## 4 Discussion  
The spatial and temporal patterns of water resources are profoundly changing in the Lancang-Mekong River basin under the impacts of climate change and human activities [16; 21; 26; 73]. A plethora of literature has reported that the Lancang-Mekong River basin has undergone increased frequency and severity of extreme floods and droughs [74; 75; 76]. Additionally, dams and reservoirs directly regulate streamflow and change water resources. The Xiaowan and Nuozhadu reservoirs can make up approximately 85% of the total storage capacity in the Lancang River, thereby playing a pivotal role of water management in the LLRB. The GRACE mission provides a potential solution for monitoring the TWSA dynamics changes in the LLRB. The TWSA of the LLRB exhibits a substantial spatial and temporal variation between the periods of 2002 to 2009 and 2010 to 2019 (Figure 3). This discrepancy is primarily due to the fluctuation in precipitation and the construction of reservoirs. From 2002 to 2009, there was no significant trend observed in either precipitation or TWSA (Figures 4a,b and 7c). However, from 2010 to 2019, while precipitation showed a significant decrease (\(-34.68\) cm/yr), the TWSA of the LLRB displayed a significant increase (8.96 cm/yr). This sudden shift in the TWSA can be attributed to the enhanced water storage capacity in the Xiaowan and Nuozhadu reservoirs after 2010. The trend of the TWSA in the LLRB over the past two decades differs from that in the Lancang and Mekong River basins (Figure 4 and Table 2). The downward trend of the TWSA observed in the Lancang-Mekong River basin is believed to be a result of climate change, characterized by decreased precipitation and increased evapotranspiration [77; 78; 79]. SWS, represented by runoff and reservoir storage, is directly impacted by the spatial and temporal distributions of precipitation. Following the impoundment in Xiaowan and Nuozhadu reservoirs after 2010, the SWS showed a dramatic increase until it stabilized, exhibiting cyclical, seasonal fluctuations (Figure 6). The persistent declining precipitation (\(-34.68\) cm/yr, Figure 7) and increasing evaporation [77; 78; 79] from 2010 to 2019 resulted in a decrease of inflow (Figure 6c). To ensure the stability of outflow in the downstream areas (Figure 6d), the volume of the reservoirs is naturally adjusted less (Figure 6a,b). Therefore, the SWS in the LLRB shows a decreasing trend (\(-5.48\) cm/yr, Figure 6d). The linear trends of the SMSA and precipitation anomaly in the study region are consistent (Figure 7) because SMS has a rapid response to precipitation [34; 80; 81]. Unlike SWS and SMS, which react to precipitation directly, the linear trend of the GWSA and TWSA after reservoir regulation in the study region is consistent (Figure 8). The Xiaowan and Nuozhadu reservoirs always maintain a certain amount of water for hydropower generation and downstream water security, even in extreme drought conditions. The maintenance of groundwater levels has a positive impact on basin drought management. Therefore, from the start of water storage in 2010 to the stabilized operation of the reservoirs, a steady rise in groundwater was observed (9.73 cm/yr). This finding aligns with the results reported by [PERSON] et al. [12]. The accuracy of EO data inevitably introduces uncertainty to the results of this study. To reduce this uncertainty, most of the data sources in this study were calculated using the average of data from multiple sources. For example, five GRACE solutions, two precipitation products, and two SM products were compared and utilized. These datasets showed high consistency in the study area. Additionally, the GRACE-derived TWSAs are more accurate for large regions; the presence of open water bodies near the basin could have affected GRACE signals. But the seasonal WSE changes in the Xiaowan and Nuozhadu reservoirs can reach 80 m, and the water volume changes are more than the order of \(10^{8}\) cubic meters.