User Manual for EXPRESS, the 

“EXAMS-PRZM Exposure 

Simulation Shell”

R E S E A R C H A N D D E V E L O P M E N T
EPA/600/R-06/095 

September 2006 

User Manual for EXPRESS, the 

“EXAMS-PRZM Exposure Simulation Shell” 

Version 1.01.02, September 30, 2006 

by 

Lawrence A. Burns


National Exposure Research Laboratory


Ecosystems Research Division


Athens, GA


U.S. Environmental Protection Agency


Office of research and development


Washington D.C. 20460

NOTICE 

The information in this document has been funded by the Unites States Environmental 

Protection Agency. It has been subject to the Agency’s peer and administrative review, and it has 

been approved for publication as an EPA document. Mention of trade names of commercial 

products does not constitute endorsement or recommendation for use. 

ABSTRACT 

The Environmental Fate and Effects Division (EFED) of EPA’s Office of Pesticide Programs 

(OPP) uses a suite of ORD simulation models for the exposure analysis portion of regulatory risk 

assessments. These models (PRZM, EXAMS, AgDisp) are complex, process-based simulation 

codes that combine chemical and physical data required by EPA of industry with agronomic, 

physiographic, and limnological descriptions of the environmental settings into which pesticides are 

released. This complexity, while providing significant sophistication and specificity in analysis, has 

posed difficulties in maintaining a transparent, reproducible process clearly devoid of arbitrariness 

in parameter selections, exposure metrics, and interpretation of ecological and human health risks. 

This project was undertaken to develop a Graphical User Interface (GUI) to combine 

regulatory simulation models, their supporting databases, regulatory “scenarios” for major crops, 

and data input screens for chemical properties into a user-friendly “simulation shell” lending itself 

to consistent analytical practices and reproducible exposure and risk assessments. The resulting 

computer software – Express (the EXAMS - PRZM Exposure Simulation Shell) – is a joint project 

of EFED and ORD. EFED designed and funded an initial draft of the Express software, and retains 

responsibility for developing and maintaining crop-specific modeling scenarios for regulatory 

analyses. ORD is responsible for continuing development, quality control, maintenance, and public 

availability of the software and its constituent models and analytical modules. 

With the completion of Express, exposure metrics (peak, 4-day, 21-day, 60-day, annual) 

for ecological and human health risk assessments can be easily and conveniently and, as 

importantly, reproducibly, developed for any or all of the agronomic scenarios developed by EFED. 

Probabilistic analyses (Weibull distribution) of these exposure metrics are derived by coupling 30­

year time-series of meteorological data to linked PRZM/EXAMS simulation studies. Express 

significantly reduces the variability, lack of reproducibility by external analysts, and perceived 

arbitrariness of EPA’s regulatory processes for determining the safety and appropriate uses of 

registered pesticides. 
Table of Contents 

Page 

1.0 Introduction to EXPRESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
1

2.0 Acquiring and Installing EXPRESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
4

3.0 Starting EXPRESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
5

3.1 Project Directories and the Master Project File. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7

3.2 Format of the Master Project File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
9

3.3 Project Manager. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
11

4.0 Creating Input Files for PRZM-EXAMS Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12

4.1 Project Design and Tier II Scenario Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13

4.2 PRZM Environmental Fate Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15

4.3 EXAMS Environmental Fate Parameters.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16

4.4 PRZM Application Data for Pesticide Use Patterns. . . . . . . . . . . . . . . . . . . . . . . . .
20

4.5 PRZM-EXAMS Input File Creation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
21

5.0 Starting and Running the PRZM-EXAMS Simulation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22

6.0 Evaluation of the PRZM-EXAMS Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22

6.1 PRZM-EXAMS Grapher.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
23

7.0 PRZM Versions.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
24

8.0 Database and Scenario Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
25

9.0 Additional Capabilities Included in EXPRESS (Index Reservoir, PCA). . . . . . . . . . . . . . . .
26

9.1 New Scenarios for Tiers I and II. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
27

9.2 Index Drinking Water Reservoir.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
27

9.3 Percent Crop Area (PCA) Adjustments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28

10.0 Using EXPRESS: Data Preparation and Input Example. . . . . . . . . . . . . . . . . . . . . . . . . .
29

Surface Water Exposure Inputs.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
29

EXPRESS Example Input Screens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
31

11.0 Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
35

APPENDIX A: Format of Daily Value Meteorological Time-Series Files. . . . . . . . . . . .
35

APPENDIX B: Added Capabilities of ORD Mode.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36

APPENDIX C: References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
37
Transport Pathways of Pesticides

1.0 Introduction to EXPRESS 

The Office of Pesticide Programs (OPP) Environmental Fate and Effects Division (EFED) 

has developed a tiered approach to determine the appropriate level of modeling needed to perform 

a risk assessment for a pesticide. This tiered approach is designed to minimize the amount of 

analysis required to evaluate any given chemical. Each of the tiers is designed to screen out 

pesticides by requiring higher, more complex levels of investigation only for those compounds that 

have not passed the previous tier. Each tier screens out a percentage of pesticides from having 

to undergo a more rigorous review prior to registration or reregistration. “Passing” a given 

assessment tier indicates that there is a low possibility of risk to the aquatic environment. “Failing” 

an assessment tier does not, however, necessarily mean the chemical is likely to cause 

environmental problems, merely that the assessment should continue on to the next higher 

assessment tier. The intent of this tiered modeling system is to produce only as thorough an 

analysis as is necessary for each pesticide, and thereby focus greatest resources and efforts 

toward areas of greatest potential ecological threat. As a matter of policy, OPP does not take 

significant regulatory action based solely upon the results of screening models. 

“Tier I” models provide simplified, highly conservative initial screening risk assessments of 

pesticides. For Tier I analyses, the Environmental Fate and Effects Division (EFED) of the U.S. 

Environmental Protection Agency's Office of Pesticides Programs (OPP) uses, inter alia, two 

“meta-models,” GENEEC2 (Parker et al. 1995) and FIRST, developed by summarization of 

standardized linked simulations using the EPA PRZM and EXAMS models. Dynamically (file 

transfer-) linked Pesticide Root Zone Model (PRZM) and Exposure Analysis Modeling System 

(EXAMS) simulations are used for a “refined” (Tier II) estimation of pesticide concentrations in 

surface waters used as drinking water sources and for aquatic ecosystem exposure assessments. 

PRZM is a one-dimensional, dynamic, compartmental model used to simulate chemical movement 

in unsaturated soil systems within and immediately below the plant root zone (Carsel et al. 1984, 

Carsel et al. 1985, Suárez 2005). EXAMS contains a set of process modules that link fundamental 

chemical properties to the limnological parameters that control the kinetics of fate and transport of 

chemicals in aquatic ecosystems (Burns et al. 1982, Burns and Cline 1985, Burns 2000). Multipleyear pesticide concentrations in the water column and benthic sediments, generated by the PRZMEXAMS simulations, are analyzed to produce, for each year, the single-day peak concentration, 

the maximum 24-hour, 96-hour, 21-day, 60-day, and 90-day mean concentrations, and the mean 

annual concentration. From simulation results encompassing the period of record for the available 

meteorological data, upper 10th percentile concentrations are made available in tabulations and 

1 
graphics for comparison with ecotoxicological and human health levels of concern (LOC). 

