Constraints on the availability of high magnitude flows for managed aquifer recharge

Abstract Floods and droughts create temporal disconnects between water supply and demand, underscoring the need to store high magnitude flows (HMFs) in depleted aquifers to alleviate these extremes. The objective of this study was to quantify the spatial and temporal variability of HMFs to inform managed aquifer recharge (MAR), accounting for water rights, instream flow requirements, and coastal inflows. Texas, USA, was selected as the study area due to its climate-driven water stress, rising water demand, and the availability of detailed data on legal constraints. Volumes of HMFs (defined as ⩾ 95th percentile) were calculated for 190 streamflow monitoring gages in all 23 river basins in Texas (1968–2022). Water availability models and coastal inflow requirements were used to consider limitations to HMF availability for permitted use, including water rights, environmental flows, and coastal inflows for select major river basins. Results show that HMFs averaged 31% of total flows across all stations in the state. Of these HMFs, 52% of flows were unappropriated (not reserved for water rights holders), representing 12% of total flows. The mean annual volume of unappropriated HMFs was 81 Mm 3 (∼0.07 million acre-feet). Mean annual unappropriated HMF volumes increased by a factor of 100 from semi-arid West Texas to humid southeast Texas, with variability driven by basin climate and watershed size. Environmental flow requirements generally did not significantly reduce the unappropriated HMFs as they were accommodated by flow reserved for downstream senior water rights. However, coastal inflow requirements limited permittable HMF volumes, especially in the semiarid Colorado River basin which includes high urban water demand up-basin. When considering volumes of HMFs that may be considered ‘available’, it is essential to consider limitations to the amount of flows that could be permitted. This study provides a template for assessing HMFs for MAR that considers water rights, environmental flows, and coastal inflow requirements that could be generally applied in many regions (particularly the western US) to address climate extremes.