Understanding Hydro-climatic Variability and Change Across World Regions and Scales: A Multi-Catchment, Multi-Dataset Approach

All societies, economic sectors, and ecosystems depend on and influence the flows and storages of terrestrial water, both as vital freshwater resources and as sources of flood and drought risk. Robust assessment of the conditions and changes of these flows and storages relies on the availability, consistency, and physical realism of hydro-climatic datasets. Here, we evaluate four widely used global hydro-climatic datasets with harmonized spatiotemporal coverage (Zarei and Destouni, 2024): (i) Obs, based solely on in-situ observations; (ii) Mixed-GLEAM, combining the same observational data with model-based GLEAM evapotranspiration; and the fully model-based reanalysis products (iii) GLDAS and (iv) ERA5. 

Comparatively across these datasets, we analyze long-term means and trends across 1,561 catchments worldwide and for four regions: the Baltic (with 69 catchments), Mediterranean (182), South America (95), and Sub-Saharan Africa (127), over the period 1980–2010. All datasets consistently show large-scale spatial trends of increasing mean temperature, precipitation, evapotranspiration, and runoff from high latitudes toward the equator. In contrast, estimates of water-storage change (DS) and its spatial patterns differ markedly among datasets. GLDAS exhibits near-zero long-term average DS, implying no systematic drying or wetting, whereas ERA5 indicates predominantly strong negative DS (systematic drying), except in the Baltic region where positive DS (systematic wetting) dominates. Temporal trend analyses further show agreement among datasets for rising temperatures, but weaker, often insignificant, and divergent change trends in precipitation, runoff, and evapotranspiration, both in magnitude and direction. Overall, the intercomparison reveals that ERA5 departs substantially from observation-based estimates and from the other datasets, with systematic biases and physically implausible implications for the terrestrial water fluxes and storage changes across regions and globally. 

Physically inconsistent storage-change implications affect inferred runoff-generation processes, hydrological memory, and model parameter transferability in large-sample applications and catchment modeling. The dataset intercomparison results raise fundamental concerns regarding the suitability of ERA5 for large-sample assessments of terrestrial water variability and change. In general, constraining the assessments by catchment-wise water-balance linkages and closure offers a valuable framework for diagnosing dataset realism and advancing unified understanding of the climate-and-water interplay across regions and scales.

Reference: Zarei, M., Destouni, G. (2024). A global multi catchment and multi dataset synthesis for water fluxes and storage changes on land. Scientific Data, 11, 1333.