## 4 Discussion
Furthermore, given their spatial resolution, GPPs will be less sensitive to short-range variations in rainfall, which may explain the somewhat poorer performance of IMERG in the present study, given that many gauges are separated from each other by less than 10 km (i.e., roughly 0.1\({}^{\circ}\), the spatial resolution of an IMERG grid box). Additionally, many factors could influence this variation of performance across studies, including the density of the rain gauge network and especially the validation technique [25; 63; 67]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**References/ Study Area**} & \multicolumn{2}{c}{**Study Period/**} & \multirow{2}{*}{**Validation**} & \multirow{2}{*}{**CC**} & \multirow{2}{*}{**RMSE**} & \multirow{2}{*}{**MAE**} & \multirow{2}{*}{**POD**} & \multirow{2}{*}{**FAR**} & \multirow{2}{*}{**CSI**} \\  & **Number of Rain** & & & & & & & \\
**Gauges for** & **Category Validation** & & & & & & & \\ \hline
[25]/East Africa & 2000–2018/36 & grid-to-grid & 0.41 & 12.4 & 7.6 & 0.88 & & \\
[66]/East Africa & 2014/37 & grid-to-grid & 0.53 & & & 0.87 & 0.04 & \\
[68]/Singapore & 2014–2016/48 & grid-to-grid & 0.53 & 11.83 & & 0.78 & 0.28 & 0.60 \\
[63]/Philippines & 2014–2017/55 & grid-to-grid & 0.81 & 5.66 & 3.74 & & & \\
[64]/Bali & 2015–2017/27 & point-to-grid & 0.32 & 17.19 & & 0.84 & 0.54 & 0.44 \\
[69]/Vietnam & 2014–2016 53 & grid-to-grid & 0.58 & & & 0.73 & 0.22 & 0.61 \\
[70]/Malaysia & 2014–2016/31 & point-to-grid & 0.54 & 14.93 & & 0.89 & 0.20 & 0.73 \\
[62]/Mexico & 2014–2015/99 & point-gridded & 0.54 & 7.93 & & 0.2–0.6 & 0.2–0.6 & 0.2–0.8 \\ \hline Ankavia & 2018–2020/14 & point-gridded & 0.80 & 12 & 5.5 & 0.5–0.6 & 0.2–0.4 & 0.4–0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of IMERG assessment studies in tropical environments at a daily time scale. According to our results, the performance of IMERG is rather uniform across the watershed, i.e., there is no evidence of a spatial trend of the statistical metrics (Figure 4). Therefore, the negative effect of the topography, which often alters the performance of the precipitation satellites, is not apparent in our study area. Nevertheless, our results are consistent with previous findings for stations located in mid- and low-altitude, with relatively mild and wet climates [67; 71; 72]. Indeed, 13 of the 14 rain gauge stations in the Ankavia catchment are located between 14 and 300 m a.m.s.l. The PDF analyses underline that most of the GPPs show the same distribution as the gauges except for the ERA5 data and, to a lesser extent, the CHIRPS data. Overall, the results reveal an overestimation for the 0-0.2 mm/day precipitation class and an underestimation of the >0.2 mm/day classes (Figure 5). Other studies have, however, reported that IMERG slightly overestimated the frequency of rainfall events between 1 and 50 mm/day [64; 68; 70]. In addition, the findings also indicate that some GPPs underestimate the precipitation classes >150 mm/day. The poor performance of GPPs at detecting extreme events was also reported in other assessment studies in tropical river basins [23; 68]. Specifically, since gridded products contain spatially-averaged rainfall values, larger grid size (0.1 \(\times\) 0.1\({}^{\circ}\)) products are more likely to smooth out the extreme rainfall values (>150 mm/day), which are especially associated with short-duration events with limited spatial extent [69; 73]. In contrast, CHIRPS and PERSIANN outperform all other GPPs in this range (>150 mm/day). Their capacities to better represent very high-intensity rainfall could be due not only to their smaller grid size (0.05\({}^{\circ}\) and 0.04\({}^{\circ}\), respectively) but also to their ability to categorize cloud-patch features based on height, areal extent, and variability of texture estimated from satellite imagery [46; 55]. These classifications aid in the assignment of rainfall values to pixels within each cloud based on a predefined curve that describes the link between rain rate and brightness temperature [55].