Historic, Current, and Future Availability of Surface Water for Agricultural Groundwater Banking in the Central Valley, California

Abstract:

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 and forecast the future availability of winter (Nov to Apr) flood flows based on current and historic daily streamflow records for 200 stream gauges on tributaries to and streams within the Central Valley, California. For each gauge, we consider flows above a stationary 90th percentile as ideal for ag-GB because reservoir operations mitigate flood risk by releasing early winter flood flows. Results based on 70 years of data show that for 25% of the gauges there are significantly decreasing flow volumes above the 90th percentile and a decreasing number of days with flows above the 90th percentile. These flows, on average, make up 20% of the total annual winter flows. The majority of gauges further show, over the past 70 years, a decrease in total annual streamflow magnitude, a decrease in the magnitude of extreme flood events, and an increase in the frequency of flood events. Variations in winter flood flows due to climate change and climate variability are a challenge to water management in California. To aid the long-term forecast of streamflow conditions in California, we present a new water year type index for the Central Valley, which considers the variation in flow percentiles over time. Together, our results suggest that flexible, coordinated efforts for the local diversion of flood flows are needed to better utilize the increasingly rare winter flood events.