## 5 Results and Discussion
### Daily Evaluation  
We start with the evaluation of daily IMERG, TMPA-3B42 and ERA-Interim data against rain-gauge observations. This is based on days with observed precipitation solely. Figures 4-6 and Figure 7a-c show the scatter plots of average daily precipitation values during March 2014 to February 2015 in four regions. In Figure 4 and Table 4, all three satellite/model products indicate underestimation according to Bias, Mbias and Ribas values but IMERG shows slight underestimation which means that all three products generate negative systematic errors. 3B42 and ERA-Interim heavily underestimate precipitation in the Guilan region (G8) with respect to Mbias and Ribas. This is more so when the precipitation is above 20 mm according to Figure 4c for ERA-Interim. On the basis of RMSE and MAE, ERA-Interim shows better results in comparison with the IMERG and 3B42.  
Figure 4: Daily precipitation events scatter plots for Guilan of IMERG (**a**); 3B42 (**b**) and ERA-Interim (**c**) against gauge data (mid-March 2014 to February 2015).  
Figure 5: Daily precipitation events scatter plots for Bushehr of IMERG (**a**); 3B42 (**b**) and ERA-Interim (**c**) against gauge data (mid-March 2014 to February 2015).  
Figure 6: Daily precipitation events scatter plots for Kermanshah of IMERG (**a**); 3B42 (**b**) and ERA-Interim (**c**) against gauge data (mid-March 2014 to February 2015).  
As seen in Figure 5 and Table 5, IMERG, 3B42 and ERA-Interim tend to underestimate while ERA-Interim underestimates heavily when precipitation is above 10 mm in Bushehr (G4). The IMERG yields Bias, Mbias and Rbias better than other products. On the other hand, ERA-Interim provides better values of RMSE and MAE in comparison with 3B42 and IMERG. In all, IMERG indicates a good agreement with the observed data.  
Figure 6 presents the scatter plot of daily precipitation over Kermanshah area (G5) where IMERG, 3B42 and ERA-Interim underestimate the observed values. As seen in Table 6, IMERG has a better  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **No. of Events** & **RMSE (mm)** & **MAE (mm)** & **Bias (mm)** & **Mbias** & **Rbias** & **CC** \\ \hline IMERG & & 19.41 & 11.59 & \(-\)**6.41** & **0.62** & \(-\)**0.37** & 0.40 \\
3B42 & 105 & 19.59 & 11.68 & \(-\)9.75 & 0.31 & \(-\)0.69 & 0.29 \\ ERA-Interim & & **17.59** & **9.88** & \(-\)9.06 & 0.35 & \(-\)0.65 & **0.55** \\ \hline \hline \end{tabular}
\end{table}
Table 4: Statistical indices of daily precipitation events in Guilan area (mid-March 2014 to February 2015).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **No. of Events** & **RMSE (mm)** & **MAE (mm)** & **Bias (mm)** & **Mbias** & **Rbias** & **CC** \\ \hline IMERG & & 13.7 & 7.92 & \(-\)**2.52** & **0.90** & \(-\)**0.10** & **0.51** \\
3B42 & 19 & **11.86** & **7.07** & \(-\)2.95 & 0.75 & \(-\)0.25 & 0.47 \\ ERA-Interim & & 14.30 & 8.04 & \(-\)7.72 & 0.22 & \(-\)0.78 & 0.40 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Statistical indices of daily precipitation events in Bushehr area (mid-March 2014 to February 2015).  
Figure 6: Density precipitation events scatter plots for Tehran of IMERG (**a**); 3B42 (**b**) and ERA-Interim (**c**) against gauge data (mid-March 2014 to February 2015).  
Figure 7: Daily precipitation events scatter plots for Tehran of IMERG (**a**); 3B42 (**b**) and ERA-Interim (**c**) against gauge data (mid-March 2014 to February 2015).  
value for RMSE, Bias, Mbias, Rbias and CC in comparison with other data sets while IMERG performs better when precipitation is greater than 10 mm.  
As seen in Figure 7, for, IMERG, 3B42 and Era-Interim underestimate Tehran (G2) daily precipitation. Furthermore, based on Table 7, 3B42 and IMERG show lower bias and better value of Mbias and Rbias in comparison with the other two datasets whereas ERA-Interim yields better in RMSE and MAE.