Bridget R. Scanlon
Bureau of Economic Geology, Jackson School of Geology
Aquifer overexploitation threatens food security in the U.S. because much of our food comes from irrigated semiarid regions. Groundwater depletion in the High Plains and California Central Valley accounts for ~50% of total depletion in the U.S. since 1900. Groundwater in the High Plains or Ogallala aquifer has traditionally been considered fossil groundwater; however, there is no depletion in the north because of high recharge in the Nebraska Sand Hills whereas depletion is focused in the central and southern regions because of much lower recharge. GRACE satellite-based estimates of groundwater depletion match those based on groundwater level data with depletion rates of ~ 8 km3/yr (2002 – 2007). Reducing irrigation could extend the lifespan of the aquifer but will not result in sustainable management of this fossil groundwater. The Central Valley is a much more dynamic, engineered system, with north-south diversions of surface water. Groundwater depletion occurs mostly in the south (Tulare Basin) and is generally restricted to droughts, which range 1 – 6 yr long with depletion from 8 to 12 km3/yr. GRACE based estimates of groundwater depletion during the 2006 – 2010 drought are the same as those based on groundwater level monitoring data, totaling 27 km3. North-south diversions amplify the impacts of drought because diversions were reduced by up to 90% during the 2004-2010 drought because of endangered species issues. Aquifer storage and recovery shows great promise for coping with climate extremes in the Central Valley by storing surface water during floods in aquifers for use during droughts. While GRACE satellites provide basin-scale estimates of water storage changes, spatiotemporal variability in storage is essential for groundwater management.