Documentation and access for these models can be found at: 

! GENEEC2: http://www.epa.gov/oppEFED1/models/water/index.htm 

! FIRST: http://www.epa.gov/oppEFED1/models/water/index.htm 

! PRZM: http://www.epa.gov/ceampubl/gwater/PRZM3/index.htm 

! EXAMS: http://www.epa.gov/ceampubl/swater/EXAMS/index.htm 

The standard PRZM-EXAMS runoff modeling scenario captures the properties and 

dimensions of an agricultural field draining into water bodies for subsequent drinking water safety 

and aquatic exposure assessments. PRZM links two subordinate models (PRZM per se and 

VADOFT), that combine differing root zone and vadose zone characteristics into a single simulation 

to predict pesticide transport and transformation in the crop root and unsaturated zones, and edgeof-field losses in runoff waters and eroded sediment. Each PRZM modeling scenario represents 

a unique combination of climatic conditions, crop-specific management practices, specific soil 

properties, site-specific hydrology, and pesticide-specific application and dissipation (fate and 

transport) processes. Each PRZM simulation is conducted for multiple years (usually 30, spanning 

the years 1961-1990) using historical weather data (Burns 2001) to provide a probabilistic exposure 

characterization for a single site. Daily edge-of-field loadings of pesticides dissolved in runoff 

waters and sorbed to entrained sediment, as predicted by PRZM, are discharged into a standard 

water body simulated by the EXAMS model. The EXAMS model accounts for subsequent 

hydrologic transport, volatilization, sorption, hydrolysis, biodegradation, and photolysis of the 

pesticide. 

EXPRESS (the EXAMS - PRZM Exposure Simulation Shell) has been created to facilitate 

a rapid and consistent Tier II assessment of aquatic pesticide exposure for multiple geographic and 

agronomic settings. EXPRESS was a joint project of the U.S. Environmental Protection Agency's 

Office of Pesticide Programs (OPP) and its Office of Research and Development (ORD). OPP 

contracted with Waterborne Environmental, Inc., for initial development of the shell code, and 

retains responsibility for development and maintenance of the crop-specific modeling scenarios. 

ORD developed and maintains the constituent Tier II models (PRZM and EXAMS) and the 

meteorological databases. Although EXPRESS provides access and output file management for 

GENEEC2 and FIRST, EXPRESS does not provide a Graphical User Interface (GUI) to facilitate 

use of these models for exposure assessment. Users wishing to run these Tier I models may wish 

to download their respective user manuals from the http sites cited above; the EXAMS and PRZM 

user manuals are provided with EXPRESS. 

2 
EXPRESS, developed using the Winteracter GUI development system (version 7.1a) from ISS Ltd., 

provides the following features: 

•	 User-friendly scenarios and parameters in a Microsoft Windows environment. 

•	 Simulation of an unlimited number of crop “scenarios” in a single run in less than 15 

seconds per crop. 

•	 Automatically runs both the ecological farm pond and the drinking water index reservoir 

scenarios. 

•	 Provides options for simulating selected groupings of scenarios for special purposes (e.g., 

cumulative assessments). 

•	 Provides flow/no-flow and steady/unsteady flow options for the farm pond and index 

reservoir, respectively. 

•	 Simulates parent pesticide and degradates. 

•	 Provides options for mixing types of applications, spray drift calculations, incorporation 

depth, etc., in the same simulation. 

•	 Runs alternative versions of PRZM with the same initial setup to permit comparisons. 

•	 Provides graphical and tabular ranked summary results for all scenarios together. 

•	 Provides graphical and tabular output for each individual scenario. 

•	 Provides graphical and tabular output for water column and benthic sediment zones. 

•	 Provides a graphical and tabular PRZM water balance summary for each scenario run. 

•	 Each graph and table can easily be printed or exported to reporting software. 

•	 Time-series files for transmission to OPP’s Health Effects Division (HED) for analysis of 

drinking water source contamination are automatically generated and inventoried. These 

files are in comma-separated-value (csv) format suitable for direct import into, for example, 

Excel for further analysis and plotting. 

•	 New scenarios can be added or existing scenarios quickly and easily changed through a 

password-protected facility. 

•	 Saves all run information in a single, email-able file from which the entire, multi-crop 

simulation can be recreated. This master file can be shared among multiple modelers 

working on the same problem set, or archived so that runs can be reproduced later or with 

future versions of PRZM and/or EXAMS. 

•	 EXPRESS also includes the Tier I screening models FIRST and GENEEC2 (Section 9), but 

does not facilitate their use beyond providing project results file management and access. 

3 
2.0 Acquiring and Installing EXPRESS 

The EXPRESS user interface software can be obtained (gratis) from the EXPRESS main 

page at EPA's Center for Exposure Assessment Modeling (CEAM): 

http://www.epa.gov/ceampubl/swater/EXPRESS/. 

EXPRESS uses standardized files of weather data that must be downloaded separately and


installed in the “...\EXPRESS\stdmet\” directory. Thirty-year (1961-1990) data files of U.S.


meteorological data are available via the “Meteorological Data” link at the EXPRESS main page,


or go directly to


http://www.epa.gov/ceampubl/tools/metdata/index.htm, and download file DVF_ALL.EXE. The


format of these data files is provided in Appendix A.


EXPRESS runs on PCs under the Windows operating system (installation has been tested 

on Windows 98, NT, 2000, and XP). To install, click on the downloaded files 

(Install_EXPRESS.EXE and DVF_ALL.EXE) and follow the on-screen directions. All required files 

will be installed automatically along the user-selected installation path ...\EXPRESS\. Download file 

“Install_EXPRESS.EXE” is 40 MB; “DVF_ALL.EXE” is 65 MB. EXPRESS requires a screen area 

setting of 1024x768 or higher, and will issue an error message if it is invoked from a lower 

resolution display. The free Adobe Reader is required to access model User Guides and location 

maps. EXPRESS is removed using the standard Windows uninstall procedure, i.e., via “Settings 

– Control Panel – Add/Remove Programs”, followed by deletion of the directory structure along with 

any remaining user-modified project or meteorological data files. 

To manually move the installation to a different directory or disk drive follow these steps: 

1) Use Windows Explorer to either copy or move EXPRESS in its entirety to the 

desired drive. The directory structure must remain exactly as installed, using 

“EXPRESS” as its base. 

2) Create a new (or modify the existing) Windows shortcut to point to the new drive (a 

modifiable shortcut file is supplied in the ...\EXPRESS\EXPRESS\ folder). 

The latest PRZM release notes are available at 

http://www.epa.gov/ceampubl/gwater/PRZM3/prz3reln.htm; the latest EXAMS release notes are 

available at http://www.epa.gov/ceampubl/swater/EXAMS/examreln.htm. 

4 
3.0 Starting EXPRESS 

Following installation, EXPRESS is started by double-clicking on EXPRESS.EXE or by 

using any shortcut to the EXPRESS executable. The starting screen (Figure 1) appears, depicting 

a stream flowing through an agricultural landscape, providing access to EXPRESS' main 

functionalities. 

Figure 2. EXPRESS opening display screen 

The functions available from EXPRESS' initial screen are: 

Navigate to a project folder, or start a new project: 

Recall, annotate, or delete existing projects: 

Select/Create Project Directory 

Project Management 

Check software and database version numbers and dates; access EXPRESS , EXAMS, and PRZM 

User Guides: Help 

5 
Operate EXPRESS in regulatory mode for submission of studies to EPA’s Office of Pesticide 

Programs: OPP 

Operate EXPRESS in ORD (Office of Research and Development) mode with additional flexibility 

and input data control (see Appendix B for details): ORD 

- access the FIRST Tier I model 

- access the GENEEC2 Tier I model 

- prepare chemical and agronomic input data

 - start PRZM/EXAMS simulations

 - analyze and display PRZM/EXAMS results 

6 
3.1 Project Directories and the Master Project File 

The first step in using EXPRESS is to select (or create) an active project directory. All 

relevant input/output data of a simulation run is saved in this directory; EXPRESS requires full 

read/write permission for the specified directory. New directories can be created on hard disks or 

network drives using EXPRESS itself, Windows Explorer, or other tools. Long filenames are 

supported. The default directory upon initiation of EXPRESS is undefined or “unset.” When the 

Select/Create Project Directory 

button is pushed, the input screen in Figure 2 is displayed. 

