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SWAT Model Application to Assess the Impact of Intensive Corn‐farming on Runoff, Sediments and Phosphorous loss from an Agricultural Watershed in Wisconsin
Citation:
Mbonimpa, E. G., Y. YUAN, M. H. MEHAFFEY, AND M. JACKSON. SWAT Model Application to Assess the Impact of Intensive Corn‐farming on Runoff, Sediments and Phosphorous loss from an Agricultural Watershed in Wisconsin. JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT. American Society of Civil Engineers (ASCE), Reston, VA, 4:423-431, (2012).
Impact/Purpose:
Nutrient runoff from US farm lands has been linked to deterioration of water quality in water bodies downstream, especially lake eutrophication and estuarine hypoxic zones [1]. As of 2004, the USEPA reported that 44% of assessed stream length, 64% of assessed lake area, and 30 % of estuarine area were impaired to a level unsuitable to support fish and unsuitable for swimming [2]. High oil prices, the desire to reduce the US dependency on foreign oil and potential increase in US agricultural profitability have triggered a need to generate biofuels from annual crops, especially corn. From 2002 to 2006 alone US ethanol production increased 125% [3]. This corn intensive farming could potentially exacerbate water, air and soil pollution since corn requires higher amounts of agricultural chemicals (nitrogen and phosphorus fertilizers, and pesticides) and tillage than some other crops [4]. In the Midwestern US many lakes suffer from excessive growth of harmful algae, believed to be promoted by excessive loss of phosphorus (P) from agricultural farms [5]. Control and reduction of P could reduce eutrophication in lakes since P is a growth-limiting nutrient for harmful algal blooms [6] [7]. The amount of P loss from farms to streams is believed to depend mainly on P content of surface soil, type of soil, tillage and crop management [8]. To reduce pollution of water bodies, regulatory agencies such as USEPA and USDA-NRCS promote agricultural best management practices (BMPs) to reduce non-point source pollution: soil erosion, nutrient and pesticide runoff [9]. Studies for various structural (e.g., buffers) and non-structural (e.g., fertilizer management) management practices to control loss of sediment and nutrients have shown mixed results. Some management practices have proven effective at controlling one pollutant while promoting the loss of another. For instance, Heatwaite et al. (2000) indicated that management of manure to control nitrogen (N) has increased soil P and generated higher P in runoff [10]. Also, they noticed that the no-till method recommended to reduce erosion and P loss could cause increased nitrate leaching. Vegetative buffers, wetlands and management of fertilizer and manure have been suggested by prior studies to be more effective in reducing sediment and P loss into streams [11] [12]. In addition, crop rotation (for instance corn-soybean instead of continuous corn) is known to reduce sediment, nutrient and pesticide loss [13]. Soybean has the ability to fix N and requires low amounts of P fertilizer. Soybean improves soil texture, making it an excellent candidate for no-till, and has low weed and insect nuisance which reduces herbicide and insecticide use. Also, soybean fields dry faster, which in wet areas helps prepare land for corn planting [14].
Description:
The potential future increase in corn-based biofuel may be expected to have a negative impact on water quality in streams and lakes of the Midwestern US due to increased agricultural chemicals usage. This study used the SWAT model to assess the impact of continuous-corn farming on sediment and phosphorus loading in Upper Rock River watershed in Wisconsin. It was assumed that farmers in the area where corn was rotated with soybean would progressively skip soybean for continuous corn as corn became more profitable. Simulations using SWAT indicated that conversion of corn-soybean to corn-corn-soybean would cause 11% and 2% increase in sediment yield and TP loss, respectively. The conversion of corn-soybean to continuous corn caused 55 % and 35 % increase in sediment yield and TP loss, respectively. However, this increase could be mitigated by applying various BMPs and/or conservation practices such as conservation tillage, fertilizer management and vegetative buffer strips. The conversion to continuous corn tilled with conservation tillage reduced sediment yield by 2% and did not change TP loss. Increase in P fertilizer amount was roughly proportional to increase in TP loss and 11% more TP was lost when fertilizer was applied four months before planting. Vegetative buffer strips, 15 to 30 m wide, around corn farms reduced sediment yield by 51 to 70 % and TP loss by 41 to 63%.