| 17 Sep 2026
The water balance quagmire: a zoom out to close in on climate forcing biases
Abstract. Non closure of the interannual water balance is a major issue in hydrological modelling. Because the potential causes (e.g. forcing biases, groundwater exchanges, anthropogenic influences, streamflow uncertainties) are so inextricably intertwined, many modelers resort to the calibration of 'sweep' parameters, leading to ad hoc corrections of water balance components to improve performance metrics. Such practices can be beneficial when applied 'for the right reasons', although this is seldom possible to verify. In this paper, we look at the untangling of water balance closure through the lens of scale: we hypothesize that the spatial variability of the water balance anomaly signal can be used to separate the different causes of non-closure and, in particular, to discriminate between those which have local determinants (such as groundwater leaks) and those involving non-local or systematic drivers (such as forcing biases). To test the idea, we compute long-term water balance anomalies for 3641 near-natural gauged catchments across Europe, using the CERRA-Land climate reanalyses (1984–2024). We then apply Thin Plate Splines to extract the mesoscale imbalance, a continuous field of low-frequency variability. We show that the extracted signal is most likely driven by the forcing biases of the CERRA-Land reanalysis. The approach is a step forward towards extracting the contribution of climate forcing biases across gauged catchments and beyond. The hypothesis and the associated methodology provide a tool to contain the uncertainties of 'sweeping', paving the way for hydrologically-grounded mesoscale corrections of the forcing variables in large-scale distributed hydrological models.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences. MR and FH are employed by Electicité de France (EDF), which partially funded this research through the CIFRE program managed by the French association for Research and technology (ANRT). The contact author has declared that none of the remaining authors has any competing interests.
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The authors have reiterated the central position of the water balance in hydrology from the perspective of the daunting problem of predicting run-off. In the process they clearly show difficulties in the level of the current discussion, pointing out that the annual water balance is affected by many factors other than what is typically considered within the Budyko formalism. The beauty of the water balance equation, however, is that the same equation extends to problems like recession which, in its simplest form (when, indeed, precipitation, evapotranspiration, and groundwater divergence as well as human intervention can be neglected) relates streamflow Q to the depletion of storage. In standard treatments of recession, the rate of decline of storage is equal to the streamflow. In the authors' equation, with increases in storage represented by a positive delta S on the opposite side of the equation from Q, this relationship would not obey conservation of mass. Obviously, this is merely a matter of a typo or some change in definition during revisions, but It would reduce distraction to clarify this relationship. Since it is in discussion forum, there should be no harm in straightening this out directly. But the advantage is to remind readers again of the various ways in which the water balance operates within the fields of hydrology, ecohydrology, climate, geomorphology, and water resources management.