The initial project directory display is always the default project folder 

“...\express\PROJECTS.” To select an existing directory, use the scroll bar to move to the desired 

drive\directory (if it is not already visible in the selection window), and then double-click on it. The 

selected folder (The terms “drive\directory” and “folder” are equivalent) will then replace the 

“...\express\PROJECTS” text initially displayed (see Figure 2). Pressing the OK button at this time 

will set the selected drive\directory to be the current project directory. To create a new project 

directory, type a “\” followed by the new project directory name on the line in Figure 2 labeled 

“create new directories here.” For example, typing “\newchem1” at the end of the folder selection 

window creates a new project directory called “...\express\projects\newchem1.” Only one 

Figure 2. Create or select the active project directory 

7 
Figure 3. Options if Master Project File 

exists in currently selected directory 

subdirectory can be created in a single step: entering, for example, “\newchem1\app1", is not 

supported. To create dependent subdirectories (i.e., folders within folders), first create \newchem1, 

press the “OK” button, then reselect the Select/Create Project Directory 

button and enter “\app1.” This will create the “...\express\projects\newchem1\app1" projects 

directory. 

This option also serves to switch between previously 

created projects (which can contain multiple crop 

scenarios). (Section 3.3 describes the “project 

manager,” a facility for cataloguing previously created 

scenarios.) Each simulation scenario can be modified, 

re-run, or re-analyzed by choosing a project directory 

in which simulations have previously been executed. 

A master project file (MPF), named “master.fpj,” is 

created within each active project directory. This file 

contains all scenario-specific information necessary to 

characterize the entire simulation project in the active 

project directory. After an active project directory has 

been chosen, EXPRESS checks to see whether an 

MPF is present in this directory. If an MPF is present, 

EXPRESS returns an output window (Figure 3) for 

choosing the action to be taken. The available options 

are: Copy, Edit, Browse, Reselect, EMail, Rerun, 

and View. The function of each of these options is 

further described in Figure 3. If no MPF is found, 

EXPRESS starts the input generator program. 

The Master Project File contains all the 

information needed to identify and replicate an EXPRESS analysis. The MPF enables convenient 

exchange of EXPRESS simulation scenarios among EXPRESS users. Once an MPF received from 

another user has been copied into a valid folder, it is available to the receiving analysts as an active 

project directory. This (MPF) feature makes it possible to easily re-create each crop-use scenario, 

pesticide chemistry, and the application-specific model inputs used to generate a particular 

EXPRESS analysis. Because every master project file is named “MASTER.FPJ,” only one MPF 

can be stored in a project folder. 

8 
3.2 Format of the Master Project File 

The definitions of the scenario, pesticide chemistry, and application-specific parameters are 

compiled according to formats used in the PRZM input parameter file (*.inp) (Suárez 2005). A 

typical example of a Master Project File is given in Figure 4. This sample Master Project File was 

created for a parent compound, a single scenario, and a single application. 

The information included in the Master Project File is complete, but omits EXPRESS default 

settings to save space. The following information is coded in the individual lines of the Master 

Project File: 

Line 1: Date on which the file was created by the EXPRESS. 

Line 2: Version and date of EXPRESS 

Line 3: Name of the parent compound 

Line 4: Identification of Group File, if any 

Line 5: Scenario type (3 = Standard Tier II) 

Line 6-9: Index of selected scenarios (1 = used; 0 = not used for this project) 

Line 10: Chemical input data controls 

Line 11: EXPRESS operational mode (OPP or ORD) 

Line 12-24: Chemical properties (parent and metabolites) 

Line 25: Number of specific application sequences 

Line 26: Application input data controls 

Line 27-34: Application parameters for each crop 

Line 35: EXAMS chemistry and flow process controls 

Line 36-47: EXAMS process parameters 

This file must not be modified by hand in any way. This description of the MPF is provided 

solely for informational purposes. Attempts to modify this file will in all probability result 

in loss of data and EXPRESS execution failure. 

The MPF is not designed to provide a complete input summary of model parameters with 

complete descriptions of all input values and their units. The MPF does, however, provide complete 

documentation of EXPRESS input values used for a specific analysis. 

9 
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Master Project File Created: 2006-08-28 at 13:02:43


EXPRESS v. 1.01.01 (2006-08-12) 


Parent Compound: Chemical A


Scenario Group File: 0 No GroupFile


Scenario Type: 3


 PRZM Crop Selector: 00000100000000000000000000000000000000000000000000


 PRZM Crop Selector: 00000000000000000000000000000000000000000000000000


 PRZM Crop Selector: 00000000000000000000000000000000000000000000000000


 PRZM Crop Selector: 00000000000000000000000000000000000000000000000000


Chemical Input Data Controls: 1 1 0 1 0


 EXPRESS Operational Mode: OPP


Chemical Name(s): Chemical A 

Molecular Weight: 

Plant Uptake Factor: 

 Partition Coefficent Method: 

 300.0 

 0.000 

| 

1

 Partition Coefficient Value: 30.00 

 Vapor Pressure: 

 Solubility : 

 Soil Degradation Half-Life: 

 % Degradate formed: 

Foliar Half-Life: 

 0.1000E-05

 100.0

 30.00 

 0.000 

 0.000 

 0.000 0.000 

 Foliar Washoff Coefficient: 0.5000 

Air Diffusion Coefficient: 4300. 

 Enthalpy of Vaporization: 

 Application DataSets Number: 

Application Parameters: 

 Days Relative/Absolute : 

Month : 

 20.00 

1 

1 1 2 2

- 1 

0 

1 0 0 87.00 0

CAM : 2 

Depi (cm)

Rate 

: 

: 

 4.000 

 0.5000 

 Drift Farm Pond % : 5.000 

 Drift Index Reservoir % : 16.00 

Efficiency % : 

 Product chem./hydr. status: 

 Aerobic Metabolism (days) : 

 Q10 Base Temperature : 

 Q10 Limnetic Bacteria: 

 95.00 

 1 1 1

 20.00 

 25.00 

 2.000 

 Anaerobic Metabolism (days): 

 Q10 Base temperature: 

 Q10 Benthic Bacteria: 

 100.0 

 25.00 

 2.000 

 Photolysis Half-Life (days): 10.00 

Number of Hydrolysis Obs.: 3

 Hydrolysis Temperatures: 25.00 

 Hydrolysis C1: 3.000E+01 3.000E+01 3.000E+01

 pH Hydrol. C1: 5.00 7.00 9.00 

Melting Point: -99.00 

10 
3.3 Project Manager 

The “Project Manager” tracks and maintains EXPRESS simulation scenarios. Information 

for each project created is stored in a database that includes the location of the project folder, the 

date and time the project was created, the name of the primary chemical, and optional usersupplied commentary/annotation (Figure 5). A record is added (or updated) in the Project Manager 

database each time the Write option is executed in the input generator. 

Figure 5. EXPRESS Project Manager 

11 
4.0 Creating Input Files for PRZM-EXAMS Simulation 

The input generator (Figure 6) creates an exposure model input data set in five consecutive 

steps. Start the input generator by pressing the inputs button on the main EXPRESS screen 

(Figure 1). (Recall that the project directory must be selected prior to starting the input generator.) 

