Variations in terrestrial water storage in the Lancang-Mekong river basin from GRACE solutions and land surface model

Measurement of regional water resource variations is significant for understanding water cycle and water management. This is especially true for transboundary river basins. This study investigated the terrestrial water storage dynamics of the Lancang-Mekong River basin, which has a total area of 795,000 km(2) and distributed between six countries in Southeast Asia, based on the terrestrial water storage anomalies (TWSA) from the Gravity Recovery and Climate Experiment (GRACE) satellites and the Global Land Data Assimilation System (GLDAS) model. A spatial weighted centroid analysis concept from geographical information science was introduced to track the movement of terrestrial water storage centroid of the basin. And the response of terrestrial water storage dynamics to climate variations throughout the basin was examined by quantifying the correlations of TWSA with a multi-scalar drought index. Results show that during 2003 to 2016, the upstream basin (Lancang River basin) experienced slight decreasing trends (ranging from - 2.45 to 0 km(3)/yr revealed by five different GRACE solutions), and the downstream basin (Mekong River basin) generally did not express statistically significant trends; five GRACE solutions are more consistent in the downstream basin than the upstream basin, and the mascons-based TWSA processed at Jet Propulsion Laboratory (JPL) produced quite different results with the other four solutions. Generally, there are good correlations between GRACE observations and GLDAS model. The TWSA is 1-2 month lagged correlated with precipitation, evapotranspiration, and runoff anomalies, because water from precipitation may take a long period to loss from evapotranspiration and infiltrate into the soil. In general, water storage dynamics are closely correlated with climatic dry and wet conditions at middle-long periods (6-12 months) in the basin, and the correlation coefficients in wet season are generally higher than those in dry season. However, a different pattern was found since 2010: in the 2010-2016 period, correlations between TWSA and the multi-scalar drought index in Lancang River basin reduced relative to the 2003-2009 period but became stronger in Mekong River basin after 2009. The results of this study highlight the differentiation of water storage dynamics' response to the climate variation between upstream and downstream of Lancang-Mekong basin in the period 2010-2016 and the possibility of examining basin storage disturbed by human activities in the basin.