DELAYED RESPONSE OF GROUNDWATER TO DROUGHTS AND IMPLICATIONS FOR A WARMER CLIMATE

Groundwater is a life-sustaining resource that supplies water to 2 billion people worldwide. The availability of groundwater is critical for agriculture, particularly during drought periods, where it serves as an alternate water supply against reduced surface water availability. Despite the importance of groundwater in sustaining human water use during drought periods, our understanding of the groundwater response to drought is limited. In this contribution, we utilize daily groundwater observations from anthropogenically unaffected, unconfined aquifers across the conterminous United States to quantify groundwater recovery time to meteorological drought. The total recovery time consists of the “time of rise”, i.e. the time necessary for the post-drought recharge to increase the storage to pre-drought conditions, plus the time-lag between the termination of the precipitation drought and the termination of groundwater storage loss. Using data from 1981-2017, we demonstrate that on average it takes 3 years for shallow aquifers to recover the storage loss from a drought, and the recovery time is longer for higher severity droughts. In addition, the average time lag of aquifer response time to drought is 15 months, and can increase up to 15 years for some aquifers. Such a considerable variation in the duration of time lag leads to a hidden legacy of drought in shallow aquifers. We find that aquifer physical properties control this time lag in regions with shallow groundwater tables, and precipitation characteristics are more important in aquifers with deep groundwater tables. Groundwater sustainability management must therefore consider this time-lag between precipitation drought and groundwater level response. Given that drought severity increases the recovery time, the projected increases to drought frequency and severity should increase groundwater recovery times and reduce buffering capacity of aquifers in times of reduced surface water availability caused by droughts.