The input data tabs (Figure 6) have the following functions. 

Figure 6. EXPRESS Input Generator 

option: description: 

Design Project: Select one or more of the predefined (EFED Standard Tier II) scenarios 

PRZM-Efate: Define the physical-chemical and environmental fate (“Efate”) pesticide 

properties needed for PRZM. 

EXAMS-Efate: Define physical-chemical and “Efate” properties needed for EXAMS. 

Application: Choose application date(s), method, rate(s) 

12 
Write:	 Write PRZM-EXAMS input files, create (or update) the Master Project File, 

and add or update a record in the scenario manager data base 

All five of these data creation steps should initially be completed in the suggested order (Design, 

PRZM-Efate, EXAMS-Efate, Application, Write) to guarantee proper creation of the input file set. 

Users of EXPRESS should be generally familiar with the use of simulation models for 

environmental risk assessments. Further information for EXAMS, including detailed model 

documentation, can be found at http://www.epa.gov/ceampubl/swater/EXAMS/index.htm, and, for 

PRZM, 
 http://www.epa.gov/ceampubl/gwater/PRZM3/index.htm . For 

simulations, process implementations and parameter definitions should follow EFED 

guidelines and reflect the intended uses of the compound. (These guidelines are available 

at authorized Tier II 

on-line at http://www.epa.gov/oppEFED1/models/water/input_guidance2_28_02.htm )

4.1 Project Design and Tier II Scenario Selection 

To begin scenario definition, click on the Project Design menu item (Figure 7). The “Project 

Design” screen (Figure 7) then presents a pick-list of predefined scenarios. The scenarios are 

defined by State, crop, soil properties, 

meteorology, and Major Land Resource Area 

(MLRA) (SCS 1981). There are approximately 

85 EFED standard scenarios currently 

included within EXPRESS. As additional 

scenarios are created and certified by EFED, 

they can be added to the set. This database 

can only be altered by an authorized database 

manager (see Chapter 8). 

One, many, or all of the currently 

available scenarios can be selected for 

inclusion in a project. In addition to scenario 

selection, the number and transformation 

product chemistry of chemicals to be 

simulated is chosen on this screen. Both 

EXAMS, and PRZM from 3.12.2 onwards, can 

13 

Figure 7. Selecting scenarios 
model the creation and subsequent fate and transport of chemical metabolites and transformation 

products of the pesticides or toxic chemicals under evaluation. EXPRESS allows for a choice 

among four possible transformation product chemistries: 

1) One Parent, No Metabolites 

2) One Parent –> One Metabolite 

3) One Parent –> Two Metabolites 

4) One Parent –> First Metabolite –> Second Metabolite 

The PRZM and EXAMS User Manuals include detailed discussions of metabolite simulation. 

The EXPRESS input generator does not permit simulation of metabolites in PRZM 3.12 beta (see 

Chapter 7 for a discussion of PRZM model versions). Because the current (3.12.2) version of 

PRZM is restricted to three chemical entities, this limitation is also enforced in EXPRESS and 

EXAMS. 

A final option on this screen, 

the “Scenario Group Functions,” allows 

for the creation or selection of scenario 

groupings. Two examples of scenario 

groupings are illustrated in Figure 8, 

“OP” (organophosphate pesticide) 

scenarios and “Standard EFED” 

scenarios. The “Standard EFED” 

scenarios (Group 1) are selected by 

default. To create a new scenario 

group, first select the desired 

scenarios (from the screen shown in 

Figure 7), and then press (mouseclick) the “Scenario Group 

Functions” button. The scenario 

group manager then opens (Figure 

8). Next, push (click) the “Save the 

currently selected scenarios to a Group File” button. The “Save Group File” panel (Figure 9) then 

opens, with space for a file name (required) and for descriptive text and annotation comments for 

the new group file (strongly recommended). 

Figure 8. Scenario Group Manager 

Figure 9. Defining a scenario group 

14 
4.2 PRZM Environmental Fate Parameters 

Figure 10. PRZM Chemical Parameters 

The PRZM Chemical Parameters screens include data entry fields for the following 

chemical properties: molecular weight, solubility, Koc/Kd, vapor pressure, degradation half-life 

(applied equally to dissolved and sorbed phases), plant uptake factor, foliar half-life, foliar washoff 

coefficient, air diffusion coefficient, and enthalpy of vaporization (Figure 10). When simulating 

transformation products or metabolites, enter a parameter set for each chemical. Vapor pressure 

units are specified at the Chemical 1 input pane; vapor pressures for other chemicals must be 

entered in the units so selected. Enter the percent transformed from the parent to the 

transformation product or metabolite as 100 × (mole of product)/(mole of parent); EXPRESS 

15 
applies the ratio of compound molecular weights prior to entry in the PRZM input files. Chemical 

dissolved in the soil solution and that sorbed with the soil particulates are parameterized with the 

same product yield in EXPRESS Tier II simulations. The product yield parameter can be entered 

as a zero (0) for those cases in which transformation processes are only relevant for the aquatic 

ecosystems (aquatic product yields are entered on the EXAMS data entry screen). 

All compound-specific parameters (parent and metabolites) can be saved in a chemical 

database provided with EXPRESS. The same chemistry can then be used for additional 

simulations with alternative soil, crop, and application scenarios. Database maintenance functions 

include a facility for deleting a single entry (Chapter 8). 

4.3 EXAMS Environmental Fate Parameters 

The EXAMS Efate Input screen provides data entry fields for transformation process halflives; these are converted to chemical dynamics parameters (rate constants) by EXAMS (Figure 

11). 

Solubility, Koc, and vapor pressure are transferred to EXAMS from the PRZM Chemical 

Parameters data entries (Figure 10). As with PRZM, when simulating transformation products and 

metabolites the product half-lives must be entered on a separate tab for each secondary chemical. 

(Although EXAMS can accommodate an arbitrary number of chemicals in a single simulation, the 

current version of PRZM, and perforce EXPRESS, is limited to three chemicals.) A 0.0 (zero) entry 

for a half-life is used to indicate that the transformation process is inactive (i.e., not, as a literal 

interpretation might suggest, that it is immeasurably rapid). Particular care is required with the 

hydrolysis inputs: at least three non-zero half-lives at three different pH values must be entered 

(conforming with EPA guidelines), or hydrolysis will not be simulated. (EXPRESS will accept up to 

10 pH-half-life observations.) In Figure 11, were the example chemical reacting by a neutral 

hydrolysis pathway only, the same half-life (e.g., 10 days) would have to be entered for pH 5, 7, 

and 9: EXPRESS requires entry of all three values for correct calculation of rate constants. 

Aerobic and anaerobic metabolism half-lives should be corrected for the contribution due to abiotic 

hydrolysis prior to entry on these data screens. 

In addition to product chemistries, the pathways of formation must also be defined. 

Transformation product information is communicated to EXAMS by setting an identifying number 

for each parent and daughter chemical, for each process generating a product, for the reactive 

form of the parent molecule, and for the process yield. The EXPRESS input parameters are: 

16 
CHPAR, the chemical parent number; TPROD, the transformation product number; NPROC, the 

identifying number of the transformation process generating the TPROD; RFORM, to identify the 

reactive form of the parent molecule, and YIELD, the (Mole/Mole) transformation process yield from 

this pathway. 

