Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides

1.0 Introduction

Appendix B. Update History for the Input Parameter Guidance
Memorandum
DATE: November 10, 2009
SUBJECT: Revision of Input Parameter Guidance
FROM: /s/ Donald Brady, Division Director, Environmental Fate and Effects Division (7507P), Office of Pesticide Programs
TO: Environmental Fate and Effects Division (7507P), Office of Pesticide Programs
This memorandum announces the release of Version 2.1 of the existing Input Parameter Guidance for a suite of models used in EFED, including SCIGROW, GENEEC, FIRST, PRZM, and EXAMS. General updates as well as modelspecific updates were made to the guidance. General updates include:

Parameter descriptions were reformatted, updated to better resemble how they appear in the models, and reordered to reflect their order in the models.

Guidance on how to calculate the 90^{th} percentile of the mean was moved to an appendix.

Soilwater partition coefficients were harmonized across the surface water models;

Incorporation depths for different application methods were added.

Output filerelated parameters not addressed in the previous guidance were addressed;

Additional editorial corrections and clarifications were made.
In general, any aquatic modeling that begins on or after the week of November 16, 2009 should be conducted with Version 2.1 of the Input Parameter Guidance (dated October 22, 2009). Aquatic exposure modeling begun prior to November 16, 2009 may be completed using Version II of the Input Parameter Guidance (dated February 28, 2002).
Attachments:

Input Parameter Guidance (Version 2.1; October 22, 2009).
Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides
Version 2.1
October 22, 2009
U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division
1.0 Introduction
EPA's Office of Pesticide Programs (OPP) has developed this guidance document to help model users select and prepare 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 that 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 environmental fate studies conducted or sponsored 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: SCIGROW, GENEEC, FIRST, PRZM, and EXAMS. More specific information about these aquatic models can be found at the Models for Pesticide Risk Assessment web site. For specific cases, in which deviation from the guidance is appropriate, the model user should document the rationale for each deviation. If additional guidance is necessary, OPP model users should consult with the Environmental Fate and Effects Division (EFED) Water Quality Technology Team (WQTT).
2.1 SCIGROW
Parameter (units) 
Input Value  Data Source  Notes 

Output file  Select any file name and extension for the output file.  SCIGROW will assign the extension ".txt" to the file name only if the "output file" button is pressed.  
Chemical name  (User's choice)  
Application rate (lbs a.i./acre)  Use the maximum single application rate allowed on the label for the modeled use.  Product labels  
Number of applications  Use the maximum number of applications allowed on the label for the modeled use.  Product labels  
K_{OC} (mL/g_{oc})  If the partition coefficients normalized for organic carbon content (K_{OC} or K_{FOC}) show greater than a threefold variation, use the lowest value. If not, then use the median value.  Adsorption/desorption data (Harmonized Test Guideline 835.1230)  SCIGROW was developed using K_{OC} values ranging from 32180 mL g_{OC}^{1} and halflives from 131000 days. Extrapolation beyond these values will increase the uncertainty of the ground water concentration. (The model will not use K_{OC} values > 9995 mL g_{OC}^{1}.) 
Soil metabolism halflife (days)  If three or less aerobic soil metabolism halflife values are available, use the mean value. If there are four or more halflives available, use the median value. If there is more than a fivefold difference, make note of the range.  Aerobic soil metabolism data (Harmonized Test Guideline 835.4100) 
2.2 GENEEC
Parameter (units) 
Input Value [Specific Parameter Guidance] 
Data Source  Notes  

