A recent assessment of terrestrial water storage depletion across the High Plains aquifer

In semi-arid to arid regions, water storage in the unsaturated and saturated zones is the main source of water for ecosystems, crops, and human needs. Recent concerns about the sustainability of agricultural systems in the US central High Plains have increased after the on-going exceptional drought that started in 2011. This work investigates the impacts of this exceptional event on water storage anomalies across the High-Plains Aquifer (HPA). Observations of the terrestrial water storage from the Gravity Recovery and Climate Experiment (GRACE) and North American Land Data Assimilation System (NLDAS) provide estimates of monthly soil moisture and groundwater anomalies from 2004 to 2012. Spatial and temporal patterns of anomalies during the 100-month observed period included the 1st phase of the recent drought. Results show a continuous negative trend of total water storage between September 2010 and April 2012. Spatial variability of soil moisture and groundwater anomalies revealed divergent patterns since March 2008. During the 1st phase of the drought, water storage depletion was more pronounced in the southern HPA while no depletion was observed in the northern HPA. The 2nd phase of the drought shows negative anomalies across the entire HPA, but higher depletion rates were evident in the central and northern parts of the HPA. We estimate that more than 15 km3 of water were lost from the unsaturated zone across the HPA since the beginning of the recent drought with regional depletion rates from 1 mm/year in the Northern HPA to 34 mm/year in the Southern HPA. Despite strong depletion rates, the change in saturated storage, estimated as the difference between total water storage and unsaturated storage varied considerably between observed soil moisture data and NLDAS products, especially during the recent drought. Such sources of bias can alter estimates of groundwater trends. Since irrigation practices were not accounted for in this preliminary study, the actual changes in in unsaturated zone storage are likely underestimates, especially in croplands with inefficient water management practices. The spatial extent and temporal duration of the current drought detected by GRACE revealed that total water storage has reached a 10-year minimum value across the HPA.