# Evaluation of Grid-Based Rainfall Products and Water Balances over the Mekong River Basin
## 4 Results and Discussion
### Evaluation of Precipitation Products' Hydrological Performance using SWAT Model
#### 4.2.2 Water Balance Components at the Sub-Region Scale  
As mentioned above, the application of a GPP is specific to the location. This uncertainty in precipitation will propagate into water balance components. Therefore, for water balance analysis, the Mekong mainstream river was divided into six regions, respectively, with six hydrological stations (Figure 7). Region 1 (R1): Lancang Jiang or Upper Mekong River in China, limited by Chiang Saen gauge; Region 2a (R2a): Chiang Saen to Luang Prabang (Upper Nam Ou in Lao PDR); Region 2b (R2b): Luang Prabang to Nong Khai (northeast Lao PDR); Region 2c (R2c): Nong Khai to Mukdahan (central Lao PDR, which includes the Nam SongKharam and Nam Cadin tributary systems); Region 3 (R3): Mukdahan to Pakse (south Lao PDR and Korat Plateau, which includes the Mun and Chi tributary systems); Region 4 (R4): Pakse to Stungtreng (Se Kong, Se San and Sre Pok and in Cambodia, Vietnam);Figure 8 shows the variability of precipitation (P), evapotranspiration (ET), groundwater recharge (GW), and runoff (R) for each GPP obtained from SWAT model simulations for sub-regions in the MRB. A significant variation of all water-balance components under different GPPs is observed for each sub-region. These uncertainties are also expressed with a wide range of standard errors (SE), as presented in Table 6. The variation of precipitation could be the main reason for the variations of other water-balance components. In the R1 sub-region, for example, the variation of P among GPPs is much smaller than those in other sub-regions, and similar variations are found for other water-balance components. As a result, the selection of relevant GPPs for a specific sub-region is important for the reliability of water-balance components and water-resource evaluation. Based on the PBIAS and NSE values, it is recommended that CPC products be used for R1, TRMM for R2a, R2b, R2c, R4, and gauge products for R3. The quantitative analysis of the water-balance components using the recommended rainfalls as inputs for respective sub-regions is displayed in Table 6.  
Figure 7: Sub-regions for the water balance analysis in the Mekong River Basin. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Components/Region** & **R1** & **R2a** & **R2b** & **R2c** & **R3** & **R4** \\ \hline Precipitation (mm) & 857 & 1615 & 1988 & 2338 & 1958 & 2247.9 \\ SE (mm) & 63.7 & 123.5 & 192.4 & 214.0 & 121.7 & 300.5 \\ \hline Evaporation and & 496.3 & 967.0 & 956.9 & 936.9 & 921.0 & 1114.9 \\ transpiration (mm) & (58\%) & (60\%) & (48\%) & (40\%) & (47\%) & (50\%) \\ SE (mm) & 27.9 & 70.1 & 98.6 & 64.4 & 49.0 & 144.4 \\ \hline Total runoff depth (mm) & 357.3 & 511.0 & 1043.6 & 1305.7 & 662.2 & 1174.5 \\ (\% compared with rainfall) & (42\%) & (32\%) & (53\%) & (56\%) & (34\%) & (52\%) \\ SE (mm) & 10.6 & 20.2 & 99.1 & 138.5 & 47.5 & 165.5 \\ Surface runoff & 14.3 & 5.1 & 73.1 & 195.9 & 390.7 & 129.19 \\ Baseflow & 343.0 & 505.9 & 970.5 & 1109.9 & 271.5 & 1045.3 \\ \hline Groundwater recharge & 3.7 & 12.1 & 17.1 & 31.7 & 21.2 & 33.52 \\ (\% compared with rainfall) & (0.44\%) & (0.75\%) & (0.86\%) & (1.35\%) & (1.10\%) & (1.50\%) \\ SE (mm) & 1.3 & 1.7 & 5.0 & 3.1 & 1.9 & 4.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Water balance components of the sub-regions in the Mekong River Basin. Figure 8: **(a)** Precipitation (P), **(b)** evapotranspiration (ET), **(c)** groundwater recharge (GW) and **(d)** total runoff (R) of the sub-regions obtained from SWAT model simulations for each of the GPPs. Box plots display the 25 th, 50 th, and 75 th percentiles. Precipitation over the basin is highly variable, ranging from 2338 mm in R2c to 857 mm in R1 (Upper Mekong River in China). The highest rainfall regions, R2c and R4, generally correspond to the areas of high elevation along the Annamite Range and are covered primarily by forest (around 60%) with a streamflow-precipitation ratio of approximately 0.6.