Run number  Select any number to associate with the results of this run in the output file.  Not applicable  
Output file name  Select up to 8 characters for the output file name followed by any 3character extension.  Not applicable  GENEEC will not assign an extension to the file name.  
Chemical name  Select any input with which to identify your chemical.  Product labels  
Crop name  Select any input with which to identify the modeled use.  Product labels  
Application rate (pounds a.i. per acre)  Use the maximum application rate specified on the label for the modeled use.  Product labels  When this information is not available on the label, determine a reasonable conservative estimate.  
Number of applications  Use the maximum number of applications specified on the label for the modeled use.  Product labels  
Interval between applications (days)  Use the minimum application interval specified on the label for the modeled use.  Product labels  
Partition Coefficient K_{d} (mL g^{1}) or K_{OC} (mL g_{OC}^{1}) 
If binding is correlated with organic carbon content, enter zero (0) for K_{d} and then enter the mean K_{OC}. Otherwise, enter the mean K_{d}.  Adsorption/ desorption data (Harmonized Test Guideline 835.1230)  Binding is correlated with organic carbon content if the coefficient of variation (i.e., the standard deviation divided by the mean) for K_{OC} values is less than that for K_{d} values.  
Soil aerobic metabolic halflife (days) 
If multiple aerobic soil metabolism halflife values are available, enter the 90^{th} percentile confidence bound on the mean halflife value (see Equation 1 in Appendix A for instructions). If a single aerobic soil metabolism halflife value is available, enter 3x the halflife value (see Equation 2 in Appendix A). If no aerobic soil metabolism data are available, assume that the compound is stable to biodegradation under these conditions, i.e., enter zero (0). 
Aerobic soil metabolism data (Harmonized Test Guideline 835.4100)  
Wetted in?  No  Product labels  In practice, "wetting in" is used to reduce the amount of pesticide available for runoff. With GENEEC, 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.  
Method of application  Select

Product labels  Lettered inputs are not case specific (e.g., (A) or (a)).  
→ Droplet size distribution [under (A)] 
Select

Product labels  Select (B) fine to medium droplet size distribution if the label does not specify a coarser spray quality.  
→ Nozzle height [under (B)]  For ground applications, select

Product labels  Select the high boom height as a default when it is not prohibited on the label.  
→ Spray quality [under (B)]  For ground applications, select

Product labels  Select the fine droplet size distribution when it is not prohibited on the label.  
→ Airblast type [under (C)]  Select

Product labels  
→ Width of nospray zone (feet) [under (A), (B), or (C)] 
If specified on the label, enter the width of the nospray (buffer) zone between the treated field and the water body.  Product labels  Select zero (0) ft if the label does not specify a buffer width.  
→ Depth of incorporation (inches) [under (B) or (D)] 
Enter a depth of soil incorporation based on the information specified on the label. If the pesticide is not incorporated, use zero (0).  Product labels  Suggested incorporation depths include:


Solubility in water (ppm)  Use the maximum available value at 20 25° C.  Water solubility data (Harmonized Test Guideline 830.7840, 830.7860)  
Aerobic aquatic metabolism halflife (days) 
If multiple aerobic aquatic metabolism halflife values are available, enter the 90^{th} percentile confidence bound on the mean halflife value for the total system (water plus sediment) (see Equation 1 in Appendix A for instructions). If a single aerobic aquatic metabolism halflife value is available, enter 3x the halflife value (see Equation 2 in Appendix A). If no aerobic aquatic metabolism data are available and the pesticide shows insignificant hydrolysis, use 2x the aerobic soil metabolism halflife input value. If no aerobic aquatic metabolism data are available and the pesticide shows significant hydrolysis, enter zero (0). 
Aerobic aquatic metabolism data (Harmonized Test Guideline 835.4300) 
This input parameter implies degradation by both metabolism and hydrolysis. If a positive value is entered, the model will NOT prompt for an independent hydrolysis rate (see hydrolysis section). Example for the case when no aerobic aquatic data are available and the pesticide is hydrolytically stable: For a single aerobic soil metabolism halflife of 30 days, the aerobic soil metabolism halflife input value is 90 days (30 days x 3); thus the estimated aerobic aquatic metabolism halflife input value is 180 days (90 days x 2). 

