## 1 Introduction  
Terrestrial water storage (TWS) refers to the total amount of water stored in the land-based water cycle, including groundwater, surface water, soil moisture, and snow and ice [1, 2]. Understanding the distribution, variability, and dynamics of terrestrial water storage is crucial for many aspects of water resources management, including water supply, water quality, and flood control [3, 4, 5, 6, 7, 8]. Particularly in international river basins [9, 10, 11, 12], such as the transboundary Lancang-Mekong River, the assessment of TWS is crucial for ensuring sustainable water resource utilization and potentially mitigating conflicts among countries [13, 14, 15, 16]. The transboundary Lancang-Mekong River, which flows through six countries in Southeast Asia, is one of the world's largest river basins and is home to a complex system of cascade reservoirs [17]. Reservoirs in the Lancang-Mekong River basin play a crucial role in managing water resources, including regulating streamflow and storing water for hydropower generation, irrigation, and other uses [18, 19, 20, 21, 22]. China has built only a small number of dams (11 out of 100 or more) in the entire Mekong River basin [18, 23]. However, these limited dams in the Upper Mekong River (Lancang) have substantial storage capacity (about 42 km\({}^{3}\)) and regulate a significant portion of the river's discharge (55% of the average annual flow in Northern Thailand) [24]. Among them, Xiaowan and Nuozhadu are the two largest reservoirs and account for massive storage capacity and adjustment ability:about 34% and 50%, respectively [14]. The two reservoirs can significantly impact the TWS of the region in several ways, including changes in soil moisture and groundwater levels, alterations in the timing and magnitude of streamflow, and the regulation of water for various uses. In turn, these changes can affect the water flow downstream, which can impact the water security of urban areas located along the river. For instance, changes in the timing and quantity of water release from the reservoirs can affect the availability of water for domestic and industrial uses in downstream areas, potentially leading to water scarcity and compromising the water security of urban populations. Therefore, understanding these changes is crucial for promoting comprehensive cooperation in managing and utilizing water resources within the basin. However, the influence of the large reservoirs on the surface and sub-surface water storages over the basin has not been comprehensively investigated due to data availability constraints and other geopolitical reasons [25; 26; 27; 28; 29; 30]. The latest Earth observation (EO) satellites, such as the Gravity Recovery and Climate Experiment (GRACE) [31], Sentinel imagery [32], and altimetry [33], provide exceptional opportunities for monitoring different water balance components with near real-time or limited delay. Hence, EO data serves as a promising solution for observing and assessing changes in water storage within the region. The GRACE mission has enabled scientists to measure TWS changes (including surface water storage, soil moisture water storage, and groundwater storage) on a global scale and provide new insights into the water balance of watersheds [12; 34; 35; 36]. For example, the GRACE satellite mission has enabled the monitoring of groundwater storage changes in regions experiencing significant depletion, including in Northwest India [37], the U.S. High Plains [38], and the California Central Valley [39]. Moreover, GRACE products have demonstrated great potential in monitoring TWS balance in response to droughts and floods [40]. Additionally, remote sensing EO data can provide complementary information to GRACE data [34], such as satellite radar altimetry, synthetic aperture radar (SAR), and optical remote sensing. Combining these data sources with GRACE data can provide a more comprehensive understanding of the Earth's system and its processes.