Figure 11. EXAMS Efate Input Screen 

The generating transformation process is specified using the following numerical codes for 

NPROC: 

1 = specific acid hydrolysis 

2 = neutral hydrolysis 

3 = specific base hydrolysis 

4 = direct photolysis 

5 = singlet oxygen 

17 
6 = free radical oxidation (e.g., hydroxyl radical)


7 = water column bacterial biolysis


8 = benthic sediment bacterial biolysis


9 = reduction


Twenty-eight reactive form numerical codes are available for specifying RFORM, in addition to 

which total dissolved etc. can be specified using an RFORM code of 

29 = all dissolved species


30 = all sediment-sorbed species


31 = all doc-complexed species


32 = all biosorbed species


(The complete list of RFORM specifications is given in Table 1.) In EXPRESS, the process yield 

must be entered as the simple mole of product per mole reacted (i.e., EXAMS temperaturedependent product yields are not currently supported). 

Table 1. Product Chemistry “Reactive Forms” for EXAMS 

Ionic species Neutral Cations Anions Total 

Valence 0 1+ 2+ 3+ 1­ 2­ 3­ (all) 

Forms: Dissolved 1 5 9 13 17 21 25 29 

Solids-sorbed 2 6 10 14 18 22 26 30 

doc-complexed 3 7 11 15 19 23 27 31 

Biosorbed 4 8 12 16 20 24 28 32 

For example, if the CHPAR parent compound (e.g., parathion) is chemical 1, and the 

TPROD product (e.g., paraoxon) is chemical 2, then a reaction scheme in which direct photolysis 

of DOC-complexed parathion yields a 50% generation of paraoxon by pathway 1, and hydroxyl 

radical oxidation of dissolved parathion produces paraoxon in mole-for-mole formation by pathway 

2, would be specified in EXPRESS as illustrated in Figure 12. 

18 
Figure 12. Example of transformation product data entry for EXAMS 

Section 2.4.1 (“Product Chemistry”) of the EXAMS manual (Burns 2000) (also available 

through the primary EXPRESS screen’s “HELP” button) should be consulted for further explanation 

on entering pathway data. 

In addition to product chemistry parameters, the user can also select whether a static pond 

and/or a steady reservoir flow rate scenario will be simulated. The current default is set for both of 

these options to be in effect. The static pond does not change volume, nor does it allow for 

hydrologic export of chemical. Turning the static pond option off (un-checking the box) lets the 

pond detention time vary (monthly) based on an EXAMS analysis of evaporation and rainfall from 

the Crop Scenario meteorological file and daily PRZM runoff volumes. The steady flow reservoir 

scenario uses a period-of-record average daily runoff from the PRZM scenario to calculate the flow. 

Turning the steady flow reservoir option off will, as with the farm pond scenario, allow EXAMS to 

calculate a monthly detention time for the reservoir based on meteorology and daily PRZM runoff. 

19 
4.4 PRZM Application Data for Pesticide Use Patterns 

The Pesticide Application Data screen contains data entry fields for the number of 

applications, application timing, application method, application rate, depth of incorporation, and 

chemical application method (Figure 13). The same values can be applied to all selected scenarios, 

Figure 13. Pesticide Application Data Entry Screen 

or, as here, specific values can be entered for each of the selected scenarios. To minimize the 

amount of data entry required, first enter any values that are common among crops, and only then 

select the option to enter specific values for each crop. At that juncture, the first-entered values will 

be copied to all additional scenarios, reducing the labor required to enter identical values across 

the selected scenario set. 

For Tier II, the aerial application default drift values have been set at 5% and 16% for the 

pond and reservoir, respectively. For aerial applications, the default efficiency is 95%. The ground 

sprayer application default drift values are 1% and 6.4% for pond and reservoir simulations, 

respectively. The default efficiency value for ground applications is 99%. 

20 
Pesticide application dates can either be set absolutely (by calendar date), or relative to 

crop planting, emergence, maturity, or harvest dates. This feature facilitates rapid configuration of 

application scenarios for multiple crops. For example, when the pesticide is intended for application 

7 days pre-planting, the user can enter -7 days relative to planting and the correct application date 

for each crop will be calculated from the crop phenology embedded in the crop scenario file. 

4.5 PRZM-EXAMS Input File Creation 

Finally, selecting the Write menu option creates all input files needed by PRZM and 

EXAMS. The current release of EXPRESS includes two PRZM versions when run in OPP mode: 

PRZM 3.12 beta, and PRZM 3.12.2. In ORD Mode, the latest version of PRZM (3.12.3) is also 

available. PRZM 3.12.2, the current regulatory model, is the default selection. Earlier versions of 

PRZM contain known bugs and limited features (See PRZM release notes at 

http://www.epa.gov/ceampubl/gwater/PRZM3/prz3reln.htm). Multiple PRZM versions are included 

within EXPRESS to facilitate comparisons of exposure analysis results across successive versions; 

PRZM 3.12.3 adds a mass balance reporting function to the suite of Express output graphics. 

Once the version of PRZM has been selected, the model input files are created. The 

information necessary to run PRZM-EXAMS is contained in several input data files: 

PRZM 

- annotated parameter file incorporating the scenario definition, *.inp 

- climate file providing the weather data used, *.dvf 

- file with definition of the PRZM run options, *.run 

EXAMS 

- parameter file including the scenario selection, *.exa 

- multiple PRZM to EXAMS transfer files, *.dyr (yr represents year of run) 

Each standard scenario is assigned an unique six character name that identifies the USA 

State and crop. An R or P is added to this file name to specify whether the input file is a reservoir 

run or pond run, respectively. All scenario-, chemical-, and application-specific information is also 

stored in the Master Project File master.fpj. In addition, a file of type *.scn is created to support 

data analysis and data visualization in EXPRESS. 

21 
The scenario and parameter definitions combine input data specified by the user in 

EXPRESS with internal EXPRESS-defined data and parameters (as developed by the OPP/EFED 

Water Quality Technical Team). The parameter definitions and values specified by EPA (e.g., in 

PRZM crop scenarios) and stored internally within EXPRESS cannot be altered by EXPRESS 

users. EXPRESS also generates some of its input data from user choices within fixed menus of 

EPA-approved alternatives. All predefined and menu-selected data within model input files 

produced by EXPRESS can be edited manually to create special scenario definitions. Under no 

circumstances, however, should data files created by EXPRESS be modified when producing EPA 

“Standard Tier II" simulations. 

With the input file set written, the data preparation phase is now complete. Select the Exit 

menu option to return to the main EXPRESS program screen. 

5.0 Starting and Running the PRZM-EXAMS Simulation 

With the input files created, simulations are started by pressing the lightning bolt icon 

(Figure 1). Current versions of PRZM run in a DOS console window. Each successive requested 

PRZM-EXAMS simulation scrolls in this window. Once simulation runs have been completed, the 

display screen reverts back to the opening menu (Figure 1), and the graphing options described 

in Section 6 become available. 

6.0 Evaluation of the PRZM-EXAMS Simulation 

After a completed simulation run, the relevant scenario output data is provided in three 

PRZM ASCII files ( *.hyd, *.cnc, and *.out), and three EXAMS ASCII files (*.xms, *.yms, and *.zms). 

These files are analyzed automatically by EXPRESS to generate tables and graphics of the results 

obtained. For Tier II runs, there is no need to analyze the output files in a different format. The 

EXPRESS grapher can also, however, export results for each simulated scenario in an ASCII file 

with an extension of “.tab.” These files are created in the current project directory whenever the 

“Display ... as Text File” option is selected. These files can be used for further data analysis; they 

include all output data required for Tier II calculations. 

