Climate Change Impacts on Groundwater: More Than Just Drought

Most professionals recognize that a lack of precipitation (AKA drought) will impact groundwater recharge. It may be less known that the proportionality between changes of precipitation and recharge is typically not the same, i.e., 10% less precipitation would not yield 10% less recharge. To illustrate this distinction even further, we will show several precipitation climate examples which produce the same amount of annual precipitation, but when applied to hydrology models they surprisingly yield quite different reductions of recharge.

We have computed statistically significant trends in precipitation timing patterns from the decades of daily climate observations from many California stations. Applying just these timing trends to basin models, but without any change in total precipitation, we show how they surprisingly produce substantial recharge reductions, e.g., nearly a 7% recharge decrease over the next 30 years.

Changes of temperature alone can cause substantial impacts on groundwater recharge. Every degree Fahrenheit of temperature increase causes a 4% increase of evaporation. Since the ET fraction of total precipitation is often quite high at 50%–70%, then even a moderate increase in that evaporation would represent a considerable water loss. This is demonstrated by applying the predicted 7°F temperature increase to California groundwater basins to show those substantial impacts on groundwater recharge.

The predictions of California sea level rise are as much as 4½ feet. Coastal basins can only be protected from the additional seawater intrusion induced by this sea level rise by a corresponding rise of inland water levels. Such a protective action can be translated into a virtual loss of recharge, e.g., for a small coastal basin a loss of 500af per year for the next 85 years.