Agroecosystems /
Carbon Sequestration
Research Focus
Iowa Final Report
Phase I: Methodology
Phase I: Methodology
Data on climate, soil, land-use and management practices were assembled from existing databases for each county as input to the Century model analysis. In the initial Phase I analyses, long-term mean climate records (1960-1990) were obtained from National Weather Station sites located in each county. Monthly temperature (mean monthly maximum and minimum) and precipitation (monthly total) were developed for each county. In subsequent re-analyses, we utilized the Parameter-Elevation Regressions on Independent Slopes Model (PRISM) monthly climate variables described in the Phase II Century Modeling section in this report (Daly et al., 1994).
County-level soil attributes were derived from the State Soil Geographic Database (STATSGO) at the component level (i.e., soil series) within soil associations (SCS, 1994). For each county, area-weighted frequency distributions of soil types were determined based on the relative proportion of component soils within each soil association. Soil types for application in the model were grouped according to surface texture (0-20 cm) and classification as hydric or non-hydric (poorly drained or well-drained soils, respectively). If a particular soil component accounted for less than 4% of the area of the county, it was not included in the analysis. In subsequent re-analyses, we included all soils that represent an area greater than 120 hectares (~300 acres) as described in the Phase II Century Modeling section in this report.
Land-use and management data were compiled from a variety of other sources including data on Conservation Reserve Program (CRP) contract acreage (USDA/FSA – A. Barbarika, personal comm.); state and county acres for crops grown by year compiled by the National Agricultural Statistics Service (Figure 12, annual state totals prior to 1943 and county totals since 1943, NASS, 1999); area by tillage practice and crop compiled by the Conservation Technology Information Center (CTIC, 1998); and field operation scheduling and fertilizer use (provided by NRCS State office in Iowa). Data on manure applications were derived from a 1974 report (USDA Economics, Statistics, and Cooperatives Service, 1978) that reported rates, manure N content, and percent of economically recoverable manure by county. These were related to livestock numbers for 1974 in order to get estimates of the manure applications as a function of livestock numbers. We then used NASS reports of annual hog and cattle numbers, to estimate the amount of manure produced and applied to cropland for the other periods of the simulation. We assumed that manure was applied to second-year corn or to first-year corn in the case of corn-soybean rotations.
Figure 12
Figure 13
Table 1: Cropping systems by management zones.
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Assumptions for tillage, fertilizer use, and harvest were based on the sources cited above, and shown in Table 2. These management scenarios were then run for each county (within an NRCS management zone), and for each of the soil types within each county, as derived from the STATSGO analysis using the mean climate data for the county.
Table 2: Tillage, fertilizer use, and harvest method for the State of Iowa.
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