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Case Study

Estimating water quality improvement from cover crops: Protecting water quality and increasing food security

The Indian Creek Watershed was selected for this project because it is an agriculturally-dominated watershed that would showcase positive water quality impacts from cover crops. In addition, it is a priority sub-watershed, as defined by the Illinois State EPA. A popular and well-tested numerical model, called SWAT (Soil Water Assessment Tool) that was developed by the USDA-ARS, was used to simulate 25 years of growing seasons.  Historical weather data from the Indian Creek watershed was used to make sure results reflected appropriate and realistic year-to-year trends. Using SWAT, we tested the impact of cover crops to reduce off-field nitrogen transport assuming (figure 1):

  • No cover crops (called a baseline)
  • 10% cover crops in the watershed
  • 25% cover crops in the watershed
  • 50% cover crops in the watershed and
  • 100% cover crops in the watershed

Water-quality-cs_cover-crop

Figure 1. Each map shows the areas of the watershed that was assumed to have cover crops.

We expected that when we simulated the least amount of cover crop acres (10% of the watershed), the smallest improvement in nitrogen loss would be seen, compared to the scenario with no cover crops (baseline) and vice versa. The modeled trend in all 4 of the cover crop cases (10%, 25%, 50%, and 100% cover crops in the watershed) showed an increase in nitrogen loss reduction (better and better in-field retention of nitrogen from cover crops) and confirmed our expectations (figure 2).

 

These modeled results also showed a slight reduction in water leaving the field (water yield) because the cover crop used it instead of it washing nitrogen out of the soil. Finally, crop yield was also estimated to increase as cover crop acres increased in the model, indicating that nitrogen that was carried over in the biomass of the cover crop from the fall to the spring was able to be used by the spring crop. This project is important because it demonstrates that cover crops may be a viable, long-term solution to improving water quality if planted in enough acres.

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