Performance Evaluation of WaPOR and ERA5 Datasets for the Purpose of Estimating Reference Evapotranspiration in the Caspian Sea Basin

To compensate for the lack, or inadequacy, of weather stations data, one of the most reliable ways is to use the remote sensing and re-analysis dataset, which provides a suitable model for such areas. In this study, the performance of two data sources, namely, WaPOR and ERA5, was evaluated in estimating reference evapotranspiration at 64 synoptic stations in the Caspian coastal region in Iran, on a daily and monthly basis. To this end, meteorological data of 64 synoptic stations with a 10-year statistical period (2011-2021) was obtained daily from the Iran Meteorological Organization. The field data used included minimum and maximum temperature, relative humidity, wind speed, and solar radiation intensity. Then, evapotranspiration and reference evapotranspiration were calculated using the Penman-Monteith equation and REF-ET software. Finally, the results were compared with the results from the WaPOR and ERA5 data bases. The results showed that, on average, the nRMSE values of the WaPOR and ERA5 datasets compared to the calculated meteorological station data were 29.6% and 29%, respectively, on a daily basis. Also, on a monthly time scale, in more than 85% of the stations, both datasets provided acceptable results. On a monthly scale, the average nRMSE value for both WaPOR and ERA5 sensors in the catchment area was 19%. The rMBE value showed that the ERA5 dataset underestimated the reference evapotranspiration in most of the stations, while the WaPOR dataset overestimated. Given that the error rate of the two sensors is different in over 30 percent of the stations, a suitable estimate of reference evapotranspiration in the Caspian Sea basin area can be obtained by combining the data from these two datasets. The results showed that in the Caspian Sea coastal areas, 34 stations in the WaPOR dataset and 28 stations in the ERA5 dataset showed the minimum error, with two stations showing the same error. Thus, both WaPOR and ERA5 are suitable databases that can be used for hydrological purposes, including estimation of reference evapotranspiration.