*4.1. Monthly Validation*

Mean monthly values for the study period were calculated for both the gauge stations (136 stations) and the two satellite precipitation products (mean values of all available corresponding grid cells within the study area). The results are illustrated in Figure 3. We notice that, overall, both IMERG and TRMM data follow very well the "climatology" of the stations, although with an underestimation during the rainy period, while IMERG is closer to the gauge values for almost the whole period of study.

**Figure 3.** Average monthly values calculated for gauges (dark blue) TRMM 3B43 estimates (light blue) and IMERG estimates (red line) for the period April 2014 to June 2018.

In order to highlight the performance of satellite precipitation (SP) products, we estimate their average difference (of their monthly values) from the corresponding gauge stations data (see Figure 4). We notice that, generally, IMERG monthly values present lower divergence than TRMM.

Next, IMERG and TRMM monthly estimates were compared to the corresponding gauge data based on a grid-level approach. Thus, for each SP product, the mean gauge station value is calculated for the comparison if more than one station was located within each SP grid.

**Figure 4.** Average difference (monthly values) between gauge stations data from 3B43 (blue line) and IMERG (red line) estimates, respectively.

The estimated correlation coefficient values between the monthly values of the gauge stations and the SP data are presented in Figure 5. Correlation seems slightly better for TRMM 3B43 cells, since there is a lower variation (minimum 0.84–maximum 0.96) than that corresponding to the IMERG cells (minimum 0.78–maximum 0.96), with the average of all cells being 0.92 and 0.91, respectively.

**Figure 5.** Correlation coefficient at grid cell level between the gauge stations and the TRMM (**a**) and IMERG (**b**) data.

Both IMERG and 3B43 products underestimate precipitation with Bias values of −1.6 mm/month and −3.0 mm/month, respectively. IMERG showed better performance than 3B43 in terms of rBias values (3.6 and −8.7, respectively), while presenting worst performance in terms of RMSE (23.8 mm/month and 20.0 mm/month, respectively) and MAE (15.1 mm/month and 12.9 mm/month, respectively) values, as presented in Table 2 (minimum and maximum respective values are provided in brackets).


