Effects of Precipitation on Nonpoint Sources of Nitrogen Contamination to Surface Waters in the U.S. Great Plains

Declining surface water quality is of great concern across the Great Plains. Recent trends in the earth’s climate can create abrupt changes in precipitation, which can alter the impact of nonpoint sources on water quality. A 2-year study dry (2009) and wet (2010) year] was initiated to assess the impact of nitrate nitrogen (N) loss from the Roca watershed on water quality in Salt Creek. The water flow and nitrate N concentration was determined weekly in Salt Creek. The predicted average nitrate N concentration in runoff during the dry year (38.3 mg L^?1) was almost five times greater than that (7.9 mg L^?1) for the wet year. However, the predicted amount of nitrate N in runoff was similar for both years because the runoff for the wet year (51.8 million m³) was about five times greater than that for the dry year (10.7 million m³). The total amount of nitrate N found in Salt Creek was 18 and 127 metric tons for the dry and wet years, respectively. These data implied that 95% (dry year) and 69% (wet year) of the nitrate N has been removed from streams water in Salt Creek. Factors responsible for removing nitrate N from water include heavy growth of algae, weeds, and aquatic plants as well as denitrification and volatilization reactions. The predicted amount of nitrate N lost from soils by leaching was almost seven times greater for the wet (1,037 metric tons) than the dry year (156 metric tons). It was concluded that high precipitation for the wet year raised both the amount of nitrate N in runoff and loading into Salt Creek and could increase the negative impact on water quality.