NUMERICAL MODELING OF THE EFFECTS OF LAND USE CHANGE AND IRRIGATION ON STREAMFLOW DEPLETION OF FRENCHMAN CREEK, NEBRASKA

A three-dimensional Control Volume Finite Difference-based numerical groundwater flow model was constructed to assess the effects of agricultural irrigation and land use change on streamflow depletion. The study area is Frenchman Creek basin located in southwestern corner of the State of Nebraska, USA. This area was subject to an increased proliferation of groundwater abstraction for agricultural purposes since industrial revolution. It has also been subject to land use change from native rangeland to dry and irrigated cropland. The groundwater flow model was spatially discretized using Voronoi cells in unstructured grid built with the USGS MODFLOW-6. Temporal discretization defined 151 time steps with varying lengths and organized in 76 non-growing season time steps alternating with 75 growing season time steps and covering a period of 75 years. Results show that the combined application of irrigation and land use change over the study area consumed up to 98% of a portion of groundwater that would overwise discharge under the influence of ambient groundwater flow as baseflow to Frenchman Creek. A run of the model in base conditions, which consists of maintaining land use and irrigation constant from early stage, shows that anthropogenic activities curtailed the amount of groundwater discharge to evapotranspiration. The study also shows the advantage of using un-structurally discretized numerical model over previously developed analytical model by Traylor and Zlotnik in accounting for aquifer heterogeneity as well as spatial and temporal changes in transmissivity. Advisor: Vitaly A Zlotni