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: (a) continuous corn; (b) corn/soybean rotation; and (c) corn/wheat rotation to assess the impact. The SWAT model results showed: (a) 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; (b) 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 (c) 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.

中文翻译：

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使用土壤和水评估工具评估近期作物扩张对密苏里河流域水质的影响

密苏里河流域 (MORB) 经历了草地转为作物生产的复苏，这引发了人们对水质的担忧。我们应用土壤和水评估工具 (SWAT) 来解决这种转换将如何影响水质。我们设计了三种作物生产情景，代表草地转化为：（a）连续玉米；(b) 玉米/大豆轮作；(c) 玉米/小麦轮作以评估影响。SWAT 模型结果显示：（a）较低的 MORB 在转化前（基线）产生了较高的总氮（TN）和总磷（TP）负荷，这主要是由于高降水量和高农业活动；(b) TN 和 TP 的最大百分比增加发生在北达科他州和南达科他州，与草地转化为农田的数量最多相吻合；(c) 草地转化为连续玉米导致 TN 和 TP 负荷增加最大，其次是转化为玉米/大豆，然后转化为玉米/小麦。尽管 TN 和 TP 的最大百分比增加发生在北达科他州和南达科他州，但这些地区在转换后对流域总负荷的贡献仍然相对较低。然而，主要位于较低 MORB 的流域仍然是“热点”，它们对流域总负荷贡献了最大量的 TN 和 TP——这是由草地转化、高降水和预先存在的农田负荷共同驱动的。在流域出口，在 2008-2016 年期间，TN 和 TP 负荷分别增加了 6.4%（13,800 吨/年）和 8.7%（3,400 吨/年），以转换为连续玉米情景。