EXPRESS generates daily time-series files for each combination of a surface drinking-water 

reservoir scenario and chemical simulated. These files are deposited in the project folder; they are 

comma-separated-value (*.csv) formatted files suitable for direct importation into risk assessment 

models (e.g., dietary exposure models in use by EPA – DEEM/CALENDEX, CARES, and a Lifeline 

22 
Excel template). An inventory of the produced files is stored in the project directory in file 

“HED_Files.txt (the name derives from the nominal target audience for these csv files, OPP's 

“HED” (Health Effects Division). 

PRZM Output 

*.out = summary of PRZM output. Contains all PRZM output 

*.cnc = mean annual soil concentration at depths of 1 meter from the soil surface and at the bottom 

of the soil column 

*.hyd = annual hydrology summary (total precipitation, runoff, erosion, and evapotranspiration) 

*.msb = annual chemical mass balance at depths of 1 meter from the soil surface and at the 

bottom of the soil column (total application, runoff flux, decay flux, erosion flux, mass 

remaining in soil column, etc.) (ORD mode with PRZM 3.12.3 only) 

Note: PRZM 3.12 beta does not create .cnc and .hyd files. 

EXAMS Output 

*.xms = EXAMS standard tabular report file 

*.yms = EXAMS EcoRisk output file containing “instantaneous” and mean 24-hour, 96-hour, 

21-day, 60-day, 90-day, and annual average aquatic concentrations of parent and 

transformation product chemicals 

*.zms = EXAMS EcoTox output files (daily concentrations of parent and product chemicals) 

EXPRESS Output 

*.csv = Comma-separated-value time-series files of dates (as MM/DD/YYY) and associated 

exposure concentrations (mg/L), each chemical/scenario in a separate file 

HED_Files.txt Inventory of .csv files generated by an exposure analysis 

6.1 PRZM-EXAMS Grapher 

Start the PRZM-EXAMS grapher by selecting the graph icon on the main EXPRESS screen 

(Figure 1) and then selecting the Standard Tier2 menu item. All EXPRESS graphing functions are 

available from the next screen display (Figure 14). Graphical or tabular output is generated by 

selecting either the Display graphs or Display tables radio buttons respectively. Selecting the 

Farm Pond or Index Reservoir radio buttons switches between the pond and reservoir results. 

Each created graphic can be printed to any installed printer or exported to a windows metafile by 

pressing the Print or Export buttons located at the bottom of the screen. The Display ... as Text 

23 
File buttons create ASCII flat files (*.tab) within the project folder; these can be imported into other 

software for additional analyses. 

Figure 14. EXPRESS graphic display 

7.0 PRZM Versions 

EXPRESS includes the obsolete PRZM 3.12 beta, and the current (3.12.2) regulatory 

release, and the most recent ORD maintenance release (3.12.3 as of 2006-09-30). This facilitates 

comparisons of results between PRZM versions. In many cases, versions are backwards 

compatible (i.e., if the input sequences are set up the same, the results will be the same), 

especially when new model versions are in the main providing new options and simulation 

capabilities. This is not, however, always the case; e.g., when errors in the code have been 

discovered and corrected, exact results may not recur unless the corrections have been applied 

to all versions. In general, EFED prefers that the older codes be maintained in an uncorrected state 

to provide a baseline for revisions to regulatory analyses. The current version of all codes accessed 

24 
by EXPRESS are available (Figure 15) by selecting the Help button on the main EXPRESS screen 

(Figure 1). For regulatory submissions, two versions of PRZM 3.12 are included in the shell at 

present. PRZM 3.12 beta was released for testing in February, 1998. PRZM 3.12.2 repairs errors 

and completes implementation of peer-review suggestions received regarding PRZM 3.12 beta, 

and adds several enhancements as well. A complete description of the several PRZM versions, and 

the modifications underpinning each, can be found on the CEAM web site at 

http://www.epa.gov/ceampubl/gwater/PRZM3/index.htm and 

http://www.epa.gov/ceampubl/gwater/PRZM3/prz3reln.htm, respectively. 

Two important changes between the 

primary (3.12 beta and 3.12.2) PRZM versions in 

EXPRESS lie in the new ability to enter up to 32 

curve numbers in 3.12.2, and correction in 

3.12.2 of errors in the parent/metabolite 

degradation kinetics coded in the older PRZM 

3.12 beta. Use of the complete set of available 

curve numbers in version 3.12.2 sometimes 

results in small differences in the results 

obtained. When simulating degradates, versions 

after PRZM 3.12 beta must be used; these input 

data are not accessible to version 3.12 beta. 

8.0 Database and Scenario Management 

One of the primary motivations 

underlying the development of EXPRESS has 

been realized by creation of the “scenario 

database”: The scenario database contains all 

the data needed to create PRZM input files, in a 

fixed form that cannot be cavalierly altered by the analyst community. This ensures that all analysts 

are using the same base data set, and helps to standardize regulatory analyses and facilitate 

archiving, public access, and peer review of exposure analysis results. Only an authorized 

“scenario manager” can change current scenarios and add or delete scenarios; the database is 

password-protected. The database as currently constructed can hold up to 200 PRZM scenarios. 

Figure 15. EXPRESS Software Versions (This 

Figure may not present latest versions or dates) 

25 
In addition, chemical parameters can be entered and stored in a separate database specific 

to each user. This database includes information used in both PRZM and EXAMS. 

9.0 Additional Capabilities Included in EXPRESS (Index Reservoir, PCA) 

In addition to the PRZM and EXAMS Tier II models, EXPRESS includes two Tier I OPP 

screening models, GENEEC2 and FIRST. GENEEC2 uses the “standard agricultural field-farm 

pond” scenario that EFED selected for all aquatic exposure assessments. This “standard pond” 

scenario assumes that ten centimeters of rain fall on a treated, 10-hectare agricultural field, 

causing pesticide-laden runoff into a one-hectare body of water of 20,000 cubic meters volume, 

2.00 meters deep. Although this “standard scenario” was designed to predict pesticide 

concentrations in a “standard farm pond,” it has been shown to be a good predictor of upper level 

pesticide concentrations in small, but ecologically important, upland streams (Effland et al. 1999; 

additional detail available on-line at http://www.epa.gov/scipoly/sap/1999/may/pca_sap.pdf ). 

For Tier I estimations of pesticide 

concentrations in drinking water, OPP uses a 

surface water model called FIRST. FIRST is 

based on an “Index Reservoir” watershed and 

includes Percent Cropped Area (PCA) factors 

which account for the percentage of a 

watershed that is planted with specific crops. 

The Index Reservoir scenario and the PCA 

have been built into standard EXPRESS Tier 

II analyses. 

The Index Reservoir is based on a 

drinking water reservoir in Illinois, Shipman City Lake. This reservoir was selected because it is 

representative of a number of reservoirs in the central Midwest that are known to be vulnerable to 

pesticide contamination. It is used similarly to the OPP “standard farm pond,” except that flow rates 

through the reservoir are set to reflect site-specific long-term average runoff. Like the standard 

pond, pesticide loadings to the water body are modeled using local soils and weather data to reflect 

crop-specific scenarios around the country. These scenarios are intended by OPP to represent 

sites at roughly the upper 90th percentile of runoff vulnerability for pesticide transport to surface 

waters. 

26 
9.1 New Scenarios for Tiers I and II 

Following passage of the Food Quality Protection Act (FQPA) by Congress in August of 

1996, EFED developed a source water modeling approach (the Tier I FIRST model, and modified 

agricultural use scenarios for EXPRESS) to supplement that already in place for ecological risk 

assessment. Recognizing that few people drink water derived from small farm ponds or from small, 

upland streams, EFED developed an “index agricultural watershed-drinking water [source] 

reservoir” or “index reservoir” scenario to supplement the “standard field-pond” ecological scenario. 