→ Hydrolysis halflife (days)  If zero (0) was entered for the aerobic aquatic metabolism halflife, enter the maximum hydrolysis halflife value at pH 7. If no hydrolysis data are available, enter zero (0) and assume the compound is stable.  Hydrolysis data (Harmonized Test Guideline 835.2120)  GENEEC prompts for the hydrolysis halflife only if a zero (0) is entered for the aerobic aquatic metabolism halflife.  
Photolysis halflife (days)  Enter the maximum darkcontrol corrected environmental aqueous phototransformation halflife value. If no aqueous photolysis data are available or if there is no evidence of photolysis, enter zero (0).  Aqueous photolysis data (Harmonized Test Guideline 835.2240) 
2.3 FIRST
Parameter (units) 
Input Value [Specific Parameter Guidance] 
Data Source  Notes  

Run number  Select any number to associate with the results of this run in the output file.  Not applicable  
Output file name  Select up to 8 characters for the output file name followed by any 3character extension.  Not applicable  FIRST will not assign an extension to the file name.  
Chemical name  Select any input with which to identify your chemical.  Product labels  
Crop name  Select any input with which to identify the modeled use.  Product labels  
Application rate (pounds a.i. per acre)  Use the maximum application rate specified on the label for the modeled use.  Product labels  When this information is not available on the label, determine a reasonable conservative estimate.  
Number of applications  Use the maximum number of applications specified on the label for the modeled use.  Product labels  
Interval between applications (days)  Use the minimum application interval specified on the label for the modeled use.  Product labels  
Percent cropped area (decimal)  Enter the maximum fraction of watershed planted in crops on which the pesticide may be applied.  PCAspecific guidance (values are suggested within the program and in the FIRST User's Manual)  
Partition coefficient K_{d} (mL g^{1}) or K_{OC} (mL g_{OC}^{1}) 
If binding is correlated with organic carbon content, enter zero (0) for K_{d} and then enter the mean K_{OC}. Otherwise, enter the mean K_{d}.  Adsorption/ desorption data (Harmonized Test Guideline 835.1230)  Binding is correlated with organic carbon content if the coefficient of variation (i.e., the standard deviation divided by the mean) for K_{OC} values is less than that for K_{d} values.  
Aerobic soil metabolism halflife (days) 
If multiple aerobic soil metabolism halflife values are available, enter the 90^{th} percentile confidence bound on the mean halflife value (see Equation 1 in Appendix A for instructions). If a single aerobic soil metabolism halflife value is available, enter 3x the halflife value (see Equation 2 in Appendix A). If no aerobic soil metabolism data are available, assume that the compound is stable to biodegradation under these conditions, i.e., enter zero (0). 
Aerobic soil metabolism data (Harmonized Test Guideline 835.4100)  
Wetted in?  No  Product labels  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.  
Method of application  Select

Product labels  
→ Depth of incorporation (inches) [under (B) or (D)] 
Enter a depth of soil incorporation based on the information specified on the label. If the pesticide is not incorporated, use zero (0).  Product labels  Suggested incorporation depths include:


Solubility in water (ppm)  Use the maximum available value at 2025° C.  Water solubility data (Harmonized Test Guideline 830.7840, 830.7860)  
Aerobic aquatic metabolism halflife (days) 
If multiple aerobic aquatic metabolism halflife values are available, enter the 90^{th} percentile confidence bound on the mean halflife value for the total system (water plus sediment) (see Equation 1 in Appendix A for instructions). If a single aerobic aquatic metabolism halflife value is available, enter 3x the halflife value (see Equation 2 in Appendix A). If no aerobic aquatic metabolism data are available and the pesticide shows insignificant hydrolysis, use 2x the aerobic soil metabolism halflife input value. If no aerobic aquatic metabolism data are available and the pesticide shows significant hydrolysis, enter zero (0). 
Aerobic aquatic metabolism data (Harmonized Test Guideline 835.4300) 
This input parameter implies degradation by both metabolism and hydrolysis. If a positive value is entered, the model will NOT prompt for an independent hydrolysis rate (see hydrolysis section). Example for the case when no aerobic aquatic data are available and the pesticide is hydrolytically stable: For a single aerobic soil metabolism halflife of 30 days, the aerobic soil metabolism halflife input value is 90 days (30 days x 3); thus the estimated aerobic aquatic metabolism halflife input value is 180 days (90 days x 2). 

