Evaluation and prediction of hydrology and nitrate-nitrogen transport in tile-drained watersheds

Implementation of artificial subsurface drainage (tile drainage) for cultivation of row crops in poorly-drained areas of the Upper Midwest of the United States has enabled the region to be one of the most agriculturally productive areas of the world; but has also resulted in loss of wetland ecosystems, altered hydrology, and increased transport of nitrate-nitrogen (NO3-N) to surface water. The direct link between subsurface tile drainage and transport of nonpoint-source pollutants, particularly NO3-N, to surface waters is a primary concern for downstream drinking water supplies and hypoxia in the Gulf of Mexico. The studies described in this dissertation include evaluation of NO3-N export from small, tile-drained watersheds typical of agricultural drainage districts on the Des Moines Lobe ecoregion of Iowa, evaluation of watershed-scale simulation of hydrology and NO3-N transport at the daily interval using the Soil and Water Assessment Tool (SWAT), investigation of important nitrogen pathways and processes simulated in SWAT, and the evaluation and improvement of SWAT algorithms for simulating water quality treatment wetlands in this landscape. Specific objectives of the first study were to quantify hydrology and NO3-N export patterns from three tile-drained catchments and the downstream river over a 5-yr period, compare results to prior plot-, field-, and watershed-scale studies, and discuss implications for water quality improvement in these landscapes. The tile-drained catchments had an annual average water yield of 247 mm yr, a flow-weighted NO3-N concentration of 17.1 mg L, and an average NO3-N loss of nearly 40 kg ha yr. Overall, water yields were consistent with prior tile drainage studies in Iowa and the upper Midwest, but associated NO3-N concentrations and losses were among the highest reported for plot studies and higher than those found in other small watersheds. More than 97% of the NO3-N export occurs