In addition, it was noted that most watersheds large enough to support a community drinking water 

system (CWS) are not entirely planted in only one crop. FIRST and EXPRESS therefore 

incorporate a method to consider a maximum percent cropped area (PCA) factor to account for this 

fact. 

9.2 Index Drinking Water Reservoir 

In July, 1998, OPP presented to the FIFRA Science Advisory Panel (SAP) a proposed 

“index reservoir” scenario to estimate pesticide concentrations in drinking water derived from 

surface water, supplementing the “field pond” scenario used in its ecological screening models. The 

concept behind use of a model of an “index” reservoir to screen pesticides was that the chosen 

reservoir – and its associated characteristics – would become the standard set of conditions by 

which EPA would judge the potential of a pesticide to contaminate drinking water derived from 

surface water. The “index” reservoir was selected from a group of actual reservoirs that are the 

source of drinking water for communities throughout the country. EPA picked from that group a 

particular reservoir having watershed characteristics associated with a higher potential for pesticide 

contamination of surface water, and uses those real-world characteristics in its mathematical 

screening model. Because the “index” reservoir model uses a set of real-world characteristics, it 

is thought likely to produce more realistic estimates of pesticide concentrations in surface drinking 

water sources than could a simple assemblage of average watershed properties. Because the 

“index” reservoir model has characteristics that are associated with a higher potential for pesticide 

contamination of surface water, the model is also likely to be protective of other drinking water 

sources less vulnerable to contamination. 

The actual reservoir chosen for simulation is a small drinking water reservoir located in 

Shipman, Illinois. Shipman City Lake is 13 acres in area, 9 feet deep, and has an input drainage 

watershed area of 427 acres. Its ratio of drainage area to capacity (volume of water in the lake) is 

approximately 12 m-1. As a comparison, the “standard farm pond” currently used has a ratio of 5 

27 
m-1. Shipman City Lake is one of a number of mid-western potable source water reservoirs that 

tend to be small and shallow with small input drainage area watersheds, and that frequently have 

Safe Drinking Water Act (SDWA) compliance problems with atrazine, a herbicide widely used on 

corn grown in these watersheds. The Index Drinking Water Reservoir scenario characteristics have 

been incorporated into the dynamically linked PRZM-EXAMS and FIRST models, and are 

implemented in conjunction with the percent cropped area adjustment (Effland et al. 1999, available 

on-line at http://www.epa.gov/scipoly/sap/1999/may/pca_sap.pdf). Although estimates of pesticide 

concentrations in drinking water based on a Midwestern “index drinking water reservoir” may not 

be wholly representative of residue levels in drinking water sources in other parts of the country, 

the scenario provides an effective screening tool to determine the need for more refined 

assessments. The modeling scenarios currently account for region-specific rainfall, soil, and 

hydrologic/runoff factors. Steps to develop additional scenarios to describe regional reservoirs for 

more advanced Tiers of modeling have been hampered by the lack of reservoir monitoring data 

outside of the Midwest that is of sufficient quality and extent to develop such scenarios. 

9.3 Percent Crop Area (PCA) Adjustments 

The PCA is a generic adjustment representing the maximum percent of any watershed that 

is planted to the crop or crops being modeled and, thus, may potentially be treated with the 

pesticide in question. PCA factors, generated from Geographic Information System (GIS) overlays 

of cropping area and watershed delineations, are applied directly to estimates of index reservoir 

surface water pesticide concentration values from the PRZM-EXAMS and FIRST models. The 

output concentration estimates generated by these models are multiplied by the maximum decimal 

fraction of cropped area in any watershed associated with the crop or crops of interest. To be 

effective as an adjustment to screening model estimates, the PCA should also result in estimated 

concentrations that are closer to, but not less than, actual observed pesticide concentrations in 

vulnerable (prone to pesticide-laden runoff) surface drinking water sources. While relaxing the 

assumption that the entire watershed would be treated at the same time with the pesticide under 

study, the use of the PCA concept is still regarded as a screening tool because it represents the 

highest percentage of crop cover of any large watershed in the U.S., and still assumes that the 

entire crop is being treated. Current PCA values and a description of how the values were derived 

and applied can be found at http://www.epa.gov/oppEFED1/models/water/regional_pca.htm. 

28 
10.0 Using EXPRESS: Data Preparation and Input Example 

Chapter 10 is a worked Tier II example of preparing data and entering them into EXPRESS 

for Forchlorfenuron. Table 10-1 presents the Efate parameters for Forchlorfenuron and indicates 

the EXPRESS input generator screen where the values are entered. (An “MRID” (Master Record 

Identification) Number is a unique cataloging number assigned to an individual pesticide study at 

the time of its submission to the Agency.) (Input parameters based on OPP 2002-02-28 Input 

Parameter Guidance (http://www.epa.gov/oppEFED1/models/water/input_guidance2_28_02.htm)) 

Table 10-1. Surface Water Exposure Inputs for Forchlorfenuron 

MODEL INPUT VARIABLE 

INPUT 

VALUE 

COMMENTS/EXPRESS SCREEN 

Scenario Selection CA Grape 

NY Grape 

Source: Label; EXPRESS Screen: Project Design 

California grapes (raisin, wine, and fresh market) 

NY grapes (wine and fresh market) 

Application Rate (kg/ha) 

CA Grape (irrig), MLRA 17 

NY Grape, MLRA 100 

0.025 

0.025 

Source: Label 

EXPRESS Screen: Pesticide Application Data 

Molecular W eight 248 g/mole 

EXPRESS Screen: PRZM-Efate 

Vapor pressure 3.5×10 -10 Torr 

EXPRESS Screen: PRZM-Efate 

Koc (ml/g) 1763 Average Koc from 4 soils range of 852-3320. 

MRIDs 44359516, 45840121 

EXPRESS Screen: PRZM-Efate 

Aerobic Soil Metabolic Half-life 

(days) 

parent 

578 Only aerobic soil metabolism study. Half-life not 

multiplied by 3 because very long. Based on 

extractable residues in MRIDs 44359515, 

45840119. Half-life based on extractable+ bound 

was 1,659 days 

EXPRESS Screen: PRZM-Efate 

Solubility (mg/L) 39 9-7-00 EUP memo 

EXPRESS Screen: PRZM-Efate 

Aerobic Aquatic Metabolic 

Half-life (days) 1156 

2X aerobic soil metabolism half-life (578 days) 

EXPRESS Screen: EXAMS-Efate 

29 
MODEL INPUT VARIABLE 

INPUT 

VALUE 

COMMENTS/EXPRESS SCREEN 

Anaerobic Aquatic Metabolism 

half-life (days) 

226 Anaerobic soil metabolism half-life (MRIDs 

44359514, 45840118) 

EXPRESS Screen: EXAMS-Efate 

Aquatic Photolysis Half-life 

(days) 

143 MRID 44394605 

EXPRESS Screen: EXAMS-Efate 

Maximum No. of Applications 

All outdoor uses 

1 Label, p. 4 

EXPRESS Screen: Pesticide Application Data 

Interval between appl. N/A For a single application 

Application Date(s) May 15 

June 15 

All California Grapes 

NY Grapes (Concord) 

Based on 2003-12-9 Email from Bernie Schneider of 

HED to Jim Breithaupt of EFED 

EXPRESS Screen: Pesticide Application Data 

Method of application 

Ground (all uses) 

Ground 

Sprayer 

CAM 2 – application to foliage 

EXPRESS Screen: Pesticide Application Data 

Incorporation depth 

All outdoor uses Default 

Can be modified for broadcast applications. 

