**Iowa Corn**

The Iowa corn scenario was developed to represent an environment in
which field corn is grown in an area where rainfall is high and soils
are more vulnerable to runoff than most places where corn is grown in
the state. Based on the 2002 USDA National Agricultural Statistics
Summary, Iowa has the most acreage of corn in the nation with 11,761,392
acres (USDA, 2005), representing 17% of the total corn acreage in the
United States. Besides field corn, two other crops are in the species of
*Zea mays:* popcorn and sweet corn. This scenario, though, is not
intended to be used for these other two forms of corn as they have
different cultural practices and tend to be more prominent in other
parts of the country than in the Midwest.

Corn culture is approximately equally distributed across Iowa from east
to west, but is most concentrated in the northern half of the state. Of
77 counties in Iowa, 24 are each planted with between 150,000 and
170,000 acres of corn, while just four have more than 170,000 acres.
Kossuth County, along the northern border of the state has the most
acreage of corn, with 287,000 acres.

Table 1 lists the candidate weather stations for Iowa, along with the
annual mean precipitation at each station. Based on data from
weather.com, Table 1 also lists the mean precipitation in June, the
highest runoff month in the Midwest. Because the corn culture is
widespread across the state, a site near one of the weather stations
(Moline, IL), was selected. This site is on the Iowa-Illinois border and
has the greatest annual rainfall of the candidate weather stations and
the second highest rainfall in June.

  -------------- ------------------ -------------------- -----------------
  **Table 1.**                                           
  Candidate                                              
  Weather                                                
  Stations for                                           
  the Iowa Corn                                          
  Scenario.                                              

  Station ID     Location           Annual Average       June Average
                                    Precipitation        Precipitation

  W14933         Des Moines, IA     33.4 in              4.6 in

  W14940         Mason City, IA     34.6 in              5.1 in

  W14943         Sioux City, IA     26.0 in              3.6 in

  W94810         Waterloo, IA       33.2 in              4.8 in

  **W14923\***   **Moline, IL**     **38.0 in**          **4.6 in**
  -------------- ------------------ -------------------- -----------------

> \* The Moline, IL weather station was selected to represent this
> scenario

The site selected for this scenario is in Clinton County and is about 30
miles north of Moline. Clinton County has the sixth highest corn acreage
in Iowa, with 169,070 acres or 1.4% of the acreage in the state. The
site is in MLRA 105, the Northern Mississippi Valley Loess Hills. Soils
in this area have dominantly formed in loess or loess of residuum (USDA,
2006). Corn, soybeans, other grains for feed, and hay are the dominant
crops. Clinton County, which is located in the Upper Mississippi River
Basin, is drained by the Wapsipinicon and Maquoketa Rivers.

The soil selected for this scenario is the Fayette silty clay loam, a
fine silty, mixed, superactive, mesic Typic Hapludalf. The Fayette soil
is a benchmark soil. It is important to note that the Fayette soil is
from Hydrologic Group B rather than Group C, which is usually selected,
according to guidance (USEPA 2004). However, Groups C and D soils only
represent 2.5% of the acreage in Clinton County, and are generally
uncommon across the northern portion of the state, where most of the
corn is grown in Iowa. Because these more runoff-prone soils are
uncommon in this region, a Hydrologic Group B soil was chosen to
represent the site. Fayette soils cover 22.9% of Clinton County, IA.
Slopes range from 2 to 40%, although the greater slopes are severely
eroded. Slopes above 18% are not currently being used for corn
agriculture to any great extent.

