Water budget closure assessment of 18 various basins combining GRACE and altimetry data

The water balance equation describes the exchange of water mass between land, ocean and atmosphere. Being able to close the water balance gives confidence in the ability to model and/or observe spatio-temporal variations in the water cycle and its components. At basin scale, the water balance equation (DTWS = P - ET - Q) compares derived total water storage (DTWS) with precipitation (P), evapotranspiration (ET) and runoff (Q). Many studies compare GRACE-based DWTS observations with P and ET datasets, and Q from Land Surface Model (LSM), due to the lack of in situ discharge observations. For some basins, human activities, glacier, reservoir and lake impact on the water cycle is not or poorly modeled by the LSM. In this case, the water budget may close due to compensation errors, for example between Q and ET.

In this study, we propose to evaluate the consistency of budget closure with Q computed from satellite altimetry data, which might have better accuracy than discharge from LSM. We will use the altimetry-based discharge products from the ESA CCI river discharge project (https://climate.esa.int/en/projects/river-discharge/), recently available. DTWS is evaluated from the CNES GRACE-GRACEFO L3 dataset. This dataset is an ensemble of 120 different solutions combining the state-of-the-art in terms of GRACE L2 data and corrections. The spread within the ensemble aims to cover the uncertainty in DTWS estimates. The dataset has a monthly resolution of 1 degree.

In order to evaluate the best combination of datasets to close the water balance, we will use more than 15 precipitation datasets using the FROGS database (https://frogs.ipsl.fr/) and more than 8 evapotranspiration datasets (GLDAS, ERA5-Land, GLEAM, SynthesizedET, SSEBop, MOD16, BESS V2, FLUXCOM). This ensemble-based approach will also enable to assess the dispersion of these precipitation and evaporation data for each basin. We evaluate the budget closure using different metrics (NSE, KGE, RMSD etc…) at 18 basins of different climate, latitude and size over 2002 to 2019.

Finally, we will compare the 18 water budget closures with those obtained with discharge computed from LSM, like GLDAS or ISBA/CTRIP, to assess the benefits of using altimeter-based discharge for the water budget closure.