Sub-Regional Groundwater Flow Modeling of the Upper Big Blue Basin Using the MODFLOW-OWHM Farm Process

The Upper Big Blue Natural Resource District and Brown and Caldwell constructed and calibrated a three-dimensional subregional groundwater flow model of a portion of the Big Blue River Basin in southeastern Nebraska using MODFLOW-OWHM including the Farm Process Version 3 (FMP3) (Hanson et al., 2014).  The Farm Process simulates the interactions of agricultural processes with groundwater and surface water including crop uptake from soil water (variably saturated to fully saturated), crop demand-driven pumping of groundwater and/or diversion of surface water for irrigation, and return flows to both groundwater as deep percolation and surface water as surface runoff.  The subregional model simulates the period from 1950 through 2010 with the six different crop types grown within the model domain with the crop mix and irrigation patterns determined each decade based on National Agricultural Statistics Service data and information from the Nebraska Department of Natural Resources groundwater well database.  In the subregional model the Farm Process is linked to the Multi-Node Well Package to provide irrigation from groundwater, the Unsaturated Zone Flow Package to simulate the timing of deep percolation reaching the water table, and the Streamflow Routing Package to receive both surface water runoff and baseflow arising from groundwater.  The model was calibrated over the simulation period to observed groundwater levels at 81 water level monitoring locations, and to both total gauged streamflows and estimated stream baseflow values at 4 stream gauging locations. The model is currently being used as a tool to delineate “10/50 zones”, which are those areas in which pumping a groundwater well over a 50 year period is predicted to result in a 10 percent or greater reduction of baseflow in adjacent streams. Future planned work includes construction and calibration of a regional-scale model of the Blue River Basin within Nebraska.