# Evaluation of Global Precipitation Products over Wabi Shebelle River Basin, Ethiopia
## 3 Methodology
### Grid-to-Point Approach  
There are two typical approaches for evaluating global precipitation products, i.e., the grid-to-grid and point-to-grid methods. The first method requires the interpolation of gauge data to grid data, whereby gauge-gridded data are compared with grid data from global precipitation estimates; however, converting points to gridded interpolated data induces an error resulting from the interpolation of an uneven geospatial distribution [46; 47; 48; 49; 50]. The second approach involves an immediate comparison of station rainfall data to the respective pixel in which the gauges are located [23; 34; 41; 42]. In an area such as the Wabi Shebelle River Basin, with a scarcely and unevenly distributed gauge network, a pixel-to-point approach is the first choice to assess the GPPs independently, considering the gauge network as representative measurements irrespective of grids from nominated GPP, without considering the location of the station in the grid. Although global precipitation products exist at coarser resolution (larger pixel size) than required by climate studies and hydrological applications, they have to be downscaled to 1 km fine spatial resolution for  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multirow{2}{*}{**Input Data**} & **Spatial Res.** & \multirow{2}{*}{**Temporal Res.**} & \multirow{2}{*}{**Start Date**} & \multirow{2}{*}{**Designation**} \\  & & **(Degree)** & & & \\ \hline \multirow{3}{*}{Raw GPPs} & CHIRPSv3 & 0.05 & 1 day & 1981 & CHIRPSv3 \\  & PERSIANN\_CDR & 0.25 & 3 h & 1983 & PERSIANN\_CDR \\  & TAMSATv3 & 0.04 & 1 day & 1983 & TAMSATv3 \\  & CHIRPSv3-Downscale & 0.01 * & 1 day & 1981 & CHIRPS\_D \\ \multirow{3}{*}{Downscaled GPPs} & PERSIANN\_CDR\_Downscale & 0.01 * & 3 h & 1983 & PERSIANN\_CDR\_D \\  & TAMSATv3-Downscale & 0.01 * & 1 day & 1983 & TAMSAT\_D \\ \hline \hline \end{tabular}  
* The raw spatial resolution of the selected GPPs was downscaled to 1 km.  
\end{table}
Table 1: Global precipitation products used for Wabi Shebelle River Basin.  
evaluation with point gauge rainfall in the desired application. The spatial downscaling method and satellite rainfall estimate are the two most critical aspects in determining the accuracy of downscaled findings. In the Upper Tekezie River Basin, bilinear downscaling performed marginally better than the nearest-neighbor method to integrate satellite products with observed rainfall [51]. Other studies also preferred the bilinear downscaling method for smooth interpolated satellite-derived rainfall [52; 53]. Therefore, bilinear downscaling was the approach chosen to downscale the spatial resolution of pixels for this study area.  
The downscaled global precipitation product is more accurate than the original coarser resolution [54; 55]. Therefore, the pixel value of raw spatial resolution GPPs in their downscaled version (0.01\({}^{\circ}\)\(\times\) 0.01\({}^{\circ}\)) was compared to gauge measurements.  
The grid-to-point method can induce uncertainty in the performance of global satellite precipitation products due to the comparison of two datasets on different spatial scales regardless of the location of gauges in the pixel. The PERSIANN_CDR (0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\)) pixel contains multiple rain gauge stations (greater than 3), which allows investigating the spatial mismatch global precipitation products against station observations for the eastern upper course (blue-colored grid box in study area map.