ESTIMATING THE CONTRIBUTION TO GROUNDWATER RECHARGE FROM STREAMFLOW ORIGINATING ON NATIONAL FOREST SYSTEM LANDS FOR THE CENTRAL VALLEY, CALIFORNIA

The Pacific Southwest Region of the United States Forest Service is actively working to restore and maintain source watersheds on National Forest System (NFS) lands. Resilient watersheds are critical to the long-term security of California’s utilized water supply. Water contributions from NFS lands to downstream areas must be quantified to demonstrate the value of NFS watersheds to distant beneficiaries. Modeling work conducted for the contiguous United States by the Rocky Mountain Research Station (RMRS) has corroborated the general importance of water yield from NFS lands to regional streamflow volumes. An effort is now underway to estimate the contribution from streamflow originating on NFS lands to groundwater recharge in California’s Central Valley. Stream loss is a main source of recharge in this system with the primary source being excess applied irrigation water. Preliminary modeling using RMRS data shows that the mean fraction of annual runoff from NFS lands is 64 percent for major surface-water inflows to the Central Valley. This data will be used in conjunction with values estimated by the USGS for surface-water loss to the groundwater system to estimate the contribution from NFS lands. Future work will attempt to estimate the proportional quantity of applied irrigation water that originates on NFS lands. This effort is expected to highlight the importance of water stewardship on NFS lands to ensure the downstream quantity and quality of both groundwater and surface-water resources to end-consumers.