Use of a Groundwater Model to Study Effects of Groundwater Pumping On Surface Water Resources in the Middle Verde River Watershed, Arizona

Groundwater is an important primary source of water for the residents of the Verde River Watershed and is an essential component among the water sources that sustain the base flow of the Verde River and its associated riparian ecosystem. The USGS Northern Arizona Regional Groundwater Flow Model provides a tool to aid stakeholder understanding of the consequences of various groundwater management scenarios on surface-water resources in the watershed. The model was constructed using the USGS MODFLOW code. In this area, the upper two model layers represent groundwater flow predominantly in the Verde and Supai Formations, in which most groundwater pumping occurs. The model can simulate effects of groundwater pumping on connected features including the Verde River and its tributaries, springs, and riparian plants. To study effects of groundwater pumping on surface water, the model was modified to start with initial predevelopment steady-state conditions followed by a 100-year transient period simulated using uniform time steps. Multiple model runs are made to show effects of pumping in the upper two model layers on surface water for different hypothetical pumping locations throughout active areas of these layers in the basin. Effects of pumping on surface water is computed as the difference in groundwater flow to or from head-dependent boundaries for the steady-state run without pumping and the transient run with pumping. These calculations are made for each time step in the transient run, thereby providing an understanding of the timing of effects of pumping for the location of the pumping well. For a particular pumping time of interest, effects of pumping on surface water can be mapped to provide an understanding of effects of pumping location on surface water.