Variations of Surface and Subsurface Water Storage in the Lower Mekong Basin (Vietnam and Cambodia) from Multisatellite Observations

In this study, we estimate monthly variationsof surface- water storage(SWS) and subsurface water storage(SSWS, including groundwater and soil moisture) within the Lower Mekong Basin located in Vietnam and Cambodia during the 2003–2009 period. The approach is based on the combination of multisatellite observations using surface-waterextent from MODIS atmospherically correctedland-surface imagery, and water-level variationsfrom 45 virtual stations (VS) derived from ENVISAT altimetry measurements. Surface-waterextent ranges from ∼6500 to ∼40,000 km 2 during low and high water stages, respectively. Across the study area, seasonal variationsof water stages range from 8 m in the upstream parts to 1 m in the downstream regions. Annual variationof SWS is ∼40 km 3 for the 2003–2009 period that contributes to 40–45% of total water-storage(TWS) variationsderived from Gravity Recovery And Climate Experiment (GRACE) data. By removing the variationsof SWS from GRACE-derived TWS, we can isolate the monthly variationsof SSWS, and estimate its mean annual variationsof ∼50 km 3 (55–60% of the TWS). This study highlights the ability to combine multisatellite observations to monitor land- water storageand the variationsof its different components at regional scale. The results of this study represent important information to improve the overall quality of regional hydrological models and to assess the impacts of human activities on the hydrological cycles.