→ Hydrolysis halflife (days)  If zero (0) was entered for the aerobic aquatic metabolism halflife, enter the maximum hydrolysis halflife value at pH 7. If no hydrolysis data are available, enter zero (0) and assume the compound is stable.  Hydrolysis data (Harmonized Test Guideline 835.2120)  FIRST prompts for the hydrolysis halflife only if a zero (0) is entered for the aerobic aquatic metabolism halflife.  
Photolysis halflife (days)  Enter the maximum darkcontrol corrected environmental aqueous phototransformation halflife value. If no aqueous photolysis data are available or if there is no evidence of photolysis, enter zero (0).  Aqueous photolysis data (Harmonized Test Guideline 835.2240) 
2.4 PRZM
The following guidance table is for running PRZM directly rather than through a graphical user interface (GUI) to estimate exposure in runoff. The main difference with using GUIs is that they typically require halflives (in values of days) rather than rate constants (in values of day^{1}) for metabolism parameters.
Parameter (units) 
PRZM Variable 
Input Value [Specific Parameter Guidance] 
Data Source  Notes 

Application date(s) (day/mo/yr) 
APD, APM, IAPYR  Use the maximum number of applications and minimum application interval for the modeled use.  Product labels or locationspecific  The model user should consider locationspecific cropping dates and relevant labelspecific information. Other relevant information may be obtained from agricultural extension agents, crop experts (landgrant universities, grower groups), and BEAD. 
Incorporation depth (cm)  DEPI  Use the information specified on the label for soil incorporation. If the pesticide is not incorporated, use zero (0).  Product labels  
Application rate (kg a.i. ha^{1}) 
TAPP  Use the maximum application rate allowed per application for the modeled use.  Product labels  
Application efficiency (decimal) 
APPEFF  Use 0.95 for aerial spray and 0.99 for ground spray and orchard airblast.  
Spray drift fraction (decimal)  DRFT  For aquatic ecological exposure assessment, use 0.05 for aerial spray, 0.03 for orchard airblast, or 0.01 for ground spray.  
For drinking water assessment, use 0.16 for aerial spray, 0.064 for ground spray, or 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 (SAP meeting, July 2930, 1998)  
Foliar extraction (cm^{1})  FEXTRC  Enter 0.5 (default value) unless field data are available.  Note that this parameter is a rate constant (cm^{1}), not a depth (cm) of 50% washoff (WD_{50}).  
Decay rate on foliage (day^{1}) 
PLDKRT 
If multiple foliar decay halflife values are available, enter the rate constant corresponding to the upper 90^{th} percentile confidence bound on the mean halflife value (see Equations 1 and 3 in Appendix A for instructions). If a single foliar decay halflife value is available, enter the rate constant corresponding to 3x the halflife value (see Equations 2 and 3 in Appendix A). If no foliar decay data are available, assume that the compound does not decay on foliage, i.e., enter zero (0). 
Magnitude of the residue or greenhouse foliar decay data  Note that this parameter is a rate constant (day^{1}), not a halflife (day). 
Volatilization rate from foliage (day^{1}) 
PLVKRT  Enter zero (0) unless field data are available.  Note that this parameter is a rate constant (day^{1}), not a halflife (day).  
Plant uptake factor (decimal)  UPTKF  Enter zero (0) unless field data are available.  
Dissolved phase pesticide decay rate in surface horizon (day^{1})  DWRATE (surface) 
DWRATE = DSRATE If multiple aerobic soil metabolism halflife values are available, enter the rate constant corresponding to the 90^{th} percentile confidence bound on the mean halflife value (see Equations 1 and 3 in Appendix A for instructions). If a single aerobic soil metabolism halflife value is available, enter the rate constant corresponding to 3x the halflife value (see Equations 2 and 3 in Appendix A). If no aerobic soil metabolism data are available, assume that the compound is stable to biodegradation under these conditions, i.e., enter zero (0). 
Aerobic soil metabolism data (Harmonized Test Guideline 835.4100) 
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. Note that these parameters are rate constants (day^{1}), not halflives (day). 
Adsorbed phase pesticide decay rate in surface horizon (day^{1})  DSRATE (surface)  
Dissolved phase pesticide decay rate in subsequent subsurface horizons (day^{1})  DWRATE (subsurface horizons) 
DWRATE = DSRATE If multiple relevant soil metabolism halflife values are available (see notes), enter the rate constant corresponding to the 90^{th} percentile confidence bound on the mean halflife value (see Equations 1 and 3 in Appendix A for instructions). If a single relevant soil metabolism halflife value is available (see notes), enter the rate constant corresponding to 3x the halflife value (see Equations 2 and 3 in Appendix A). If no relevant soil metabolism data are available, assume that the compound is stable to biodegradation under these conditions, i.e., enter zero (0). 
Aerobic or anaerobic soil metabolism data (Harmonized Test Guideline 835.4100, 835.4200) 
Pesticide degradation below 2 cm does not influence PRZM runoff concentrations, so parameterization of horizon degradation below 2 cm is rarely necessary (scenarios developed for the PRZM volatilization routines are an exception). Note that these parameters are rate constants (day^{1}), not halflives (day). 
Adsorbed phase pesticide decay rate in subsequent subsurface horizons (day^{1})  DSRATE (subsurface horizons)  
Pesticide partition or distribution coefficients for each horizon (cm^{3} g^{1})  KD 
If binding is correlated with organic carbon content, use the mean K_{OC}:
If binding is not correlated with organic carbon content, use the mean K_{d}:

