## 6 Conclusions
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & **POD** & & **FAR** & & & **CSI** & \\ \hline \multirow{3}{*}{
\begin{tabular}{c} **Number of** \\ **Precipitation** \\ **Days >15 mm** \\ \end{tabular} } & \multirow{3}{*}{**IMERG**} & \multirow{3}{*}{**3B42**} & \multirow{3}{*}{**ERA**} & \multirow{3}{*}{**IMERG**} & \multirow{3}{*}{**3B42**} & \multirow{3}{*}{**ERA**} & \multirow{3}{*}{**IMERG**} & \multirow{3}{*}{**3B42**} & \multirow{3}{*}{**ERA**} \\  & & & & & & & & & \\ \hline
**Guilan (G8)** & **20** & **0.36** & 0.14 & 0.09 & 0.53 & 0.69 & **0.38** & **0.23** & 0.10 & 0.08 \\ \hline
**Bushehr (G4)** & **2** & **0.42** & 0.37 & 0.00 & 0.70 & 0.60 & — & 0.26 & **0.30** & 0.00 \\ \hline Kermanshah (G5) & **2** & **0.36** & 0.29 & 0.02 & **0.67** & 0.72 & 0.86 & 0.21 & 0.17 & **0.39** \\ \hline Tehran (G2) & **2** & **0.24** & 0.08 & 0.00 & **0.70** & 0.87 & 1.00 & **0.09** & 0.05 & 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 18: Spatially averaged POD, FAR and CSI metrics for precipitation above 15 mm/day. precipitation on the daily scale. With respect to contingency table metrics, IMERG outperformed ERA-Interim on the basis of POD, FAR, RMSE, Bias, Mobias, Ribias and CC (correlation coefficient) while ERA-Interim performed best in terms of MAE values on the daily scale. 3B42 outperformed other products on a monthly scale. Moreover, all three products showed rather the same behavior in seasonal scale but IMERG indicates a better CC. 3. Just northwest of the Persian Gulf, Bushehr region is subject to warm and subtropical arid climate in low latitudes. In this area, on the daily scale, IMERG was more accurate in terms of Bias, Mobias, Ribias and CC while 3B42 was better in RMSE and MAE. Also, 3B42 outperformed other products on monthly and seasonal scales. On the other hand, IMERG showed reasonable results with a POD of 0.70 that could be due to the dual-frequency sensor based on the GPM which can detect light rain. 4. Tehran is located in a semi-arid climate with the towering Alborz Mountains to its North and the central desert to the South. In this region, according to the daily scale precipitation, IMERG outperformed other precipitation products with POD, FAR and CSI. 5. ERA-Interim yields weak results of POD, FAR and CSI for precipitation above 15 mm/day over Iran while IMERG is far superior to the other products in all study areas. ERA-Interim in Guilan region shows a significant value of POD for precipitation below 15 mm/day. 6. Generally, in Guilan, Kermanshah, Tehran and Bushehr regions, all three products (IMERG, 3B42 and ERA-Interim) underestimate precipitation based on daily scale, which might be due to an inadequate number of gauges which are provided by the Global Precipitation Climatology Centre (GPCC) and used for bias correction in satellite products or/and the inability to measure available water in the air profiles. Moreover, in semi-arid and hot climates, rain drops may evaporate before reaching the ground [40]. 7. Based on monthly and seasonally scale, in Guilan, all products tend to underestimate while in Bushehr and Kermanshah, IMERG and 3B42, and in Tehran, 3B42 indicated overestimate. In summary, the overestimation or underestimation over mountainous regions indicates that accurate estimation by satellite-based precipitation products remains a challenge. Such inaccuracy may be rooted in the inadequate number of gauges, provided by the Global Precipitation Climatology Centre (GPCC) and used for bias correction in satellite products or the non-uniform beam filling (NUBF) problem for remote sensing instruments. Another cause might be the spatial resolution of the satellite product, since precipitation within a region may occur in smaller scales than the pixel size of satellites. Overall, this preliminary accuracy assessment highlights that the IMERG product can adequately substitute 3B42 products despite its limited historic record.