Integrated Management of Groundwater and Surface Water Resources in the Bates Creek Alluvial Aquifer near Casper, Wyoming

Management of conjoined surface and ground waters in alluvial aquifers within the prior appropriation water rights doctrine requires predictive abilities of the influence of pumping of individual wells on streamflow.   This knowledge is needed by regulatory bodies who must decide when junior groundwater rights are infringing on senior surface water rights.   A study was performed using MODFLOW to simulate the coupled streamflow and subsurface hydraulics of the Bates Creek irrigation area southwest of Casper, Wyoming.  The setting is unique considering stream flows in Bates Creek are typically small (< 1 m³/s), while the volume of storage in the alluvium is approximately 15 years of average annual streamflow.  Furthermore, annual potential evapotranspiration is over twice annual precipitation.   The influence of irrigation-induced recharge is important but highly uncertain in magnitude.   In this study response functions were developed to identify the influence of individual well pumping on surface streamflows both in terms of quantity of induced stream bed infiltration and lag time from the onset of pumping to the onset of induced infiltration, as well as the lag time from pumping cessation to reduction in induced infiltration.   These response functions were combined in an optimization framework to explore the maximum pumping that could be achieved by junior groundwater right holders while insuring senior surface water right holders will receive their appropriation.  In addition, the sensitivity of the results to assumed rates of recharge due to surface irrigation was explored.