Predicted Impacts of Conjunctive Water Management on Late Summer Streamflow in an Agricultural Groundwater Basin with Limited Storage, Scott Valley, CA
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Late summer streamflow for the Scott River in northern California has decreased approximately 50% since the mid 1960’s, resulting in increased water temperatures and disconnection of the stream. This negatively impacts aquatic habitat of fish species such as coho and fall-run Chinook salmon. In collaboration with local stakeholders, the Scott Valley Integrated Hydrologic Model has been developed, which combines a water budget model and a groundwater-surface water model (MODFLOW) of the 200 km2 basin. The goal of the integrated model is to better understand the hydrologic system of the valley and explore effects of different conjunctive management scenarios on streamflow during the critically dry months (Aug-Oct). The groundwater model has a 100 m lateral resolution with aggregated monthly stresses over a 21 year simulation period (1990-2011). The Scott River and tributaries are represented using the streamflow routing (SFR) package. Sensitivity analysis and calibration were performed using 812 head observations from 50 wells in the basin and average daily streamflow observations from a USGS stream gauge during the simulation period. The calibrated model was used to evaluate two different management scenarios: 1) in-lieu recharge where surface-water instead of groundwater is used to irrigate fields near the river while streamflow is sufficiently high, and 2) managed aquifer recharge during the winter months (Jan-Mar) on agricultural fields located in gulches on the eastern side of the valley using existing infrastructure. Preliminary results indicate that managed aquifer recharge may increase streamflow during the critically dry months at the basin outlet by 1-2 cubic feet per second (cfs), while in-lieu recharge may increase flows during the same period by 10-30 cfs. The greater increase in flow from the in-lieu recharge scenario is largely due to reductions of groundwater pumping by 10-25% from the base case scenario, with greater pumping reductions during wetter years. This increase in flow during the critically dry months decreases the length of dry reaches both spatially and temporally, allowing for earlier reconnection of the Scott River and decreased stress on fish.