In arid and semi-arid regions of the world, the occurrence of prolonged drought events (megadroughts) associated with climate change can seriously affect the balance between water supply and demand, thereby severely increasing the susceptibility of such regions to adverse impacts. In this study, a simple framework is introduced to estimate renewable water volumes (RW) to mitigate the challenges of megadrought events by managing the groundwater resources. The framework connects a weighted annual hydrological drought index (wSPEI) to RW, based on the short time-scale precipitation volume. The proposed framework, which was in a proof-of-concept case study applied to the Neishaboor watershed in the semi-arid part of Iran, showed that developing the weighted drought index can be valuable to estimate RW. The results suggested that the wSPEI, aggregating hydrological drought index (HSPEI) with the time scale k = 5 days and the regional coefficient s = 1.3 can be used to estimate RW with reasonable accuracy (R2 = 0.73, RMSE = 11.5 mm year−1). This indicates that in the Neishaboor watershed, the best estimation of RW can be determined by precipitation volumes (or the lack thereof) falling over 5-day aggregation periods rather than by any other time scales. The accuracy of the relationship was then investigated by cross validation (leave-one-out method). According to the results, the proposed framework performed fairly well for the estimation of RW, with R2 = 0.75 and RMSE = 12.2 mm year−1 for k = 5 days. The Overall agreement between the wSPEI, the RW derived from water balance calculations, and the estimated RW by the proposed framework was also assessed for a period of 34 years. It showed that the annual RW followed closely the wSPEI, indicating a reasonable relationship between wSPEI and the annual RW. Accordingly, the proposed framework is capable to estimate the renewable water of a given watershed for different climate change scenarios.

Due to the effects of global climate change on duration, frequency and number of drought events, the occurrence of prolonged droughts, referred to as “megadroughts” (lasting for two decades or longer) will become more probable in the future. Thus, it is crucial for countries especially in arid and semi-arid regions of the world to develop appropriate preparedness plans for megadrought risk management. Since groundwater is the key water resource in these regions, it is important to reliably quantify the maximum sustainable extraction to ensure a sufficient groundwater reserve, i.e. the Strategic Groundwater Reserve, for a probable future megadrought event. For this purpose, a new concept of Probable Maximum Drought is proposed in this study, based on the concept of Probable Maximum Flood. As the spillways of large dams are designed based on the Probable Maximum Flood to minimize the probability of failure and the associated casualties and damages, the Probable Maximum Drought concept is proposed to estimate Strategic Groundwater Reserves to limit the consequences of prolonged droughts, including damage and threats to societal stability. This will allow water resources managers and policymakers to develop appropriate strategies to adapt and restrict development plans of a given region based on a sustainable megadrought risk management.