Adsorption/ desorption data (Harmonized Test Guideline 835.1230) 
Binding is correlated with organic carbon content if the coefficient of variation (i.e., the standard deviation divided by the mean) for K_{OC} values is less than that for K_{d} values. Use of the mean Kd may not be appropriate for certain chemicals with binding not correlated with organic carbon content, such as those that are ionic at environmental pH values. In these cases, the model user should document the rationale for the selected model input values. Additional guidance may be sought at the EFED WQTT. 
2.5 EXAMS
The following guidance table is for running EXAMS directly rather than through a graphical user interface (GUI). The main difference with using GUIs is that they typically require halflives (in units of days) rather than rate constants (in units of hour^{1}) for metabolism, hydrolysis, and photolysis parameters.
Parameter (units) 
EXAMS Variable  Input Value [Specific Parameter Guidance] 
Data Source  Notes 

Henry's Law Constant (atmm^{3}/ mole) 
HENRY (1) 
Use the measured Henry's Law Constant from submitted product chemistry data. If a measured value is not available, calculate the value from HENRY = (VAPR/760)/(SOL/MWT), 
Product chemistry data  
Bacterial biolysis in water column (cfu/mL)^{1} hour^{1}  KBACW (*,*,1) 
If multiple aerobic aquatic metabolism halflife values are available, enter the rate constant (in units of hour^{1}) corresponding to the 90^{th} percentile confidence bound on the mean halflife value for the total system (water plus sediment) (see Equations 1 and 3 in Appendix A for instructions). If a single aerobic aquatic metabolism halflife value is available, enter the rate constant (in units of hour^{1}) corresponding to 3x the halflife value (see Equations 2 and 3 in Appendix A). If no aerobic aquatic metabolism data are available and the pesticide shows insignificant hydrolysis, use (1/48)x the PRZM aerobic soil metabolism rate constant input value (DWRATE), i.e., use the rate constant corresponding to 2x the halflife corresponding to the PRZM aerobic soil metabolism rate constant input value and convert units from day^{1} to hour^{1}. If no aerobic aquatic metabolism data are available and the pesticide shows significant hydrolysis, assume that the compound is stable to aerobic aquatic metabolism, i.e., enter zero (0). 
Aerobic aquatic metabolism data (Harmonized Test Guideline 835.4300) 
When both aquatic metabolism and hydrolysis rate data are included, the metabolism rate needs to be corrected for the hydrolysis rate at the pH of the aquatic metabolism study. EXAMS calls for secondorder rate constants normalized by bacterial "colony forming units". Colony forming units are fixed at 1 cfu ml^{1}. Note that this parameter is a rate constant (hour^{1}), not a halflife (hour). Also note that EXAMS rate constants are in units of hour^{1}, not day^{1}, as in PRZM. 
Bacterial biolysis in benthic sediment (cfu/mL)^{1} hour^{1}  KBACS (*,*,1) 
If multiple anaerobic aquatic metabolism halflife values are available, enter the rate constant (in units of hour^{1}) corresponding to the 90^{th} percentile confidence bound on the mean halflife value for the total system (water plus sediment) (see Equations 1 and 3 in Appendix A for instructions). If a single anaerobic aquatic metabolism halflife value is available, enter the rate constant (in units of hour^{1}) corresponding to 3x the halflife value (see Equations 2 and 3 in Appendix A). If no anaerobic aquatic metabolism data are available and the pesticide shows insignificant hydrolysis, use (1/48)x the PRZM anaerobic soil metabolism rate constant input value (DWRATE), i.e., use the rate constant corresponding to 2x the halflife corresponding to the PRZM anaerobic soil metabolism rate constant input value and convert units from day^{1} to hour^{1}. If no anaerobic aquatic metabolism data are available and the pesticide shows significant hydrolysis, assume that the compound is stable to anaerobic aquatic metabolism, i.e., enter zero (0). 
