Assessment of continental water mass balance components 2003--2022 over EURO-CORDEX

Abstract

Observational and modelled components of the continental water cycle still show considerable variation between available data products. At the same time, however, the research community is trying to identify processes and changes across several domains of the hydrosphere in increasingly more detail.Within the Collaborative Research Centre (CRC) 1502 DETECT, we have developed an analysisframework in order to assess the mass budget consistency of observational and modelled water flux products. We evaluate essential climate variables such as precipitation (P), evaporation from land (E), terrestrial water storage (TWS) change (dTWS/dt), and river discharge (Q) by means of kernel-integrated monthly water mass fluxes in the GRACE/-FO era. Specifically, we focus on the terrestrial water balance equation as in dTWS/dt+Q=P−E, which states that net flux P−E between atmosphere and surface must be balanced by the combination of dTWS/dt and Q.In our major assessment 2003-01/2022-12 over the EURO-CORDEX region, we assessed budget flux components for 30+ major river catchments, including a wide selection of observational and reanalysis products, such as e.g. GPCC, MSWEP, ERA5, GLEAM, GRDC and COST-G (and other GRACE-related products). While individual data products can vary considerably on a per-month basis and in their relative offsets, we find that clear majority (~74%) of catchments show drying P−E behaviour, while the balance-term dTWS/dt+Q shows mostly consistent behaviour (~67%). Notably, out of the basins with a maximum P−E net flux of +25 mm/month, >90% show drying trends in P−E, and >85% show negative trends in the combination of TWS change and river discharge.While dP/dt, across Europe, is heterogeneously distributed spatially and across data products, E more consistently increased by 1 mm/month per decade.  On average, TWS losses increased and discharge declined. In contrast to absolute fluxes, dTWS/dt+Q change appears, on average, ~equally caused by a decline in discharge and TWS; the trends, however, exhibit large uncertainties. In total, a flux budget misclosure remains below 2 mm/month.We again point out that researchers must be aware of the notable variability between available data products and avoid 'cherry-picking' of analysis products.