## 4 Results  
### GPM IMERG-E Product Evaluation  
The GPM IMERG-E product is evaluated against the rain gauge data using six statistical indices considered (CC, ME, RMSE, BIAS, POD and FAR) at both basin scale and the individual grid-cells corresponding to rain gauges during the available period from January 2011 to December 2018. The statistical evaluation indices of the IMERG-E product are presented at the basin scale in Table 3 and at the grid-cell scale in Table 4.  
At the basin scale, satellite precipitation data overestimates the amount of rainfall with a relative Bias of +35.61%. Several studies found that the IMERG precipitation products overestimate the amount of rainfall; in Italy [57], in Peru and Ecuador [58], in Iran [59] and in Spain [60]. About the overall agreement between the gauge observations and satellite precipitation data, the Pearson's correlation coefficient is equal to 0.10 and 0.14 at the hourly and daily time scale, respectively. Therefore, the accuracy of IMERG-E data improved over the daily time scale. In terms of precipitation detection capability, the near real time IMERG-E precipitation data presents satisfactory performance and can be used to detect rain events over the Ghdat basin. The same results have been found in Spain with a low bias in the IMERG-E product that allows near-real time applications [61]. The daily estimates had a higher POD equal to 0.68 and a smaller FAR equal to 0.63 compared to the hourly (POD = 0.36 and FAR = 0.86) time scale. For the grid-cell evaluation, the comparison of IMERG-E data with the data at the Sidi Rahal rain-gauge presents higher performance compared with others pixels; in which the RMSE is lower at the hourly time scale. Concerning the agreement between observed and satellite precipitation data, Sidi Rahal also shows the higher correlation coefficients compared to the other gauges. The GPM IMERG-E presents slightly higher performance in the downstream part of the studied basin. These results may be reflected the impact of the complex topography in the upstream portions of the basin. The good agreement between observed and satellite precipitation at the Adrar station is probably due to the very good quality of the data at this station; this gauge is located at a relatively high altitude but in an open area where there is no relief effect affecting rain measurements. This gauge is also \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Pixel** & **Time Scale** & **CC** & **ME (mm)** & **RMSE (mm)** & **BIAS (\%)** & **POD** & **FAR** \\ \hline \multirow{2}{*}{GPM-Adrar} & Hourly & 0.03 & 0.00 & 2.05 & \multirow{2}{*}{95.67} & 0.24 & 0.91 \\  & Daily & 0.08 & 0.07 & 24.15 & & 0.57 & 0.72 \\ \hline \multirow{2}{*}{GPM-Sidi Rahal} & Hourly & 0.26 & 0.03 & 0.61 & \multirow{2}{*}{3.99} & 0.25 & 0.90 \\  & Daily & 0.37 & 0.63 & 5.73 & & 0.47 & 0.76 \\ \hline \multirow{2}{*}{GPM-Tijdante} & Hourly & 0.00 & 0.02 & 0.84 & \multirow{2}{*}{143.29} & 0.11 & 0.97 \\  & Daily & \(-\)0.02 & 0.51 & 9.32 & & 0.40 & 0.83 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Evaluation indices of IMERG-E at the grid-cell scale for the entire period (2011–2018). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Time Scale** & **CC** & **ME (mm)** & **RMSE (mm)** & **BIAS (\%)** & **POD** & **FAR** \\ \hline Hourly & 0.10 & 0.02 & 0.84 & \multirow{2}{*}{35.61} & 0.36 & 0.86 \\ Daily & 0.14 & 0.44 & 9.88 & & 0.68 & 0.63 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Evaluation indices of IMERG-E at basin scale for the entire period (2011–2018). considered by the Tensift Hydraulic Basin Agency as reference station, that is also used to relay data transmission from the other gauges with a poorer network coverage. Several studies presented similar or comparable results. In terms of elevation effects on the IMERG product performance, in the Ebro River basin in Spain the IMERG performance strongly depends on elevation [60].