+-----------------+-----------+----------------------------------------+
| **Table 2.**    |           |                                        |
| PRZM 3.12.2     |           |                                        |
| Climate and     |           |                                        |
| Time Parameters |           |                                        |
| for Clinton     |           |                                        |
| County Corn in  |           |                                        |
| Iowa.           |           |                                        |
+-----------------+-----------+----------------------------------------+
| Parameter       | Value     | Source/Comments                        |
+-----------------+-----------+----------------------------------------+
| Starting Date   | Jan. 1,   | Meteorological File from Moline, IL    |
|                 | 1961      | (W14923)                               |
+-----------------+-----------+----------------------------------------+
| Ending Date     | Dec. 31,  | Meteorological File (W14923)           |
|                 | 1990      |                                        |
+-----------------+-----------+----------------------------------------+
| Pan Evaporation | 0.76      | PRZM Manual Figure 5.1. (USEPA, 2006)  |
| Factor (PFAC)   |           |                                        |
+-----------------+-----------+----------------------------------------+
| Snowmelt Factor | 0.36 cm   | Number range CHAPTER Maximum value of  |
| (SFAC)          | °C^-1^    | minimum range of PRZM Manual Table 5.1 |
|                 |           | (USEPA, 2006)                          |
+-----------------+-----------+----------------------------------------+
| Minimum Depth   | 17.5 cm   | PRZM Manual (USEPA, 2006); average of  |
| of              |           | 15-20 cm                               |
|                 |           |                                        |
| Evaporation     |           |                                        |
| (ANETD)         |           |                                        |
+-----------------+-----------+----------------------------------------+
| Pan Factor Flag | 0         | Pan Factor Flag set to read from       |
| (IPEIND)        |           | weather data as per guidance (USEPA,   |
|                 |           | 2004)                                  |
+-----------------+-----------+----------------------------------------+

+-----------------+----------+-----------------------------------------+
| **Table 3.**    |          |                                         |
| PRZM 3.12.2     |          |                                         |
| Erosion and     |          |                                         |
| Landscape       |          |                                         |
| Parameters for  |          |                                         |
| Corn in Clinton |          |                                         |
| County, Iowa.   |          |                                         |
+=================+==========+=========================================+
| Parameter       | Value    | Source/Comments                         |
+-----------------+----------+-----------------------------------------+
| Method to       | 4 (MUSS) | PRZM Manual (USEPA, 2006)               |
| Calculate       |          |                                         |
| Erosion         |          |                                         |
| (ERFLAG)        |          |                                         |
+-----------------+----------+-----------------------------------------+
| USLE K Factor   | 0.37     | USDA NRCS Soil Data Mart                |
| (USLEK)         | tons     | (http://soildatamart.nrcs.usda.gov/)    |
|                 | EI^-1\*^ |                                         |
+-----------------+----------+-----------------------------------------+
| USLE LS Factor  | 3.73     | Value listed for 12.2% slope at 400     |
| (USLELS)        |          | feet using equation in Hann and         |
|                 |          | Barfield (1978)                         |
+-----------------+----------+-----------------------------------------+
| USLE P Factor   | 0.8      | 12.2% slope row crop PRZM Manual Table  |
| (USLEP)         |          | 5.6 (USEPA, 2006)                       |
+-----------------+----------+-----------------------------------------+
| Field Area      | 172 ha   | Area of Shipman Reservoir watershed     |
| (AFIELD)        |          | (USEPA, 1999)                           |
+-----------------+----------+-----------------------------------------+
| NRCS Hyetograph | 3        | PRZM Manual Figure 5.8 (USEPA, 2006)    |
| (IREG)          |          |                                         |
|                 |          | Type 2, IREG=3                          |
+-----------------+----------+-----------------------------------------+
| Slope (SLP)     | 6%       | Set to 6%, as slope is greater than 6%, |
|                 |          | as per guidance (USEPA, 2004)           |
+-----------------+----------+-----------------------------------------+
| Hydraulic       | 600 m    | Shipman Reservoir (USEPA, 1999)         |
| Length (HL)     |          |                                         |
+-----------------+----------+-----------------------------------------+
| Irrigation Flag | 0        | Irrigation off                          |
| (IRFLAG)        |          |                                         |
+-----------------+----------+-----------------------------------------+
| \* EI = 100     |          |                                         |
| ft-tons \* in/  |          |                                         |
| acre\*hr        |          |                                         |
+-----------------+----------+-----------------------------------------+
|                 |          |                                         |
+-----------------+----------+-----------------------------------------+

