Abstract

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 evaluate two alternative approaches to quantify the 2015 groundwater drought over two regions in southern Germany and eastern Netherlands. The first approach is based on spatially explicit relationships between meteorological conditions and historic groundwater level observations. The second approach uses the Gravity Recovery Climate Experiment (GRACE) terrestrial water storage (TWS) and groundwater anomalies derived from GRACE-TWS and (near-)surface storage simulations by the Global Land Data Assimilation System (GLDAS) models. We combined the monthly groundwater observations from 2040 wells to establish the spatially varying optimal accumulation period between the Standardised Groundwater Index (SGI) and the Standardized 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. 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 during 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. GRACE-TWS and GRACE-based groundwater anomalies did not capture the spatial variability of the 2003 and 2015 drought events satisfactorily. GRACE-TWS did show that both 2003 and 2015 were relatively dry, but the differences between Germany and the Netherlands in 2015 and the spatially variable groundwater drought pattern in 2003 were not captured. This could be associated with the coarse spatial scale of GRACE. The simulated groundwater anomalies based on GRACE-TWS deviated considerably from the GRACE-TWS signal and from observed groundwater anomalies. The uncertainty in the GRACE-based groundwater anomalies mainly results from uncertainties in the simulation of soil moisture by the different GLDAS models. The GRACE-based groundwater anomalies are therefore not suitable for use in real-time groundwater drought monitoring in our case study regions. The alternative approach based on the spatially variable relationship between meteorological conditions and groundwater levels is more suitable to quantify groundwater drought in near real-time. 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 2 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.

Highlights

  • In the summer of 2015, large parts of Europe were affected by a severe drought (Van Lanen et al, 2016; Orth et al, 2016)

  • We focus on two regions in Europe, the eastern part of the Netherlands and southern Germany, for which groundwater level data were available and historical relationships between precipitation and groundwater levels can be determined following the method of Kumar et al (2016)

  • The grid cells we used in this study are finer than those used in that previous study (0.25◦ instead of 0.5◦ spatial resolution), which means that fewer groundwater boreholes are used in the determination of the optimal accumulation period of Standardized Precipitation Evapotranspiration Index (SPEI) to match Standardised Groundwater Index (SGI), and this has the advantage that less spatial variability in groundwater is averaged out

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Summary

Introduction

In the summer of 2015, large parts of Europe were affected by a severe drought (Van Lanen et al, 2016; Orth et al, 2016) This drought event has been analysed from climatological (Ionita et al, 2017; Orth et al, 2016) and hydrological (Laaha et al, 2016) perspectives, which give a useful overview of the causes, development, extent, and severity of the drought event. Van Lanen et al (2016) and Laaha et al (2016) note that hydrological information is key to understanding and managing the impacts of drought events and that hydrological drought needs to be monitored. Hydrological drought encompasses both below-normal river flow and below-normal groundwater levels (Van Loon, 2015). Groundwater is linked to other hydrological variables such as soil moisture and streamflow; monitoring groundwater during drought gives important information about drought persistence in these related variables

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