Are we approaching water budget closure at basin scale for the right reasons?

Abstract. The water cycle represents the continuous circulation of water through the atmosphere, the continents and the oceans. At the basin scale, this cycle can be decomposed into incoming and outgoing fluxes as well as changes of water stored within the basin. The relationship between these fluxes and changes in storage is called the water budget. Closing the water‑budget at basin scale, using observations, model outputs or a combination of both, provides a deeper understanding of water fluxes, storage changes as well as the underlying hydrological processes.

Satellite measurements, especially those from the Gravity Recovery And Climate Experiment (GRACE, 2002–2017) and GRACE Follow-On (GRACE-FO, 2018–present) missions, now allows to estimate the water budget closure level for large basins worldwide, including those poorly or not instrumented at all. One of the outgoing fluxes for exorheic basins, river discharge, is not available for many basins, particularly during the twenty‑first century due to the spatial and temporal heterogeneity of openly accessible in‑situ river discharge. To address this issue, several studies have attempted to close the basin‑scale water‑budget by estimating river discharge as the simple sum of runoff calculated from Land Surface Models. Despite well-documented significant errors in runoff estimation, uncertainties associated with these approaches are often neglected as river discharge is, in most cases, the flux with the smallest amplitude in the water budget. In this study, using an ensemble approach to assess the state‑of‑the‑art estimates of water cycle fluxes and stocks, we precisely evaluate the impact of river discharge errors on the conclusions drawn from the water budget analysis of 82 exorheic basins worldwide. When ensembles of products are used to estimate combination of precipitation, evapotranspiration and discharge that are the closest to the GRACE storage change variations, we demonstrate compensations between the components of the water budget can occur. Although river discharge is the flux with the smallest amplitude, its dispersion (36 %), when derived from aggregated runoff, is comparable to that of precipitation (44 %) and evapotranspiration (34 %) and a seemingly high level of water budget closure can be achieved with, for some basin, errors of more than 40 % in the annual volume assessment of precipitation and evaporation. Errors caused by these compensations are larger for continental, sub‑arctic and polar basins (around 15  %) than for tropical, arid and temperate basins (around 6  %). Moreover, applying a simple routing scheme to the runoff does not significantly reduce these errors. In this study, we mitigate and assess these compensations by using a combination of in situ and satellite-based discharge estimates. Future studies should explore methods that further limit the compensation effect when attempting to approach a water budget closure at basin scale.