ASSESSING DROUGHT IMPACTS ON THE COLORADO RIVER BASIN USING GRACE SATELLITES AND GROUND-BASED DATA

GRACE satellites monitor changes in total water storage (TWS) in large basins globally; however, interpretation of the hydrologic significance of these changes is sometimes complicated. Here we evaluate GRACE TWS changes in the Colorado River Basin within the context of longer-term monitoring and modeling data to evaluate controls on water storage changes. The monitoring period (1980 – 2014) is generally characterized by decadal droughts (around 1990, early 2000s, and 2010s). Rates of TWS depletion during the droughts are generally similar (~ 6 km³/yr) totaling ~50 – 75 km³ over drought periods of 2 – 6 years. However, the big difference between droughts since 2000 and earlier droughts is limited recovery because of only 1-2 anomalously wet years since 2000 versus ~4 years in the 1990s. GRACE data show slight recovery of TWS in 2005 and 2011 followed by large depletion of almost 47 km³ over 2-3 yr. In the Upper Colorado River Basin TWS depletion is dominated by reservoir and soil moisture storage changes whereas in the Lower Colorado River Basin groundwater storage depletion is also important. The primary approach to managing these wet and dry climate extremes is through storing water, with current remaining reservoir storage representing ~3 years of water demand. Water management strategies adopted to cope with drought stress include conjunctive use of Colorado River water and groundwater and managed aquifer recharge in the Lower Colorado River Basin. While water allocations have been met to date, continued drought may jeopardize junior water rights and require more intensive drought-management strategies.