Terrestrial water storage response to the 2012 drought estimated from GPS vertical position anomalies

Historically, drought monitoring systems have not incorporated observations of terrestrial water storage (TWS). Global Positioning System (GPS) instruments can be used to estimate TWS variations in near real time because the solid Earth responds elastically to changes in hydrologic loading. Here GPS vertical position data, u, are used to assess the timing and duration of TWS anomalies in the High Plains caused by the 2012 drought; u indicates an initial TWS minimum in 2012, consistent with GRACE TWS, several months after the most severe meteorological forcing. Standard drought indices show recovery from drought during spring 2013. In contrast, u indicates that the TWS anomaly intensified by approximately 15% during summer 2013, an interval when no GRACE data are available. Hydrologic observations indicate that depletion of groundwater, not soil moisture, is the source of the persistent TWS anomaly. These results show that GPS data could be used to monitor drought-induced variations in TWS in near real time.