parameters for FIRST Version 1.0
Parameter (units) 
Input Value [Specific Parameter Guidance] 
Data Source  Notes  

Application rate (pounds a.i./ acre) 
Use the maximum application rate.  Product label  When this information is not available on the label, the OPP scientist may need to work with local extension agents, crop experts (landgrant universities, grower groups, USDA etc), BEAD, and other sources to determine a reasonable estimate.  
Number of applications  Use the maximum number of applications  Product label  
Interval between applications (days)  Use the minimum application interval  Product label  
Partition coefficient K_{d} or K_{oc} (mL/g) 
Use the lowest nonsand Koc or lowest nonsand K_{d} [Nonsand soil textures are all soil textural classes except sand and loamy sand that is, classes with more than 70% sandsized separates  0.05 to 2.00 diameter (USDA texture classification)]  Acceptable mobility data (Guideline 1631) 

Aerobic soil metabolism halflife (in days) 
If only one halflife value is available, use the following formula: t_{input} = 3xt_{1/2} If more than one halflife value is available, use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at If no data is available, the modeler should assume that the compound is stable. 
Acceptable or supplemental aerobic soil metabolism data (Guideline 1621)  This method calculates the upper confidence bound on the mean metabolism halflife, not the upper percentile of the distribution of halflife values.
Some relevant t values:


Wetted in?  No  Product label  In practice, "wetting in" is used to reduce the amount of pesticide available for runoff. With FIRST, though, selecting the "wetting in" option changes the timing of the storm from two days after last application to immediately after the last application. The result is that pesticide concentrations may be greater, not less, than wetting in especially for pesticides with a short halflife.  
Depth of incorporation (inches)  Use the information specified on the label for depth of soil incorporation. If the pesticide is not incorporated, use zero (0).  Product label  
Method of application  Select aerial or ground application as applicable.  Product label  
Percent cropped area (decimal)  Use the maximum fraction of watershed planted in crop(s)  Select the value indicated in the program or in the FIRST User's Manual  
Solubility in water (mg/L) 
Use the maximum available value at 20 25°C.  Acceptable product chemistry data (Guideline 638)  
Aerobic aquatic metabolism halflife (in days)  Total system mass half life (water plus sediment). This value should include both metabolism and hydrolysis. If only one halflife value is available, use the following formula: t_{input} = 3xt_{1/2} If more than one halflife value is available, use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at If no data is available, assume that the compound is stable. If no aerobic aquatic metabolism data is available and the pesticide is hydrolytically stable, use 2x aerobic soil metabolism halflife input value. If no aerobic aquatic metabolism data is available and the pesticide shows significant hydrolysis, assume that the compound is stable for metabolism [Hydrolysis will be the ratedetermining degradation process]. If no aerobic soil metabolism data is available, assume that the compound is stable. 
Acceptable or supplemental aerobic aquatic metabolism data (Guideline 1624)  This input parameter implies degradation by both metabolism and hydrolysis. If the modeler enters a value, he/she will NOT be prompted for an independent hydrolysis rate (see hydrolysis section). Example for aquatic metabolism with no data: For a single aerobic soil halflife of 30 days, the aerobic soil metabolism halflife input parameter is 90 days ( 30 days x 3); thus the estimated aerobic aquatic halflife is 90 days x 2 or 180 days. 

Hydrolysis halflife (in days) 
Use the maximum value at pH 7. If no data is available or the compound is stable, use zero (0).  Acceptable abiotic hydrolysis data (Guideline 1611)  FIRST prompts for the hydrolysis halflife only if a zero (0) is entered for the aerobic aquatic halflife.  
Aquatic photolysis halflife (in days) 
Use the maximum darkcontrol, corrected value. If no data is available or if the compound is stable, use "0".  Acceptable aqueous photolysis data (Guideline 1612) 
Table 2.4 Input Parameters for PRZM Version 3.12
Variable (units)  Variable Description  Input Value [Specific Parameter Guidance] 
Data Source  Notes  

APD, APM, IAPYR (day/mo/yr) 
Application date(s)(day/mo/yr)  Use the maximum number of applications and minimum application interval.  Product label or locationspecific  The modeler should give consideration to locationspecific cropping dates (emergence, maturity, harvest dates) or other relevant factors as specified on the label. Cropping dates are included in the standard scenarios for PRZM modeling. In developing new scenarios, OPP will work with local extension agents, crop experts (landgrant universities, grower groups), and BEAD to develop reasonable estimates. If the application interval is not specified, OPP will consult with local extension agents or crop experts. 