Anaerobic aquatic metabolism data (Harmonized Test Guideline 835.4400) 
When both aquatic metabolism and hydrolysis rate data are included, the metabolism rate needs to be corrected for the hydrolysis rate at the pH of the aquatic metabolism study. The model calls for secondorder rate constants normalized by bacterial "colony forming units". Colony forming units are fixed at 1 cfu ml^{1}. Note that this parameter is a rate constant (hour^{1}), not a halflife (hour). Also note that EXAMS rate constants are in units of hour^{1}, not day^{1}, as in PRZM. 
Direct photolysis (hour^{1})  KDP (*,1)  Enter the minimum darkcontrol corrected environmental aqueous phototransformation rate constant (corresponding to the maximum halflife value). If no aqueous photolysis data are available or if there is no evidence of photolysis, enter zero (0).  Aqueous photolysis data (Harmonized Test Guideline 835.2240)  Note that this parameter is a rate constant (hour^{1}), not a halflife (hour). Also note that EXAMS rate constants are in units of hour^{1}, not day^{1}, as in PRZM. 
Simulated latitude of photolysis test (degrees)  RFLAT  Use the latitude referenced in the aqueous photolysis study.  Aqueous photolysis data (Harmonized Test Guideline 835.2240)  
Neutral hydrolysis (hour^{1})  KNH (1,*,1) 
Use the minimum hydrolysis rate constant at pH 7 (corresponding to the maximum halflife value). If no hydrolysis data are available, assume that the compound is stable, i.e., enter zero (0). 
Hydrolysis data (Harmonized Test Guideline 835.2120) 
Technically, this value represents the neutral hydrolysis rate coefficient. However, if KAH and KBH are set to zero, then the KNH variable represents the overall pH 7 hydrolysis rate. The standard water bodies are set to pH 7. Note that this parameter is a rate constant (hour^{1}), not a halflife (hour). Also note that EXAMS rate constants are in units of hour^{1}, not day^{1}, as in PRZM. 
Partition coefficient for sediment keyed to organic carbon (mL/g_{OC})  KOC (1)  If binding is correlated with organic carbon content, enter the mean K_{OC} and do not enter a K_{d} value for KPS.  Adsorption/ desorption data (Harmonized Test Guideline 835.1230) 
Binding is correlated with organic carbon content if the coefficient of variation (i.e., the standard deviation divided by the mean) for K_{OC} values is less than that for K_{d} values. Note that EXAMS calculates a partition coefficient in sediment using the KOC input if the KPS input is not used. The KOW input may be used if both KOC and KPS inputs are not used. 
Partition coefficient for sediment (mL/g)  KPS (*,*)  If binding is not correlated with organic carbon content, enter the mean K_{d}.  Adsorption/ desorption data (Harmonized Test Guideline 835.1230)  Binding is correlated with organic carbon content if the coefficient of variation (i.e., the standard deviation divided by the mean) for K_{OC} values is less than that for K_{d} values. 
Molecular weight (g/mole)  MWT (1)  Enter the value for the modeled pesticide.  Calculated  
Aqueous solubility (mg/L)  SOL (*,*)  Enter the maximum value from product chemistry data for the temperature closest to that of the modeled water body.  Product chemistry data  
Vapor pressure (Torr)  VAPR (1)  Enter the maximum value from product chemistry data for the temperature of the modeled water body.  Product chemistry data  
Sediment bacteria temperature coefficient (decimal)  QTBAS (*,*,1)  Enter 2.  Standard value  
Water bacteria temperature coefficient (decimal)  QTBAW (*,*,1)  Enter 2.  Standard value 
Appendix A
Calculating the 90^{th} percentile confidence bound on the mean halflife value

