Rathbun Lake Watershed assessment and water quality implications of switchgrass biomass production

Neppel, Jerome
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Land use distribution by subbasin was analyzed to obtain the maximum acreage of forest with the minimum number of hydrologic response units (HRUs). The soils threshold for the HRUs was selected based upon experience. Using average annual stream discharge, the model was calibrated for 1966-1986 and validated for 1987-1999. The model ranked the 61 subbasins on their relative production of sediment yield and nitrogen, phosphorus, and atrazine loading. In general, subbasins that ranked the highest had a high percent of row cropland and 4-5% average subbasin slope. Growing switchgrass for biomass was shown to have several environmental benefits. A switchgrass scenario defined as growing switchgrass on approximately 38% of the row crop area, reduced sediment yield and nutrient loading more than a third compared to the baseline (current conditions) scenario. The quantity of sediment-bound atrazine delivered to Rathbun Lake is predicted to be reduced 84%.Rathbun Lake is a 4,455-hectare multipurpose water resource in southern Iowa. Its long-term ability to meet all of its designated uses is threatened by excessive siltation, nutrient enrichment, and pesticide runoff. A comprehensive watershed assessment is necessary to identify the sources and locations of these pollutants. The Soil and Water Assessment Tool (SWAT) was selected for this study. The two objectives were to: (1.) rank the 61 subbasins of Rathbun Lake Watershed as to their relative environmental impact on runoff water quality, and (2. evaluate the runoff water quality implications of using switchgrass for biomass production. The ArcView[Registered trademark symbol] SWAT interface version 1.601 was selected for this study. The ArcView[Registered trademark symbol] geographic information system (GIS) was desired to be used to demonstrate the utility of this technology and to automate the data entry workload. The digital elevation model (DEM), land use/land cover, and soils GIS coverages were obtained from government agencies. Weather, crop, fertilizer, and pesticide database information was supplied by the SWAT model or were obtained through literature review, subject matter experts, or existing site-specific databases. Management practice schedules were obtained by interviewing watershed farmers and local agency personnel familiar with farming practices in the watershed.