Impacts of climate versus human intervention on water storage changes based on GRACE satellites in major U.S. aquifers

Climate extremes (droughts and floods) and human intervention (groundwater pumping and reservoir storage) play critical roles in water resources. Here we evaluate long-term trends and interannual variability in terrestrial total water storage (TWS) from GRACE satellites relative to climate forcing and human water use in major U.S. aquifers for 2002-2017. Results show that climate is the dominant driver of TWS changes at interannual timescales in many of the major aquifers. Large declines in TWS have been recorded in response to long-term droughts in the southwestern U.S. (e.g. California Central Valley, Arizona Alluvial Basins) with limited recovery during shorter wet periods. TWS slightly increased in the northwest U.S. (Columbia and Snake River basins). TWS in the northern High Plains shows a net increase but declined in the central + southern High Plains. Aquifers in the more humid southern and eastern U.S. show interannual variability with little net change in TWS (Texas Edwards Plateau and Gulf Coast, Mississippi Embayment, Pennsylvania, and Florida aquifers).

Human intervention amplifies or dampens effects of interannual climate cycles on TWS changes. In the southwest U.S. (Central Valley), TWS declines are amplified during drought because of switching from mostly surface water irrigation to groundwater during drought. The northwest U.S. has predominantly surface water irrigation, limiting groundwater depletion to local areas. More sustainable water resource development in the northern High Plains is attributed to conjunctive use of surface water and groundwater and higher recharge relative to mostly groundwater irrigation and lower recharge in the central + southern High Plains. Some other regions (e.g. Arizona Alluvial Basins, Texas Gulf Coast) have shifted from mostly groundwater irrigation in the 1990s to 2000s to surface-water irrigation since then because of overdraft and subsidence, dampening the effect of climate cycles on TWS. Aquifers in the more humid regions in the eastern U.S. have not been subjected to extreme droughts and are not intensively irrigated. The results show regional trends in TWS in response to climate extremes and human intervention and highlight approaches to more sustainable water resource development through conjunctive use of surface water and groundwater.