Long-term dynamics of total water storage deficit recovery in Germany

Germany experienced exceptional multi-year water storage deficits starting in 2018. Recurring years below average precipitation and above average temperatures lead to water deficits that accumulated primarily in slow-response subsurface water storages such as (deep) soil moisture or groundwater levels. Total water storage (TWS) anomalies estimated by the GRACE satellite mission show a strong decline for Germany since measurements began in 2002, but these time series began with a relatively wet period, ended in an exceptional drought situation, and are still relatively short. Therefore the resulting trends are not representative for long-term TWS dynamics and should not be used to extrapolate the development into the future (Güntner et al, 2022). In addition, the GRACE signal does not allow partitioning of the water storage and flux components. Hydrological simulations provide a suitable tool to analyse the long-term dynamics of water storages. We analyse the long-term monthly TWS anomalies estimated with the hydrological model mHM (Samaniego et al. (2010), Kumar et al. (2013)) from a data reconstruction starting in 1766 (Rakovec et al, 2022). Comparison of the monthly total water storage estimates between the hydrological simulations and two GRACE solutions (JPL RL06M MSCNv02CRI, GFZ COST-G RL01) show good agreement for both anomalies (R²=0.78-0.84) and the residuals after removing the seasonal cycle (R²=0.69-0.72) on the scale of Germany (spatial mean). We specifically examine the periods of recovery from total water storage deficits from a water balance perspective. Besides precipitation (P) being the main driver of changes in the TWS, the progress of recovery from water storage deficits is controlled by the main water fluxes evapotranspiration (E) and runoff (Q) that are the loss terms in the water balance equation deltaTWS = P - E - Q. Decadal evaluations indicate that the water balance in Germany in recent decades has been increasingly driven by above-average E as a result of the temperature rise. While the cumulative precipitation deficits in the last decade 2011 - 2020 are less exceptional compared to other historical decades, cumulative E residuals account for a much larger part of the water deficits than in other historical decades. The results will contribute to an improved understanding how TWS deficit recovery are affected by long-term changes in the water balance.   References: Güntner, A., Gerdener, H., Boergens, E., Kusche, J., Kollet, S., Dobslaw, H., Hartick, C., Sharifi, E., and Flechtner, F. (2022): Changes of water storage in Germany since 2002 observed with GRACE/GRACE-FO, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-93, https://doi.org/10.5194/gstm2022-93. Rakovec, O., Samaniego, L., Hari, V., Markonis, Y., Moravec, V., Thober, S., Hanel, M., & Kumar, R. (2022): The 2018–2020 Multi‐Year Drought Sets a New Benchmark in Europe. In Earth’s Future (Vol. 10, Issue 3). American Geophysical Union (AGU). https://doi.org/10.1029/2021ef002394 Samaniego L., R. Kumar, S. Attinger (2010): Multiscale parameter regionalization of a grid-based hydrologic model at the mesoscale. Water Resour. Res., 46,W05523, doi:10.1029/2008WR007327 Kumar, R., L. Samaniego, and S. Attinger (2013): Implications of distributed hydrologic model parameterization on water fluxes at multiple scales and locations, Water Resour. Res., 49, doi:10.1029/2012WR012195