Impact Of Agricultural Land Use On Stream Nitrate Phosphorus And Sediment Concentrations At The Watershed And Field Scale

Water quality is directly impacted by the landscape through which it travels. As such, land use, including summer annual and winter annual/perennial agriculture, has dramatic influence on the water quality of downstream aquatic and terrestrial ecosystems. I examined the impact of agricultural land use on water quality through two projects, one at a watershed scale and one at a field scale. In my first project, I investigated the impact of agricultural land use and climate on water quality in 13 HUC10 watersheds across Nebraska using public data from US Geological Survey (USGS), US Department of Agriculture National Agricultural Statistics Service (USDA-NASS), and National Oceanic and Atmospheric Administration (NOAA). I focused on spring concentrations of nitrate, phosphorus, and suspended sediment in streams from 1980-2017. Results showed that each of the pollutants is impacted differently by agricultural land use and climate. Watersheds with higher percentages of summer annual (corn and soybean) acres generally had higher and more variable concentrations of pollutants. Additionally, watersheds with lower percentages of summer annual acres and higher percentages of grassland/pasture were found to have consistently lower pollutant concentrations across flood and drought conditions. In the second project, my main objective was to create a field scale sampling protocol using rainfall simulators to investigate the impact of riparian area runoff on stream chemistry. Using a conservative tracer in the "rain" water, I was able to confirm the effectiveness of the proposed rainfall simulator protocol as a method for investigating riparian runoff impact on stream chemistry and pilot the protocol in the riparian areas of summer annual and grassland fields. Results of water quality analysis found that stream chemistry constituents (nitrogen, phosphorus, and sediment) increased during the rainfall simulation, indicating that the runoff generated carried additional nutrients and sediment into the stream. Overall, these results from both the field and watershed scale suggest that variability in water quality under summer annual is higher than in perennially-based land uses.