DEPI (cm)  Incorporation depth  Use the information specified on the label for soil incorporation. If the pesticide is not incorporated, use zero ( 0).  Product label  
TAPP (kg a.i. ha^{1}) 
Application rate  Use the maximum application rate allowed per application  Product label  
APPEFF (decimal) 
Application efficiency  Use 0.95 for aerial spray and 0.99 for ground spray.  Spray Drift Task Force data  Application efficiencies were taken from Spray Drift Task Force studies.  
DRFT (decimal) 
Spray drift fraction  For aquatic ecological exposure assessment, use 0.05 for aerial spray or 0.01 for ground spray.  
For drinking water assessment, use 0.16 for aerial spray, 0.064 for ground spray, and 0.063 for orchard airblast.  Spray Drift Task Force Data  Spray drift fractions for drinking water exposure modeling are based upon the Spray Drift Task Force studies. See https://www.epa.gov/oscpmont/sap/meetings/1998/index.htm#072998 (SAP meeting, July 2930, 1998)  
FEXTRC  Foliar extraction  0.5  
PLDKRT  Decay rate on foliage  0  
PLVKRT (day ^{1}) 
Volatilization rate from foliage  0.0 is the default value unless field data are available  Default or field data  
UPTKF () 
Plant uptake factor  0.0 is the default value unless field data are available  Default or field data  
DWRATE (surface) (day^{1}) 
Dissolved phase pesticide decay rate in surface horizon  DWRATE = DSRATE Set this parameter equal to the simulated lumped firstorder degradation rate. If only one halflife value is available, use the following formula: t_{input} = 3xt_{1/2} If more than one halflife value is available, use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at α = 0.1(i.e., 1.9) 
Acceptable or supplemental aerobic soil metabolism data (Guideline 1621)  Although EFED rarely receives horizonspecific studies,separate chemicalspecific inputs can be created for each horizon, The surface horizon is the most critical horizon for modeling runoff in PRZM. This method calculates the upper confidence bound on the mean metabolism halflife, not the upper percentile of the distribution of halflife values. Some relevant t values:


DSRATE (surface) (day^{1}) 
Adsorbed phase pesticide decay rate in surface horizon  
DWRATE (subsurface horizons) (day^{1}) 
Dissolved phase pesticide decay rate in subsequent subsurface horizons  DWRATE = DSRATE Set this parameter equal to the simulated lumped firstorder degradation rate. 
Acceptable or supplemental aerobic or anaerobic soil metabolism data (Guideline 1621)  Calculating this value for soil metabolism halflives below the surface layer is difficult because subsurface horizons are often not anaerobic and microbial activity varies with depth among soils. Pesticide degradation below 4 cm does not influence PRZM runoff. Metabolism is faster in surface soils which are typically higher in organic matter and microorganisms than in subsurface horizons. In selecting the appropriate halflife for DWRATE in the subsurface horizons, the modeler should use data on the soil type and soil horizons modeled and the nature of the use. For a Mollisol, the aerobic halflife may be appropriate for certain depths before the organic carbon content decreases. For soil horizons that are saturated for long periods during the growing season, an anaerobic halflife may be more appropriate. For subsurface horizons which are low in organic matter, an assumption of no degradation may be warranted, unless soil metabolism studies are available for low organic matter subsurface horizons.  
DSRATE(subsurface horizons) (day^{1}) 
Adsorbed phase pesticide decay rate in subsequent subsurface horizons  
KD ^{5} ( mL g^{1} ) 
Pesticide partition or distribution coefficients for each horizon  The modeler can use two options: Option 1: enter the Koc value and let PRZM calculate the Kd
Option 2: input each Kd value

Mobility Adsorption/Desorption study (Guideline 1631) 
See accompanying text for guidance on determining the appropriateness of a Koc model for a specific chemical. If Koc is appropriate, use the average Koc from batch experiments. If Koc is not appropriate for a chemical, seek additional guidance from the Water Quality Tech Team. 
Table 2. 5 Input Parameters for EXAMS 2.97.5
Variable (units)  Variable Description  Input Value [Specific Parameter Guidance] 
Data Source  Notes 

