The Missouri River Basin (MORB) has experienced a resurgence of grassland conversion to crop production, which raised concerns on water quality. We applied the Soil and Water Assessment Tool (SWAT) to address how this conversion would impact water quality. We designed three crop production scenarios representing conversion of grassland to: 1) continuous corn; 2) corn/soybean rotation; and 3) corn/wheat rotation to assess the impact. The SWAT model results showed: 1) the lower MORB produced high total nitrogen (TN) and total phosphorus (TP) load before conversion (baseline) due mainly to high precipitation and high agricultural activity; 2) the greatest percentage increases of TN and TP occurred in the North and South Dakotas, coinciding with the highest amount of grassland conversion to cropland; and 3) grassland conversion to continuous corn resulted in the greatest increase in TN and TP loads, followed by conversion to corn/soybean and then conversion to corn/wheat. Although the greatest percentage increases of TN and TP occurred in the North and South Dakotas, these areas still contributed relatively low TN and TP to total basin loads after conversion. However, watersheds, predominantly in the lower MORB continued to be "hotspots" that contributed the greatest amounts of TN and TP to the total basin loads-driven by a combination of grassland conversion, high precipitation, and loading from pre-existing cropland. At the watershed outlet, the TN and TP loads were increased by 6.4% (13,800 t/yr) and 8.7% (3,400 t/yr), respectively, during the 2008 -2016 period for the conversion to continuous corn scenario.

Abstract. The United States Corn Belt region, which primarily includes two large basins, namely, the Ohio-Tennessee River Basin (OTRB) and the Upper Mississippi River Basin (UMRB), is responsible for the Gulf of Mexico hypoxic zone. Climate patterns such as El Niño can affect the runoff and thus the water quality over the Corn Belt. In this study, the impacts of eastern Pacific (EP) and central Pacific (CP) El Niños on water quality over the Corn Belt region were analyzed using the Soil and Water Assessment Tool (SWAT) models. Our results indicated that at the outlets, annual total nitrogen (TN) and total phosphorus (TP) loads decreased by 13.1 % and 14.0 % at OTRB, 18.5 % and 19.8 % at UMRB, respectively, during the EP-El Niño years, whereas during the CP-El Niño years, they increased by 3.3 % and 4.6 % at OTRB, 5.7 % and 4.4 % at UMRB, respectively. On the sub-basin scales, more sub-basins showed negative (positive) anomalies of TN and TP during EP- (CP-) El Niño. Seasonal study confirmed that water quality anomalies showed opposite patterns during EP- and CP-El Niño years. At the outlet of OTRB, seasonal anomalies in nutrients matched the El Niño-Southern Oscillation (ENSO) phases, illustrating the importance of climate variables associated with the two types of El Niño on water quality in the region. At the UMRB, TN and TP were also influenced by agriculture activities within the region and their anomalies became greater in the growing seasons during both EP- and CP-El Niño years. Quantitative analysis of precipitation, temperature, and their effects on nutrients suggested that precipitation played a more important role than temperature did in altering water quality in the Corn Belt region during both types of El Niño years. We also found specific watersheds (located in Iowa, Illinois, Minnesota, Wisconsin, and Indiana) that faced the greatest increases in TN and TP loads, were affected by both the precipitation and agricultural activities during the CP-El Niño years. The information generated from this study may help proper decision-making for water environment protection over the Corn Belt.