Using GRACE Satellites to Estimate Impacts of Climate and Irrigation on Water Storage Changes in Major U.S. Aquifers

Managing water resources sustainably requires a comprehensive understanding of the effects of climate extremes (floods and droughts) and human water use (particularly irrigation) on water storage. In this study we examined the relative importance of climate and human drivers on variability in total water storage (TWS) from GRACE satellites compared with drought indices and irrigation water use in 14 major aquifers within the U.S. Results show marked depletion of TWS, tracked by GRACE satellites, was restricted to the semi-arid southwestern Central Valley and south-central High Plains, totaling ~90 km³, ~3× greater than the capacity of Lake Mead, the largest U.S. reservoir. Water storage depletion in the Central Valley was linked to long-term droughts (≤5 years) that were amplified by changing water sources from predominantly surface water irrigation during wet periods to groundwater irrigation during droughts. Interannual variability in TWS dominates long-term variability in major aquifers throughout the rest of the U.S. In the eastern U.S., aquifers in humid regions show low TWS trends related to low drought intensity. Although groundwater pumpage for irrigation in the humid Mississippi Embayment aquifer exceeded that in the semi-arid California Central Valley, no TWS depletion was recorded in the GRACE satellites in the Mississippi Embayment aquifer. The lack of TWS depletion is attributed to groundwater pumpage capturing streamflow in this humid region. Low or slightly rising trends in TWS in the northwest U.S., Columbia and Snake River Basins are attributed to surface water irrigation dampening drought impacts and disconnecting storage from climate forcing. The analysis of GRACE data shows synergies between climate and irrigation, with amplified water storage depletion in the semi-arid southwest and southcentral U.S., little impact on water storage in the humid east, and dampened water storage depletion in the northwest and north central U.S. To enhance the sustainability of water resources, groundwater and surface water should be used conjunctively, with inefficient surface water irrigation increasing groundwater recharge and efficient groundwater irrigation minimizing storage depletion. The use of managed aquifer recharge has been expanding within the past decade in different aquifers. Results of this study have important implications for managing water resources more sustainability within the context of climate extremes and intense irrigation globally.