Henry's Constant (atmm^{3}mole^{1}) 
Henry's Law Constant  Either use the measured Henry's Law Constant provided by the registrant or calculate it from Henry = (VAPR/760)/(SOL/MWT), where VAPR is vapor pressure in torrs, MWT is the molecular weight, and SOL is the actual solubility (not multiplied by 10). 
From registrant or product chemistry data  
KBACW(*,*,1) (cfu/mL)^{1} hour^{1} 
Bacterial biolysis in water column  Use total system mass halflife (water + sediment) from aerobic aquatic metabolism studies. If only one halflife value (n) is available, use the following formula: t_{input} = 3xt_{1/2} If more than one halflife value is available (n>1), use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at If no aerobic aquatic metabolism data are available and the compound is stable to hydrolysis, use 0.5 x PRZM aerobic soil metabolism rate constant(DWRATE) If no aerobic aquatic metabolism data are available and there is significant hydrolysis, assume that the compound is stable to aquatic metabolism. 
Acceptable or supplemental aerobic aquatic metabolism data (Guideline 1624)  When both aquatic metabolism and hydrolysis rate data are included, the metabolism rate needs to be corrected for the hydrolysis rate. The model calls for secondorder rate constants normalized by bacterial "colony forming units". Colony forming units are fixed at 1 cfu/ml. 
KBACS(*,*,1) (cfu/mL)^{1} hour^{1} 
Bacterial biolysis in benthic sediment  Use anaerobic aquatic metabolism data. If only one halflife value (n) is available, use the following formula: t_{input} = 3xt_{1/2} If more than one halflife value is available (n>1), use the formula below: t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}] where ‾t‾_{1/2} = mean of sample half lives s = sample standard deviation n = number of half lives available t_{90,n1} = onesided students t value at If no anaerobic aquatic metabolism data are available and the compound is stable to hydrolysis, use 0.5 x anaerobic soil metabolism rate If no anaerobic aquatic metabolism data are available and significant hydrolysis occurs, assume that the compound is stable to aquatic metabolism. If no soil metabolism data are available, assume that the compound is stable. 
Acceptable or supplemental anaerobic aquatic metabolism data (Guideline 1624)  See note above for KBACW. 
KDP(*,1) (hour^{1}) 
Direct photolysis  Use the darkcontrol corrected aqueous photolysis rate constant. If acceptable data are not available, assume that the chemical is stable (value = 0). 
Acceptable or supplemental aqueous photolysis data (Guideline 1612).  
RFLAT (degrees) 
Simulated latitude of photolysis test  Use the latitude referenced in the supplied by aqueous photolysis study.  Acceptable or supplemental aqueous photolysis data (Guideline 1612)  
KNH(1,*,1) (hour^{1}) 
Technically this value represents the neutral hydrolysis rate coefficient. However, if the modeler sets KAH and KBH to zero, then the KNH variable can represent the overall pH 7 hydrolysis rate.  Use the overall hydrolysis rate from the pH 7 test. If acceptable data are not available, assume that the compound is stable (value = 0). 
Acceptable or supplemental abiotic hydrolysis data (Guideline 1611)  The standard water bodies are set to a pH of 7. 
KPS(*,*) (mL g^{}) 
Partition coefficient for sediments  If K_{OC} is valid, use this value and do not set KPS. If K_{OC} is inappropriate, then set KPS to the average K_{d}. 
Mobility Adsorption/Desorption study(Guideline 1631)  The modeler should use appropriate judgement when selecting a Kd value, as the Kd for pond sediment may be quite different than the Kd for agricultural soils. 
MWT(1) (g mole^{1}) 
Molecular weight  From registrant or product chemistry data  
SOL(*,*) (mg L^{1}) 
Aqueous solubility  Multiply the water solubility by 10. Use Product Chemistry Guideline 638 data for the temperature most closely resembling that of the water body being modeled.  From registrant or product chemistry data  Mass is not necessarily conserved when solubility is set too low. 
VAPR(1) (torr) 
Vapor pressure  Use Product Chemistry Guideline 639 data.  From registrant or product chemistry data  
QTBAS(*,*,1)  Sediment bacteria temperature coefficient  2  Standard value  
QTBAW(*,*,1)  Water bacteria temperature coefficient  2  Standard value 