A Tool for Dynamically Estimating Groundwater Recharge

Ensuring sustainability of groundwater supplies in the face of changing environmental demands, such as climate and land use, is a challenge for all water purveyors relying on groundwater.  Many basins operate on the premise of an average safe yield number.  During periods of drought, pumping the average safe yield will stress the aquifer if the amount pumped exceeds groundwater recharge.  By pumping the average safe yield, the hope is that over the long-term, the basin will be replenished during high rainfall years.  Operating a basin this way can have problems, especially during dry years when groundwater levels can be lowered to the point of damaging wells and pumps.  More reliable recharge estimates are necessary for planning.  Estimating groundwater recharge near the end of each water year based on that year’s rainfall gives basin managers the upper hand when planning the following year’s production.

Water purveyors in the Soquel-Aptos basin, Santa Cruz County, California needed data to assist them making groundwater management decisions.  Specifically, they needed to justify pumping curtailments during low rainfall periods and to plan for recharge impacts related to future land use changes.  They wanted a tool that would use rainfall to estimate groundwater recharge in real-time.  The USGS’s Precipitation-Runoff Modeling System (PRMS) was selected as the tool to determine the rainfall-recharge relationship for the basin.

PRMS simulates streamflow from precipitation, after evapotranspiration, and groundwater recharge are accounted for.  Model input is daily climatic data: precipitation and temperature.  Hydrologic and physical characteristics are assigned to the model area. Calibration using average monthly solar radiation, potential evapotranspiration, and daily streamflow as targets improved model credibility.  

The main observation from the modeled hydrologic response was that as precipitation increases, recharge increases proportionally faster.  The significance to basin managers is that in a series of average rainfall years, basin recharge decreases.