## 3 Results
### Surface Water Storage Variations  
WSE of the reservoirs are monitored by Jason-2, Sentinel-3, and Cryosat-2 (Figure 5c,d). Additionally, SWEs provided by the JRC and GRAS are compared (Figure 5e,f). WSE and SWE experienced a sharp rise starting in 2010 (Xiaowan) and 2012 (Nuozhadu), which coincides with the start of the reservoir impoundment phase. It took roughly three years for Xiaowan and Nuozhadu to reach stability after impoundment, which occurred in 2013 and 2015, respectively. The significant annual fluctuations of both Xiaowan and Nuozhadu reservoirs (>60 m) indicate a wide range of operating conditions (filling or draining the reservoirs).  
The presence of extreme values and anomalies in mountainous regions such as Xiaowan and Nuozhadu reservoirs due to cloud cover and mountain shadow can impact the quality of optical images and cause errors in JRC's data results [52]. Therefore, SWE from GRAS and WSE derived from Jason-2, Sentinel-3, and Cryosat-2 from 2017 to 2019 were utilized to construct Equation (6). Additionally, the fitted curves fitted by the digital elevation model (DEM) [72] were used to validate the WSE-SWE curves shown in Figure 5e,f. The  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Trends in TWSA (cm/yr)**} \\
**Basin** & **2002-2009** & **2010-2019** & **2002-2019** \\ \hline Study Region & 0.16 & 8.96 * & 2.95 * \\ Lancang River basin & \(-\)2.21 & 1.85 & \(-\)1.40 * \\ Mekong River basin & \(-\)8.65 & \(-\)2.48 & \(-\)3.36 * \\ \hline \hline \end{tabular} Note. An asterisk (*) indicates that the trends are statistically significant (\(p\)-value \(<\) 0.05).  
\end{table}
Table 2: The total water storage anomaly (TWSA) trends derived from GRACE in the lower Lancang River basin, Lancang River basin, and Mekong River basin.  
Figure 4: Temporal variations of GRACE-derived total water storage anomalies (TWSA) in (**a,b**) Xiaowan and Nuozhadu reservoirs basin, (**c,d**) Lancang River basin, and (**e,f**) Mekong River basin. LOESS trend is the de-seasonalized TWSA using the LOESS (locally estimated scatterplot smoothing) method. The gray bars indicate the intermission gap between GRACE and GRACE-FO.  
WSE and SWE pairs are more or less scattered on the teams derived from DEM topography, which proves the reliability of this research method.  
The WSE from 2010 to 2019 was interpolated to a daily resolution according to a bilinear interpolation method. Then, the volume changed the time series from 2010 to 2019 in Xiaowan and Nuozhadu reservoirs, which was constructed (Figure 6a,b) based on the SWE-WSE relation curves fitted in Figure 5g,h. Climate change had a very limited impact on the inflow (Figure 6c). Since the reservoir impoundment had intercepted a portion of the runoff, there had been a noticeable decrease in outflow after 2010 (Figure 6d). Reservoir regulation significantly limits peak flows and supplements dry season flows, resulting in less seasonal variation in outflows in the study area. The SWSA time series is shown in Figure 6e. Due to the operation of the reservoir, the SWSA from 2010 to 2019 fluctuated sharply (range -20 to 20 cm) and had a significantly decreasing trend (\(-5.48\) cm/yr) compared to 2002 to 2009. This result is consistent with the precipitation trend from 2010 to 2019 in Figure 3.  
Figure 5: The water bodies of **(a)** Xiaowan and **(b)** Nuozhadu reservoirs. The water surface elevations (WSE) derived from Jason-2. Sentinel-3, and Cryosat-2 in **(c)** Xiaowan and **(d)** Nuozhadu reservoirs. The surface water extents (SWE) provided by JRC and GRAS in **(e)** Xiaowan and **(f)** Nuozhadu reservoirs. The relation curves fitted by altimetry-derived WSE and GRAS-derived SWE in **(g)** Xiaowan and **(h)** Nuozhadu reservoirs.