Accuracy assessment of GPM-IMERG satellite precipitation data on half-hourly and daily time scales (Case study: Gorganroud Basin)

Precipitation is one of the most important factors affecting water and energy balance in the world and important meteorological variables. To accurately estimate precipitation, various methods are used, including the direct use of meteorological ground station data and direct observations, the use of remote sensing satellite data, or the use of interpolation methods based on geo-statistical methods. Therefore, developing innovative approaches for accurate estimation of precipitation in areas with inadequate or inadequate data is critical. The use of radar remote sensing technologies in the accurate estimation of precipitation is crucial as the most important factor affecting water and energy balance in areas with unsuitable and inadequate data. Therefore, this research was conducted to evaluate GPM-IMERG satellite precipitation data and compare it with the data of observatory stations in the Golestan province-Gorganroud basin.

To do this research, after obtaining the GPM satellite data and processing it, we performed a comparison between half-hourly daily satellite data set with the ground-based (stability and normal) observational data. Concerning the spatial (0.1*0.1) and temporal (daily and half-hourly) resolutions of GPM-IMERG satellite data, we employed enough and valid ground-based rainfall records dated 20/03/2014-20/03/2016 (for daily series) and 20/03/2014-21/09/2016 (for half-hourly series). To assess the accuracy of GPM data in rainfall estimation, some statistical indicators such as FAR , CSI , POD , RBias and some other validation indicators were used.

The results showed that the half-hour rainfall IMERG records with CC values equal to 0.05-0.23 and CSI equal to 0.20-0.52 were relatively acceptable. Validation of GPM satellite rainfall data using MAE, RMSE, and MBE statistical indicators has also been relatively acceptable. Based on the validation analysis of daily records, the RBias index showed the highest level of accordance of GPM data with observational data at 0.74 , and the lowest level corresponding to 2.27, that belongs to Nodeh station. The POD index also showed that Nodeh and HagholKhajeh stations had the highest and lowest correspondence with ground stations with the values of 0.5 and 0.25, respectively. The values of the CSI index in all stations were calculated to be between 0.13 and 0.22, which were related to Zarrin Gol and Shirabad stations, respectively. Based on the values of the FAR index, it was observed that the lowest value of FAR in Bagh Salian and Zarringol stations was 0.64 and the highest value was 0.80 in Shirabad station. Therefore, to improve the data obtained from the IMERG algorithm, especially in arid regions with the extensive spatial distribution and temporal changes in precipitation, satellite precipitation products should be calibrated to improve their accuracy in measuring daily precipitation.

In this study, the calculation of statistical and matching indicators was performed for the first time to compare half-hour data of the GPM satellite with observational data. It was found that the IMERG algorithm of the GPM satellite is relatively consistent with the recorded values of ground stations daily, as well. Given the FAR values at all stations, it can be said that there is a relative correspondence between satellite data and observed data from ground stations. POD values also showed acceptable performance of this satellite's data. The results of this study also showed that there was a relative correlation between the data of ground stations and GPM satellite data. Therefore, considering the non-evaluation of precipitation data of the GPM satellite system with data of ground stations in many regions of Iran, including the study area of Gorganroud, the results of this study can be very useful for innovation and increasing the efficiency in water resources management.