Using coupled hydrogeophysical modeling to assess the likely value of proposed gravity observations to support water resources decision making

Tristan E. Dicke and T.P.A. Ferré
Department of Hydrology and Atmospheric Sciences
The University of Arizona

Decisions regarding the permitting of new groundwater extractions often depend on the perceived impact of those withdrawals on groundwater levels in wells and streamflow. These decisions can be informed using an ensemble modeling approach, which quantifies both the most likely outcome and the associated uncertainty given limits on subsurface hydrogeologic information. Additional data can constrain forecasts and reduce risk, thereby improving decision making. Groundwater levels in wells are one of the most common hydrologic measurements, but it can be prohibitively expensive to drill wells to add new observation points to inform decision making. Time-lapse gravity measurements provide a proxy method to gain insight into the subsurface hydrologic conditions. While gravity measurements are less direct than groundwater levels, it can be considerably less expensive to add monitoring points.  In this study, an ensemble of MODFLOW models is developed for a hypothetical catchment. Forecasts of drawdown in one well due to the addition of another well are the prediction of interest for decision making. The accuracy and uncertainty of the forecasts are calculated with and without added observations (water levels or gravity change). The result is a map of the basin showing the relative value of observations of each type at each location for improving the prediction of interest. This map can be used to choose between water levels and gravity change observations on the basis of effectiveness and cost. The same approach could be extended to consider multiple measurements of both types or other available observation modalities.

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