Historic, Current, and Future Availability of Surface Water for Agricultural Groundwater Banking in the Central Valley, California
University of California, Davis
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Groundwater banking, the intentional recharge of groundwater from surface water for storage and recovery, is an important conjunctive use strategy for water management in California. A largely unexplored approach to groundwater banking, agricultural groundwater banking (ag-GB), utilizes flood flows and agricultural lands for recharging groundwater. Understanding the availability of excess streamflow (e.g., the magnitude, frequency, timing, and duration of winter flood flows) is fundamental to assessing the feasibility of local-scale implementation of ag-GB. In this study, we estimate the current availability flows based on current and historic daily streamflow records for about 100 stream gauges on tributaries to and streams within the Central Valley, California by quantifying the magnitude, frequency, duration, and timing of winter flood flow events. For each gauge, we consider flows above a stationary 90th percentile as ideal (available) for ag-GB because reservoir operations mitigate flood risk by releasing early winter flood flows. Additionally, we investigate the future availability of flood flows by determining the long-term trends in the magnitude, frequency, duration, and timing of winter flood flow events. Results suggest that on average across all year types, there are 5 million acre-feet of flows above the 90th percentile available from rivers in the Sacramento, San Joaquin, and Tulare basins between December and February. Trend analyses indicate that Sierra watersheds show 1) increasing trends in the average flood flow volume, 2) increasing trends in the average number of days, and 3) increasing trends in the average number of flood peaks above the 90th percentile. These positive trends suggest ongoing climate change effects on snowpack and the progressive shift from snow- to rain-dominated runoff generation. In contrast, CV gauges show predominantly negative trends in all metrics. Finally, we compare the quantified available water to the existing water rights (eWRIMS database from the State Water Resources Control Board) for each station to determine the amount of surface water that could potentially be allocated for groundwater recharge through a temporary water permit.