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Abstract:
Since 2018, large parts of Europe have experienced below-average annual precipitation and above-average air temperatures. These phenomena were accompanied by falling water levels in lakes and groundwater, low flow conditions in rivers, damage to ecosystems and negative impacts in various economic sectors, leading to a public debate on the current and future availability of water resources. In Germany, this debate has been boosted by media reports on drastically decreasing terrestrial water storage (TWS) based on satellite gravimetry of GRACE and GRACE-FO, in which results based on data of the U.S. analysis center JPL (JPL Mascons data) indicate a TWS decrease of -2.4 Gt/year for Germany from 11/2002 to 10/2021. To provide a more robust scientific basis for the ongoing
debatethis paper first introduces the concept of satellite gravimetry, including its potential and limitations. Besides the JPL Mascons
data, we then analyze three other GRACE and GRACE-FO data products (COST-G, GFZ and ITSG Graz / University of Bonn), resulting in
German-wide TWS trends of -0.7 to -1.3 Gt/year for this period. Due to the measurement and processing principles, satellite gravimetry also captures mass changes beyond the area of interest, so that Alpine glacier mass loss leads to spuriously more negative TWS trends for Germany, which were corrected in the present analysis. The spread of results based on different data products illustrates
the uncertainty of GRACE data so that a comparative analysis of different data sets is recommended. The markedly different results of the JPL-Mascons data set may be due to its processing method that differs from the other data sets in several aspects. In view of extreme positive TWS anomalies in 2002 due to high rainfall and very negative anomalies in the drought years 2018 and 2019, the resulting trend values are strongly dependent on the selected time period. A longer TWS time series for Germany simulated with
a hydrological model shows a good correspondence to the TWS observations and indicates that the trend values for the period of satellite gravimetry are not representative of the long-term dynamics. Extrapolating future water storage trends from them is thus not recommended.