EXPRESS Screen: Pesticide Application Data 

Application Efficiency 

Ground Sprayer 0.99 

Per 2000-11-7 Input Parameter Guidance 

EXPRESS Fixed parameter 

EXPRESS Screen: Pesticide Application Data 

Spray Drift (Farm Pond) 

Ground Sprayer 0.01 

Per 2000-11-7 Input Parameter Guidance 

EXPRESS Fixed parameter 

EXPRESS Screen: Pesticide Application Data 

Spray Drift (Drinking W ater) 

Ground Sprayer 0.064 

Per 2000-11-7 Input Parameter Guidance 

EXPRESS Fixed parameter 

EXPRESS Screen: Pesticide Application Data 

30 
EXPRESS Example Input Screens 

1. EXPRESS Screen: Project Design 

31

2: EXPRESS Screen: PRZM Efate 

32

3: EXPRESS Screen: EXAMS Efate 

33

4: EXPRESS screen(s): Pesticide Application Data 

34

11.0 Appendices


APPENDIX A: Format of Daily Value Meteorological Time-Series Files


Field Columns Description Units Type Format 

1 1 blank N/A Character 1x 

2 38389 Date mmddyy Integer 3i2 

3 38580 Precipitation cm/day Real f10.2 

4 18-27 Pan Evaporation cm/day Real f10.2 

5 28-37 Temperature (mean) Degrees 

Centigrade 

Real f10.1 

6 38-47 Wind Speed @ 10 meter cm/second Real f10.1 

7 48-57 Solar Radiation Langleys/day Real f10.1 

8 58-63 FAO Sho 0 rt Grass Et mm/day Real f6.1 

9 64-73 Daylight Station Pressure kiloPascal Real f10.1 

10 74-77 Daylight Relative Humidity Percent Integer I4 

11 78-80 Daylight Opaque Sky Cover Tenths of Sky 

Covered 

Integer I3 

12 81-90 Daylight Temperature Degrees 

Centigrade 

Real f10.1 

13 91-96 Daylight Broadband 

Aerosol 

Optical Depth Real f6.3 

14 97 -102 Daylight Prevailing Wind 

Speed at 10 meter height 

meter/second Real f6.1 

15 103-106 Daylight Prevailing Wind 

Direction 

degrees (N=0, 

E=90, ...) 

Integer i4

 Daily values file format: 

(1x, 3i2, t8,f10.2, t18,f10.2, t28,f10.1, t38,f10.1, & 

t48,f10.1, t58,f6.1, t64,f10.1, t74,i4, t78,i3, & 

t81,f10.1, t91,f6.3, t97,f6.1, t103,i4) 

Fields 03 - 08 represent daily totals or mean values (Wind Speed and Temperature) 

Fields 03 - 07 units preserved from earlier PRZM met files for compatibility 

Fields 09 - 15 represent mean values for daylight hours only. 

35 
APPENDIX B: Added Capabilities of ORD Mode 

In ORD mode, several additional capabilities are available to the analyst: 

1. Sorption parameters include the octanol-water partition coefficient, K . K is passed through ow ow 

directly to EXAMS. For use in PRZM, the soil K is calculated as 0.175 K and applied to the soil oc ow 

organic carbon content in each soil horizon, as in the following code fragment:

 do M=1,SSNHORIZ


select case (KDTYP(J))


case (1) ! entered soil Koc


KD(J,M,Zone)=(KDSP(J)*SSOC(M))/100.0


case (2) ! direct Kd


KD(J,M,Zone)=KDSP(J)


case (3) ! Kow (see p. 13 of EXAMS manual (EPA/600/R-00/081))


KD(J,M,Zone)=(0.175*KDSP(J)*SSOC(M))/100.0


end select


 end do


2. The initial default parameters for foliar processes correspond to the ORD-recommended values 

(Suárez 2005: foliar washoff coefficient FEXTRC=0.10 (p 5.12), plant uptake factor UPTKF=1.0, 

representing passive movement in the transpiration stream (p. 5-22)). As in OPP mode, these can 

be altered at the user’s discretion. 

3. For aquatic chemistry, additional parameters are made available: Q10 for aerobic and anaerobic 

aquatic biolysis, the temperature of the biolysis experiments, and melting point of the chemicals. 

4. For a more detailed view of spray drift, AgDrift program outputs tailored to the standard farm 

pond and index reservoir are displayed. These include several droplet size categorizations for aerial 

applications and ground sprayers, and orchard/vineyard airblast applications. The protective impact 

of spray buffers of various widths, separately for the farm pond and the index reservoir, can be 

evaluated in ORD mode. 

5. Available versions of PRZM include 3.12.3, which also provides a mass balance analysis and 

display by EXPRESS. 

36 
APPENDIX C: References 

Burns, L. A. 2000. Exposure Analysis Modeling System (EXAMS): User Manual and System 

Documentation. EPA/600/R-00/081, U.S. Environmental Protection Agency, National 

Exposure Research Laboratory, Athens, Georgia. 

Burns, L. A. 2001. Probabilistic Aquatic Exposure Assessment for Pesticides. I: Foundations. 

EPA/600/R-01/071, U.S. Environmental Protection Agency, National Exposure Research 

Laboratory, Athens, Georgia. 

Burns, L. A., and D. M. Cline. 1985. Exposure Analysis Modeling System: Reference Manual for 

EXAMS II. EPA/600/3-85/038, U.S. Environmental Protection Agency, Athens, Georgia. 

Burns, L. A., D. M. Cline, and R. R. Lassiter. 1982. Exposure Analysis Modeling System (EXAMS): 

User Manual and System Documentation. EPA-600/3-82-023, U.S. Environmental 

Protection Agency, Athens, Georgia. 

Carsel, R. F., L. A. Mulkey, M. N. Lorber, and L. B. Baskin. 1985. The Pesticide Root Zone Model 

(PRZM): A procedure for evaluating pesticide leaching threats to ground water. Ecological 

Modeling 30:49-69. 

Carsel, R. F., A. E. Smith, L. A. Mulkey, J. D. Dean, and P. Jowise. 1984. User's Manual for the 

Pesticide Root Zone Model (PRZM): Release 1. U.S. Environmental Protection Agency, 

Athens, Georgia. 

Effland, W. R., N. C. Thurman, and I. Kennedy. 1999. Proposed Methods For Determining 

Watershed-Derived Percent Cropped Areas and Considerations for Applying Crop Area 

Adjustments To Surface Water Screening Models. USEPA Office of Pesticide Programs, 

Presentation to FIFRA Science Advisory Panel, May 27, 1999, Washington, D.C., USA. 

Parker, R. D., R. D. Jones, and H. P. Nelson. 1995. GENEEC: A Screening Model for Pesticide 

Environmental Exposure Assessment. Pages 485-490 in International Exposure 

Symposium on Water Quality Modeling. Proceedings of the American Society of 

Agricultural Engineers, Orlando, Florida, USA. 

SCS. 1981. Land Resource Regions and Major Land Resource Areas of the United States. 

Agriculture Handbook 296, United States Department of Agriculture Soil Conservation 

Service, Washington, D.C. 

Suárez, L. A. 2005. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root 

and Unsaturated Soil Zones: Users Manual for Release 3.12.2. EPA/600/R-05/111. U.S. 

Environmental Protection Agency, National Exposure Research Laboratory, Athens, 

Georgia. 

37