# Evaluation of Grid-Based Rainfall Products and Water Balances over the Mekong River Basin
## 4 Results and Discussion
### Statistical Evaluation of GPPs
#### 4.1.2 Monthly Comparison  
The monthly average precipitation time series in the basin for the GPPs during the period 1998-2006 were evaluated against the rain gauge data at different spatial scales (i.e., basin and pixel scales). Relatively good agreements with the rain gauge data were observed in the CC indicator for all of the GPPs at the basin scale, with 0.83 for TRMM 3B42, 0.79 for GSMaP, 0.77 for CMORPH, and 0.76 for CPC. In terms of the five statistical indicators (Table 4), TRMM 3B42 had the best performance at the monthly scale, but with a slightly overestimated precipitation value, and CMORPH, CPC, and GSMaP had large errors with all indicators underestimating the precipitation. At the pixel scale, the spatial distributions of the indicators (CC, RMSD, and PBIAS) at a monthly scale over the study area are shown in Figure 3a-l). From the simulated data, it can be seen that TRMM 3B42 has the best CC values for the whole basin. The CC values of CPC and GSMaP are worst in the south-east region, ranging from 0.2 to 0.4, while those of CMORPH are relatively low in the upstream and south-west regions of the basin, in the range of 0.4-0.6 (Figure 3a-d). \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Time** & **Precipitation** & **Mean** & **SE** & **RMSD** & **MAE** & **CC** & **PBIAS** & **POD** & **FAR** & **CSI** \\ \hline \multirow{4}{*}{Daily} & Gauge & 4.61 & 0.21 & - & - & - & - & - & - & - \\  & CMORPH & 3.73 & 0.15 & 11.75 & 4.87 & 0.40 & \(-\)16 & 0.82 & 0.29 & 0.50 \\  & CPC & 3.65 & 0.14 & 11.44 & 4.83 & 0.40 & \(-\)15 & 0.91 & 0.41 & 0.48 \\  & GSMaP & 3.52 & 0.13 & 10.73 & 4.56 & 0.44 & \(-\)12 & 0.92 & 0.44 & 0.46 \\  & TRMM 3B42 & 4.69 & 0.18 & 12.19 & 5.24 & 0.42 & 4 & 0.81 & 0.28 & 0.50 \\ \hline \multirow{4}{*}{Monthly} & Gauge & 140.33 & 2.62 & - & - & - & - & - & - & - \\  & CMORPH & 113.60 & 1.82 & 97.05 & 65.09 & 0.77 & \(-\)16 & - & - & - \\ \cline{1-1}  & CPC & 110.96 & 1.85 & 99.58 & 68.20 & 0.76 & \(-\)15 & - & - & - \\ \cline{1-1}  & GSMaP & 104.63 & 1.79 & 91.71 & 63.70 & 0.79 & \(-\)12 & - & - & - \\ \cline{1-1}  & TRMM 3B42 & 142.57 & 2.33 & 82.93 & 53.59 & 0.83 & 4 & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average indexes of the rain gauge observations and precipitation products, at both the daily and the monthly time scales. The RMSD shows a similar pattern, but to a different extent in all of the GPPs (Figure 3e-h). The RMSD of TRMM 3B42 is the lowest error (around 50-100 mm/month) at the monthly scale, compared to all of the other GPPs (250-300 mm/month). Very high RMSD values are seen in CPC and GSMaP in the south-east of the river basin, as shown in the Figure 3e-h, which is probably because this area is a high rainfall zone in the river basin. At the basin scale, TRMM 3B42 also has the smallest RMSD value (82.93 mm/month), while the RMSD values of GSMaP, CMORPH, and CPC are 91.71, 97.05, and 99.58 mm/month, respectively. Figure 3: Spatial distributions of the correlation coefficient (CC) (**a–d**), the root-mean-squared deviation (RMSD) (mm/month) (**e–h**), and the percentage bias (PBIAS) (%) (**i–l**) between the GPPs and the gauge observations at the monthly scale during 1998–2006. The distribution of PBIAS indicates that TRMM 3B42 performs the best compared to the others, since PBIAS shows the lowest values at both the grid scale, around 10% (Figure 3i-l), and the watershed scale, at 4% (Table 4). Considering CPC and GSMaP, their PBIAS values are consistently the worst in the east of basin--40% underestimation was seen--but the indicators are improved in the south-west of the basin, with PBIAS values of around 10% overestimation. In contrast, the PBIAS of CMORPH has no clear relationship with the gauge observations, sometimes overestimating or underestimating the observation by 20%-40%, as seen in Figure 3i-l. As expected, at the watershed scale, the PBIAS of GSMaP (-12%) and of CPC (-15%) can be considered to perform comparably better than CMORPH (-16%).