+-----------------+----------+-----------------------------------------+
| **Table 4.**    |          |                                         |
| PRZM 3.12.2     |          |                                         |
| Crop Parameters |          |                                         |
| for Corn in     |          |                                         |
| Clinton County, |          |                                         |
| Iowa.           |          |                                         |
+=================+==========+=========================================+
| **Parameter**   | *        | **Source/Comments**                     |
|                 | *Value** |                                         |
+-----------------+----------+-----------------------------------------+
| Initial Crop    | 1        | Set to one for all crops (EPA, 2004).   |
| (INICRP)        |          |                                         |
+-----------------+----------+-----------------------------------------+
| Initial Surface | 1        | 1= fallow, default -- parameter is      |
| Condition       |          | ignored as ERFLAG \> 0.                 |
|                 |          |                                         |
| (ISCOND)        |          |                                         |
+-----------------+----------+-----------------------------------------+
| Number of       | 1        | Set to number of crops in simulation.   |
| Different Crops |          |                                         |
| (NDC)           |          |                                         |
+-----------------+----------+-----------------------------------------+
| Number of       | 30       | Set to weather data in meteorological   |
| Cropping        |          | file: Moline, IL (W14923).              |
| Periods (NCPDS) |          |                                         |
+-----------------+----------+-----------------------------------------+
| Maximum         | 0.25 cm  | Low end of corn heavy canopy; PRZM      |
| Rainfall        |          | Table 5-4.                              |
| Interception    |          |                                         |
| Storage of Crop |          |                                         |
| (CINTCP)        |          |                                         |
+-----------------+----------+-----------------------------------------+
| Maximum Active  | 90 cm    | Middle of range for corn, Table 5.9     |
| Root Depth      |          |                                         |
| (AMXDR)         |          |                                         |
+-----------------+----------+-----------------------------------------+
| Maximum Canopy  | 100%     | Set to 100 for row crops, as per        |
| Coverage        |          | guidance                                |
| (COVMAX)        |          |                                         |
+-----------------+----------+-----------------------------------------+
| Soil Surface    | 3        | 3 = residue, as according to guidance   |
| Condition After |          | (USEPA, 2004)                           |
| Harvest (ICNAH) |          |                                         |
+-----------------+----------+-----------------------------------------+
| Date of Crop    | 25/05/61 | Middle of most active period;           |
| Emergence       |          | Associated with first RUSLE date of 10  |
|                 |          | days after planting.                    |
| (EMD, EMM,      |          |                                         |
| IYREM)          |          |                                         |
+-----------------+----------+-----------------------------------------+
| Date of Crop    | 24/07/61 | 60 days after planting (USDA, 1997)     |
| Maturity        |          |                                         |
|                 |          |                                         |
| (MAD, MAM,      |          |                                         |
| IYRMAT)         |          |                                         |
+-----------------+----------+-----------------------------------------+
| Date of Crop    | 19/10/61 | Middle of most active period (USDA,     |
| Harvest (HAD,   |          | 1997)                                   |
| HAM, IYRHAR)    |          |                                         |
+-----------------+----------+-----------------------------------------+
| Maximum Dry     | 0.0      | Not used in scenario                    |
| Weight (WFMAX)  |          |                                         |
+-----------------+----------+-----------------------------------------+
| Maximum Crop    | 300 cm   | Upper end for corn from PRZM Manual     |
| Height (HTMAX)  |          | Table 5.16                              |
+-----------------+----------+-----------------------------------------+
| SCS Curve       | 86, 79,  | PRZM Table 5.10, B Soil, fallow, and    |
| Number (CN)     | 86       | row crop, contoured, poor condition     |
+-----------------+----------+-----------------------------------------+
| Manning's N     | 0.014    | RUSLE Project; M99CGCSN; corn for       |
| Value (MNGN)    |          | grain, Des Moines weather station;      |
|                 |          | mulch tillage(USDA, 2000); dates        |
|                 |          | adjusted to match planting and harvest  |
+-----------------+----------+-----------------------------------------+
| USLE C Factor   | 0.0      | RUSLE Project; M99CGCSN; corn for       |
| (USLEC)         | 16-0.307 | grain, Des Moines weather station;      |
|                 |          | mulch tillage(USDA, 2000); dates        |
|                 |          | adjusted to match planting and harvest  |
+-----------------+----------+-----------------------------------------+

