Changes of water storage in Germany since 2002 observed with GRACE/GRACE-FO

A recent sequence of years with below-average precipitation and above-average air temperatures in large parts of Europe, going along with decreasing lake and groundwater levels, low flow conditions in rivers, damage to forest ecosystems and reduced or failing crop yields in agriculture, raised the public debate on the current and future availability of water resources. In Germany, the public debate has been boosted by reports on drastically decreasing Terrestrial Water Storage (TWS) based on satellite gravimetry, with the JPL Mascon data indicating a TWS decrease of indicating a TWS loss of -2.4 Gt / year for the area of Germany from 11/2002 to 10/2021. To provide additional scientific evidence to this debate, we analyzed the GRACE and GRACE-FO data products of 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. Signal leakage of Alpine glacier mass loss leads to spuriously more negative TWS trends for Germany. Furthermore, we stress that given extreme positive TWS anomalies in 2002 and very negative anomalies in 2018 and 2019, the resulting trend values are very sensitive to the selected time period. A longer TWS time series for Germany simulated with a hydrological model indicates that the trend values for the period of satellite gravimetry are not representative of the long-term dynamics. Thus, they are not suited for extrapolating to future TWS trends.