Citation:

Yuan, Y. AND P. Chen. SWAT Model Application for Assessing Recent Crop Expansion Impacts on Missouri River Basin Water Quality. 2023 International SWAT Conference & Workshop, Aarhus, N/A, DENMARK, June 26 - 30, 2023.

Impact/Purpose:

Water bodies and coastal areas around the world are threatened by excessive amounts of nitrogen (N) and phosphorous (P) from upstream watersheds, which can cause rapid proliferation of algae. These algal blooms negatively impact drinking water sources, aquatic species, and recreational services of water bodies by producing toxins, also called harmful algal blooms (HABs). Understanding how land use/land cover changes impact water quality is paramount important for EPA program offices and regional partners to make informed decisions to better control nutrient losses from agricultural fields. This is a large-scale environmental assessment to address concerns raised on water quality due to cropland increase of corn and soybean production from 2008 to 2016, which has displaced grasslands in the Missouri River Basin.

Description:

The Missouri River Basin (MORB) has experienced a resurgence of grassland conversion to crop production, due partially to the increased biofuel feedstock production of corn and soybeans. 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 impacts. The model setup, calibration and validation were time-consuming and challenging for this large system. To balance the need for capturing the detailed land use changes and computational requirement, 8-digit hydrologic units (HUC8s), defined by the USGS, were selected as SWAT subbasins. We quantified water quality changes resulting from the recent land use shift for the entire MORB and identified “hotspots”, or watersheds, that experienced the greatest increase in nutrient loadings. 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. Watersheds predominantly in the lower MORB, however, continued to be “hotspots” that contributed the greatest amounts of TN and TP to total basin loads—driven by a combination of grassland conversion, high precipitation, and loading from pre-existing cropland. At the watershed outlet, 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 conversion to the continuous corn scenario. This information is critical to ensure proper decision making for watershed protection.

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