If more than one halflife value is available, use Equation 1 to calculate the 90^{th} percentile confidence bound on the mean halflife value:
Equation 1
t_{input} = ‾t‾_{1/2} + [(t_{90,n1}s) / n^{1/2}]
where,
t_{input} = halflife input value (time)
‾t‾_{1/2}= mean of sample halflives (time)
s = sample standard deviation (time)
n = number of halflives available ()
t_{90,n1} = onesided Student's t value at α = 0.1 (i.e., 1.00.9) ()
This equation does not calculate the 90^{th} percentile of the distribution of halflife values.
Some Student's t values include n1 1 2 3 4 5 6 7 8 9 10 11 12 ∞ t_{90} 3.078 1.886 1.638 1.533 1.476 1.440 1.415 1.397 1.383 1.372 1.363 1.356 1.282 
If only one halflife value is available, use Equation 2 instead of Equation 1 to account for uncertainty in the environmental variability:
Equation 2
t_{input} = 3 x t_{1/2}

If a firstorder rate constant is needed, use Equation 3 to convert the halflife input value from Equation 1 or 2 to a rate constant input value:
Equation 3
k_{input} = ln(2) / t_{input}
where,
k_{input} = rate constant input value (time ^{1})
Appendix B
Update History for the Input Parameter Guidance
October 22, 2009 (Version 2.1): The following updates were implemented.

Updated parameter descriptions to better resemble how they appear in the models and reformatted tables to place all parameter descriptions in the left column and order them as they occur in the models.

Moved to an appendix guidance on calculating the upper 90^{th} percentile of the mean, which is referenced by the guidance for many parameters.

Harmonized across all surface water models the guidance for soilwater partition coefficients, i.e., to use mean K_{OC} when binding is correlated to organic carbon content and to use mean K_{d} otherwise.

Added suggested incorporation depths for different application methods.

Addressed output filerelated parameters not addressed in the previous version (i.e., run number, file name, chemical name, crop name).

Added other clarifications and made editorial corrections.
GENEEC

Added guidance for followup parameters (e.g., nozzle height, spray quality, etc.) after that for the "method of application" parameter.
PRZM

Added the spray drift fraction (0.03) for ecological exposure assessment of airblast spray.

Clarified that this guidance is for using PRZM directly rather than through a graphical user interface (GUI) to estimate exposure in runoff.
EXAMS

Clarified that when multiple rate constants are available from hydrolysis or aqueous photolysis studies, the minimum value should be used.

Clarified that the photolysis input value should reflect the environmental dark controlcorrected value, not simply the laboratory dark controlcorrected value.

Removed the instruction to multiply aqueous solubility values by 10.

Added instruction that if K_{OC} is used, the K_{d} input should be left blank.

Clarified that this guidance is for using EXAMS directly rather than through a GUI.