+---------------------+---------------------+--------------------------+
| **Table 5.** PRZM   |                     |                          |
| 3.12.2 Fayette      |                     |                          |
| Silty Clay Loam     |                     |                          |
| Soil Parameters for |                     |                          |
| Corn in Clinton     |                     |                          |
| County, Iowa.       |                     |                          |
+---------------------+---------------------+--------------------------+
| **Parameter**       | **Value**           | **Source/Comments**      |
+---------------------+---------------------+--------------------------+
| Total Soil Depth    | 152 cm              | NRCS Soil Data Mart      |
| (CORED)             |                     | (SDM)                    |
|                     |                     | (http://soi              |
|                     |                     | ldatamart.nrcs.usda.gov) |
+---------------------+---------------------+--------------------------+
| Number of Horizons  | 4                   | NRCS Soil Data Mart      |
| (NHORIZ)            |                     | (SDM)                    |
+---------------------+---------------------+--------------------------+
| Horizon Thickness   | 10 cm (HORIZN = 1)  | NRCS Soil Data Mart      |
| (THKNS)             |                     | (SDM). The top horizon   |
|                     | 5 cm (HORIZN = 2)   | was split into two       |
|                     |                     | horizons as per PRZM     |
|                     | 125 cm (HORIZN = 3) | Scenario Guidance (EPA,  |
|                     |                     | 2004).                   |
|                     | 12 cm (HORIZN = 4)  |                          |
+---------------------+---------------------+--------------------------+
| Bulk Density (BD)   | 1.4 g/cm^3^ (HORIZN | NRCS Soil Data Mart      |
|                     | = 1)                | (SDM)                    |
|                     |                     | ([                       |
|                     | 1.4 g/cm^3^ (HORIZN | http://soildatamart.nrcs |
|                     | = 2)                | .usda.gov](http://soilda |
|                     |                     | tamart.nrcs.usda.gov/)). |
|                     | 1.37 g/cm^3^        | Midpoint of the reported |
|                     | (HORIZN = 3)        | range. PRZM Scenario     |
|                     |                     | Guidance (EPA, 2004).    |
|                     | 1.48 g/cm^3^        |                          |
|                     | (HORIZN = 4)        |                          |
+---------------------+---------------------+--------------------------+
| Initial Water       | 0.310 cm^3^/cm^3^   | NRCS Soil Data Mart      |
| Content (THETO)     | (HORIZN =1)         | (SDM); values are 15 bar |
|                     |                     | water plus mean of       |
|                     | 0.310 cm^3^/cm^3^   | available water          |
|                     | (HORIZN =2)         | capacity.                |
|                     |                     |                          |
|                     | 0.279 cm^3^/cm^3^   |                          |
|                     | (HORIZN = 3)        |                          |
|                     |                     |                          |
|                     | 0.309 cm^3^/cm^3^   |                          |
|                     | (HORIZN = 4)        |                          |
+---------------------+---------------------+--------------------------+
| Compartment         | 0.1 cm (HORIZN = 1) | NRCS Soil Data Mart      |
| Thickness (DPN)     |                     | (SDM)                    |
|                     | 1 cm (HORIZN = 2)   | ([                       |
|                     |                     | http://soildatamart.nrcs |
|                     | 5 cm (HORIZN = 3)   | .usda.gov](http://soilda |
|                     |                     | tamart.nrcs.usda.gov/)). |
|                     | 6 cm (HORIZN = 4)   | PRZM Scenario Guidance   |
|                     |                     | (EPA, 2004).             |
+---------------------+---------------------+--------------------------+
| Field Capacity      | 0.310 cm^3^/cm^3^   | NRCS Soil Data Mart      |
| (THEFC)             | (HORIZN =1)         | (SDM); values are 15 bar |
|                     |                     | water plus mean of       |
|                     | 0.310 cm^3^/cm^3^   | available water capacity |
|                     | (HORIZN =2)         |                          |
|                     |                     |                          |
|                     | 0.279 cm^3^/cm^3^   |                          |
|                     | (HORIZN = 3)        |                          |
|                     |                     |                          |
|                     | 0.309 cm^3^/cm^3^   |                          |
|                     | (HORIZN = 4)        |                          |
+---------------------+---------------------+--------------------------+
| Wilting Point       | 0.120 cm^3^/cm^3^   | NRCS Soil Data Mart      |
| (THEWP)             | (HORIZN =1)         | (SDM), soil              |
|                     |                     | characterization data;   |
|                     | 0.120 cm^3^/cm^3^   | values are mean 15-bar   |
|                     | (HORIZN =2)         | water contents of        |
|                     |                     | Fayette silty clay loam  |
|                     | 0.079 cm^3^/cm^3^   |                          |
|                     | (HORIZN = 3)        |                          |
|                     |                     |                          |
|                     | 0.119 cm^3^/cm^3^   |                          |
|                     | (HORIZN = 4)        |                          |
+---------------------+---------------------+--------------------------+
| Organic Carbon      | 0.93% (HORIZN = 1)  | NRCS SDM; values for     |
| Content (OC)        |                     | horizons 1 to 3 = mean   |
|                     | 0.93% (HORIZN = 2)  | %OM / 1.724. PRZM        |
|                     |                     | Scenario Guidance (EPA,  |
|                     | 0.14% (HORIZN = 3)  | 2004).                   |
|                     |                     |                          |
|                     | 0.14% (HORIZN = 4)  |                          |
+---------------------+---------------------+--------------------------+

