An unconfined groundwater model of the Death Valley Regional Flow System and a comparison to its confined predecessor

SummaryThe MODFLOW version of the United States Geological Survey (USGS) Death Valley Regional Flow System (DVRFS) in California and Nevada is conceptually inaccurate in that it models an unconfined aquifer as a confined system and does not accurately simulate unconfined drawdown in transient pumping simulations. The transfer of geologic and hydrologic information from the confined MODFLOW DVRFS model to an unconfined MODFLOW–SURFACT (SURFACT) version was accomplished by maintaining cell structure between models and computing effective cell properties to translate the HUF2 package used in MODFLOW to the BCF4 package used by SURFACT. The confined version of the DVRFS was compared to the unconfined SURFACT version by examining head contour maps and the ability of the SURFACT model to match the 4900 observations of hydraulic head/drawdown, 49 observations of groundwater discharge, and 15 estimates of groundwater fluxes into/out of the model domain. Resultant weighted root mean squared error (ωRMSE) for the unconfined SURFACT model was lower than the USGS confined model. Despite a lower ωRMSE, unconfined conditions simulated with SURFACT did produce greater heads in mountainous regions compared to the confined MODFLOW with differences most pronounced in regions where cell thickness is large, horizontal conductivity small and recharge large. Difference in computed heads reflects computation schemes employed by both models to estimate interblock conductance. Specifically, interblock conductance for the unconfined SURFACT model is dependent on the relative saturation of a modeled cell while MODFLOW’s confined system is not. Despite head differences, SURFACT simulates comparable flux estimates to MODFLOW (e.g. observed ET, groundwater spring flow, and groundwater flux across model boundaries), while significantly improving transient well drawdown estimates. SURFACT is also capable of producing more realistic estimates of water availability from proposed groundwater development and resultant potential impacts to the region.