Drought in groundwater—drought distribution and performance indicators

In order to investigate how droughts are changed by the groundwater system and to analyse the performance of groundwater during drought, 10 time series of 1000 years of recharge and groundwater discharge were generated. The 10×1000 years of synthetic daily data were generated using Nearest Neighbour resampling based on 37 years of observed daily meteorological data. The root zone was simulated by a non-linear water balance model and the groundwater system by a linear reservoir model. The size and thus the response time of the reservoir was characterised by a reservoir coefficient. Subsequently, the deficit and duration of the droughts were derived from the time series of recharge and groundwater discharge using the threshold level approach. An analysis of the distribution of these droughts shows that for droughts with small return periods, the deficit in the groundwater discharge is smaller than in the recharge. For droughts with large return periods, the deficit in the groundwater discharge is larger than in the recharge. The performance of groundwater systems with respect to droughts was evaluated using three classical performance indicators (reliability, resilience and vulnerability), a combination of these three indicators (Loucksʹ sustainability index) and three newly defined overall performance indicators. The newly defined indicators combine the severity and frequency of the droughts, instead of analysing these separately in reliability and vulnerability. The performance is estimated for the groundwater recharge and for the discharge of groundwater systems with three different values of the reservoir coefficient. Of all the performance indicators used, one of the newly introduced overall performance indicators with a strong emphasis on droughts with a high return period appeared to characterise the groundwater droughts best. This indicator shows a more or less constant performance for low and medium high reservoir coefficients and an increasing performance for higher reservoir coefficients.