Abstract
In California, groundwater is a critical resource that supplies 40-60% of the state's fresh water supply. Groundwater will become even more valuable as a reliable, year-round water source as climate change creates greater uncertainty in the surface water supply. Higher temperatures will cause more precipitation to fall as rain rather than snow and the snowpack to melt earlier and faster, leaving less fresh surface water available for irrigation in the summer. Climate change may also alter groundwater recharge by changing precipitation patterns and river flow, and increasing evapotranspiration. We modeled groundwater-surface water interaction in the Sacramento region to investigate the effect that climate change will have on groundwater supply. Sacramento County is home to the capitol of California and generates over $500 million annually in agriculture. The model was calibrated with field data including mean groundwater ages, recharge temperatures, stable water isotopes, and historic groundwater level data. The model simulates groundwater recharge through the year 2100 for four different downscaled climate projections from a CO (sub 2) emissions scenario where emissions peak in 2040 (Cal-Adapt.org). Future work will include modeling a more pessimistic CO (sub 2) emissions scenario in which emissions rise strongly until 2050 and plateau in 2100. Model results indicate that overall groundwater recharge will vary by 4.5% depending on the climate projection, with recharge from precipitation varying by 30% and recharge from the lower American River varying by 2.5%. These results suggest that climate change could create new challenges for groundwater sustainability in the Sacramento area over the next century. In an already tight groundwater budget, a 4.5% difference in natural recharge could necessitate offsets through adaptive groundwater management such as artificial recharge and demand reduction by pumping restrictions.