Controls on recharge in thick vadose zones under climate variability and change

An improved understanding of recharge mechanisms and rates in relatively thick (14-38 m) vadose zones under irrigated and rangeland land use/land cover (LULC), climate variability, and future climate change can help inform sustainable groundwater management. Vadose-zone monitoring data from the High Plains aquifer in central USA were used to calibrate Hydrus-1D models and simulate recharge and total potential profiles during historical (1950-2018) and future (1950-2100) periods using an ensemble of nine downscaled global climate models (GCMs) at representative concentration pathways (RCPs) 4.5 and 8.5. Results show that historical recharge lag correlates with the Palmer Drought Severity Index, and LULC was a major control on the recharge lag time to hydroclimatic perturbations; recharge lag times were 20-24 months at irrigated sites and 5-31 years at rangeland sites. The historic total potential profile dynamics were most sensitive to location along the west-east gradient in average annual precipitation. Results also show that future recharge is greater under irrigated sites compared to rangeland sites along the west-east gradient. An important finding is that future total potential profiles were projected to have relatively more seasonal upward gradients under all LULCs, which explains the projected decrease in recharge rates by the mid- to late-21st century of more than 40% at some sites, particularly under RCP 8.5. These findings indicate that total potential profiles that control recharge rates may fundamentally change under future climate change across a range of LULC, which may further challenge the sustainability of some groundwater resources.