**References**

Haan, C.T., and B. J. Barfield. 1978. Hydrology and Sedimentology of
Surface Mined Lands. Office of Continuing Education and Extension,
College of Engineering, University, University of Kentucky, Lexington,
KY 40506. pp 286.

USDA. 1997. Usual Planting and Harvest Dates for U. S. Field Crops.
<http://www.usda.gov/nass/pubs/uph97.htm>

USDA. 2006. Land Resource Regions and Major Land Resource Areas of the
United States, the Caribbean, and the Pacific Basin. USDA Handbook 296.
<ftp://ftp-fc.sc.egov.usda.gov/NSSC/Ag_Handbook_296/Handbook_296_low.pdf>

USDA. 2000. Revised Universal Soil Loss Equation (RUSLE) EPA Pesticide
Project. U.S. Department of Agriculture, National Resources Conservation
Service (NRCS) and Agricultural Research Service (ARS).

USDA. 2005. 2002 Census of Agriculture. U.S. Department of Agriculture,
National Agricultural Statistics Service (NASS). Online at:
<http://www.nass.usda.gov/index.asp>.

USDA. 2006. Official Series Description -- Fayette Series. U.S.
Department of Agriculture, Natural Resources Conservation Service
(NRCS). Sept. 2006. Online at:
<http://ortho.ftw.nrcs.usda.gov/osd/dat/F/FAYETTE.html>

USEPA. 1999. Jones, R.D., J. Breithaupt, J. Carleton, L. Libelo, J. Lin,
R. Matzner, and R. Parker. *Guidance for Use of the Index Reservoir in
Drinking Water Exposure Assessments.* Environmental Fate and Effects
Division, Office of Pesticide Programs, U.S. Environmental Protection
Agency, Washington, DC.

USEPA. 2004. Abel, S.A. *Procedure for Conducting Quality Assurance and
Quality Control of Existing and New PRZM Field and Orchard Crop Standard
Scenarios.* Environmental Fate and Effects Division, Office of Pesticide
Programs, U.S. Environmental Protection Agency, Washington, DC.

USEPA. 2006. Carsel, R.F., J.C. Imhoff, P.R. Hummel, J.M. Cheplick, and
A.S. Donigian, Jr. *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.* National Exposure Research Laboratory, Office of
Research and Development, U.S. Environmental Protection Agency, Athens,
GA.