UC Davis

With declining groundwater resources worldwide, there is growing interest in replenishing aquifers through the intentional recharge of excess surface water. However, data on the annual and seasonal availability of source water for groundwater recharge projects remain limited. In the contiguous United States (CONUS), increased intensity and frequency of extreme hydrologic events present new opportunities to divert high-magnitude flows (HMFs, defined as the largest 5% or 10% of flows on record) from rivers for managed aquifer recharge. To support the design, planning, and funding of groundwater recharge projects, we developed a nationwide database of HMF characteristics, including volume, duration, frequency, and timing, evaluated for the 30-year (1990–2020) and 50-year (1970–2020) streamflow records from gages across the CONUS. Our analysis reveals that the top 25 most-pumped aquifers collectively generate an average annual HMF volume of 91 to 164 km³, depending on the HMF threshold and period of record selected - volumes equal to three to five times the capacity of Lake Mead (32 km³), the largest reservoir in the U.S. For individual aquifers, utilizing as little as 1% or as much as 92% of the annual HMF volume would be required to offset depletion rates. In the Texas-Gulf Coast aquifer, only 13–23% of the HMF volume would suffice to balance depletion rates, whereas in the High Plains aquifer, a much larger share, 63–92%, would be needed. A larger distribution of HMF volumes, 36–83%, would be required to offset depletion rates in the Central Valley of California, highlighting the influence of the selected threshold on the utility of HMFs for groundwater recharge. HMF volumes manifest differently in their timing, frequency, and duration based on their region. Long-duration, low-frequency events, typical of aquifers in the Southwest and South-Central regions, offer extended opportunities to capture flows after the onset of events, even in areas with lower HMF volumes. In contrast, high-frequency, short-duration events in the Eastern U.S. may require infrastructure capable of rapidly capturing and storing flows. Recognizing the legal and institutional complexities of acquiring water rights for HMF diversions, this research addresses permitting structures for groundwater recharge projects in the U.S., their treatment of water rights, and approaches to account for environmental flows. By identifying where conjunctive management of surface and groundwater supplies can most effectively supplement declining groundwater levels, this work aims to provide technical and legal recommendations for optimizing HMF utilization while strengthening the protection of instream flows.