Modeling the effects of agricultural management practices on groundwater quality and quantity

The purpose of this research was to develop a methodology to assess the impact that changes in groundwater recharge, nitrate leaching from the root zone of crops, and pumping rates for irrigation have on groundwater quality and quantity.; A vadose zone model (VZM) was developed and linked to existing models for groundwater flow (MODFLOW) and solute transport (MT3D) to simulate the movement of nitrate from the root zone to and through the groundwater system.; VZM was developed from a solution of the Richards Equation, using the method of decomposition. The equation was separated into its linear and non-linear components. An analytical solution was found for the linear component while a series solution was used to solve the non-linear components. This method produced an approximated analytical solution that underestimated the vertical infiltration velocity. An iteration procedure was necessary to select the surface boundary condition for the vadose zone so that infiltration into the vadose zone matched deep percolation from the crop root zone.; The inputs to VZM are values of deep percolation and nitrate leaching. These were calculated with an external watershed-plant model. VZM simulates the transport of water and nitrate through the vadose zone, calculating the volume of water and the mass of nitrate that reach the groundwater. With that information, VZM generates the recharge files used by MODFLOW and MT3D.; The methodology was applied to an area in the Central Platte Valley of Nebraska, near the town of Shelton. The groundwater system in the area is a shallow, sand-gravel aquifer that is highly contaminated by nitrate-nitrogen. The groundwater flow model was calibrated using historic groundwater level data for 1981–1996.; The combined models were used to predict changes in groundwater quality resulting from reduced nitrate leaching at a farm scale. Simulations showed concentration reductions on the order of 10 percent in the upper third of the groundwater body after 10 years of improved irrigation and nitrogen fertilizer management.