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The field used to represent alfalfa production in Texas is located in Milam County in the Texas Claypan region. According to the 1997 Census of Agriculture, Texas is not ranked in the top 20 states in production of alfalfa in the U.S. Alfalfa is a perennial crop, grown on a variety of soils, but performs best in Texas on well drained soils with a pH of 6.5. It is planted late in the year (August) and maintained under continuous cultivation on a 3- to 5-year cycle at which time a new crop is planted. Planting depths range from 0.25 to 1.5 inches, depending on soil texture, on level seed beds. Row spacing is approximately 30 inches; approximately 70 percent of the crop is irrigated by sprinkler (55%) and flood (15%). Cuttings range from 3 to 5 per year. Most production is used in state to supply dairies and feedlots. The soil selected to simulate the field is a benchmark soil, Lufkin loam. Lufkin loam, is a fine, smectitic, thermic, Oxyaquic Vertic Paleustalfs. These soils were in general crop production in the past, but are now mostly pasture. Crops currently planted on these soils include grain sorghum, hay crops or small grain for grazing. Lufkin loam is a very deep, moderately well drained, very slowly permeable soil with medium to low runoff depending on slope. The soil has a very slow internal drainage due to the claypan at approximately 12 to 18 inches. The series formed in slightly acid to alkaline clayey sediments at elevations of 75 to 125 feet above major flood plains on slopes of mainly less than one percent, but may range up to 3 percent. The series is extensive in the Texas Claypan region of MLRA 87A and 87B and of to lesser extent in MLRA 86A. Lufkin loam is a Hydrologic Group D soil.

Table 1.
PRZM 3.12 Climate and Time Parameters for Milam County, Texas - Alfalfa
Starting Date January 1, 1948Meteorological File - Austin, TX (W13958)
Ending Date December 31, 1983Meteorological File - Austin, TX (W13958)
Pan Evaporation Factor (PFAC) 0.71PRZM Manual Figure 5.1 (EPA, 1998)
Snowmelt Factor (SFAC) 0.36 cm C- 1PRZM Manual Table 5.1 (EPA, 1998)
Minimum Depth of Evaporation (ANETD) 25 cmPRZM Manual Figure 5.2 (EPA, 1998)

Table 2.
PRZM 3.12 Erosion and Landscape Parameters for Milam County, Texas - Alfalfa
Method to Calculate Erosion (ERFLAG) 4 (MUSS)PRZM Manual (EPA, 1998)
USLE K Factor (USLEK) 0.43 tons EI-1* GLEAMS Manual, table of Representative Soils (USDA, 1990)
USLE LS Factor (USLELS) 0.109GLEAMS Manual, table of Representative Soils (USDA, 1990)
USLE P Factor (USLEP) 1.0PRZM Manual, Table 5.6 (EPA, 1998)
Field Area (AFIELD) 172 haArea of Shipman Reservoir watershed (EPA, 1999)
NRCS Hyetograph (IREG) 4PRZM Manual Figure 5.12 (EPA, 1998)
Slope (SLP) 1%Dominant slope for soil series (EPA, 2001)
Hydraulic Length (HL) 600 mShipman Reservoir (EPA, 1999)

* EI = 100 ft-tons * in/ acre*hr

Table 3.
PRZM 3.12 Crop Parameters for Milam County, Texas - Alfalfa
Initial Crop (INICRP) 1Set to one for all crops (EPA, 2001)
Initial Surface Condition (ISCOND) 1Set to fallow prior to new crop planting.
Number of Different Crops (NDC) 1Set to crops in simulation - generally one
Number of Cropping Periods (NCPDS) 36Set to weather data. Austin, TX (W13958)
Maximum rainfall interception storage of crop (CINTCP) 0.25PRZM, Table 5.4 (EPA, 1998)
Maximum Active Root Depth (AMXDR) 100 cm Set to soil profile maximum depth. Roots may grow to 20 feet; 8-12 feet is common (EPA, 2001)
Maximum Canopy Coverage (COVMAX) 100 Set to default for row crops (EPA, 2001). Consistent with Ag. Agents from other Alfalfa growing regions.
Soil Surface Condition After Harvest (ICNAH) 3 Set to residue for winter months after last harvest during multi-year growth and during winter of last years of growth.
Date of Crop Emergence
30/08 TX generally restricted to Fall plantings. Dates set to mid-points.
Emergence 7-10 days after planting; Maturation occurs approximately 60 days after planting..
First harvest after maturation is generally May.
Three and sometimes 4 cutting per season. Harvest set to last event assuming 4 cuttings.
Each cutting may occur 28-30 days after last. http://texaserc.tamu.edu/ Exit EPA Disclaimer
http://forage.cas.psu.edu/docs/species/alfalfa.html Exit EPA Disclaimer
Date of Crop Maturity
Date of Crop Harvest
Maximum Dry Weight (WFMAX) 0.0Set to "0" Not used in simulation
SCS Curve Number (CN) 90, 88, 89 Gleams Manual Table A.3, pasture/range, non-CNT, poor condition (USDA, 1990)
Manning's N Value (MNGN) 0.110 RUSLE Project, J94HGHGC; Hay, Grass, conventional tillage, Waco, TX (USDA, 2000)
USLE C Factor (USLEC) 0.000 - 0.004 RUSLE Project; J94HGHGC; Hay, Grass, conventional tillage, Waco, TX (USDA, 2000)

Table 4.
PRZM 3.12 Lufkin Soil Parameters for Milam County, Texas - Alfalfa
ParameterValue Verification Source
Total Soil Depth (CORED) 100 cm NRCS, National Soils Characterization Database (NRCS, 2001)
Number of Horizons (NHORIZ) 3 (Top horizon split in two)
First, Second, and Third Soil Horizons (HORIZN = 1,2,3)
Horizon Thickness (THKNS)
  • 10 cm (HORIZN = 1)
  • 8 cm (HORIZN = 2)
  • 82 cm (HORIZN = 3)
NRCS, National Soils Characterization Database (NRCS, 2001) http://soils.usda.gov/survey/nscd/ Exit EPA Disclaimer
Bulk Density (BD)
  • 1.55 g cm-3 (HORIZN = 1,2)
  • 1.6 g cm-3 (HORIZN = 3)
Initial Water Content (THETO)
  • 0.215 cm3-H2O cm3-soil (HORIZN =1,2)
  • 0.320 cm3-H2O cm3-soil (HORIZN =3)
Compartment Thickness (DPN)
  • 0.1 cm (HORIZN = 1)
  • 1 cm (HORIZN = 2)
  • 2 cm (HORIZN = 3)
Field Capacity (THEFC)
  • 0.215 cm3-H2O cm3-soil (HORIZN = 1,2)
  • 0.320cm3-H2O cm3-soil (HORIZN = 3)
Wilting Point (THEWP)
  • 0.105 cm3-H2O cm3-soil (HORIZN = 1,2)
  • 0.200 cm3-H2O cm3-soil (HORIZN = 3)
Organic Carbon Content (OC)
  • 1.16% (HORIZN = 1,2)
  • 0.29 (HORIZN = 3)

EPA. 1998. 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 3.0. National Exposure Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.

EPA. 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. D.C.

EPA. 2001. 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, D.C.

USDA. 1990. Davis, F.M., R.A. Leonard, W.G. Knisel. GLEAMS User Manual, Version 1.8.55. USDA-ARS Southeast Watershed Research Laboratory, Tifton GA. SEWRL-030190FMD.

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).

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