The European 2015 drought from a groundwater perspective: estimation in absence of observed groundwater data
Authors: Van Loon Anne F. , Kumar Rohini , Mishra Vimal 
Published: November 2016

Abstract. In 2015, central and eastern Europe were affected by a severe drought. This event has recently been studied from meteorological and streamflow perspective, but no analysis of the groundwater situation has been performed. One of the reasons is that real-time groundwater level observations often are not available. In this study, we use spatially-explicit relationships between meteorological conditions and historic groundwater level observations to quantify the 2015 groundwater drought over two regions in southern Germany and eastern Netherlands. We use the monthly groundwater observations from 2040 wells to establish the spatially-varying optimal accumulation period between the Standardised Groundwater Index (SGI) and the Standardised Precipitation Evapotranspiration Index (SPEI) at a 0.25 gridded scale. The resulting optimal accumulation periods range between 1 and more than 24 months, indicating strong spatial differences in groundwater response time to meteorological input over the region. The Standardised Precipitation Index (SPI) showed similar results, which point to a limited influence of potential evaporation in determining the period of influence of meteorological conditions on groundwater levels in our study regions. Based on the estimated optimal accumulation periods and available meteorological time series, we reconstructed the groundwater anomalies up to 2015 and found that in Germany a uniform severe groundwater drought persisted for several months in this year, whereas the Netherlands appeared to have relatively high groundwater levels. The differences between this event and the 2003 European benchmark drought are striking. The 2003 groundwater drought was less uniformly pronounced, both in the Netherlands and Germany. This is because slowly responding wells (the ones with optimal accumulation periods of more than 12 months) still were above average from the wet year of 2002, which experienced severe flooding in central Europe. We also tested the applicability of the Gravity Recovery Climate Experiment (GRACE) Terrestrial Water Storage (TWS) and GRACE-based groundwater anomalies to capture the spatial variability of the 2003 and 2015 drought events. GRACE-TWS does show that both 2003 and 2015 were relatively dry, but the difference between Germany and the Netherlands in 2015 and the spatially-variable groundwater drought pattern in 2003 were not captured. This could be associated to the coarse spatial scale of GRACE. The simulated groundwater anomalies based on GRACE-TWS deviate considerably from the GRACE-TWS signal and from observed groundwater anomalies. These are therefore not suitable for use in real-time groundwater drought monitoring in our case study regions. Our novel approach based on the spatially-variable relationship between meteorological conditions and groundwater levels allow to quantify groundwater drought in near real-time. We found that the 2015 groundwater drought in southern Germany was more severe than the 2003 drought, because of preconditions in slowly responding aquifers. Compared to the meteorological drought and streamflow drought (described in previous studies), the groundwater drought of 2015 had a more pronounced spatial variability in its response to meteorological conditions, with some areas primarily influenced by short-term meteorological deficits and others influenced by meteorological deficits accumulated over the preceding two years or more. In drought management, this information is very useful and our approach to quantify groundwater drought can be used until real-time groundwater observations become readily available.