Shoukat Ali Shah - Institute of Water Resources Engineering and Management, Mehran University of Engineering and Technology, Jamshoro, Sindh, Pakistan.

Reference Evapotranspiration (ETo) counted as the main factor for assessing the amount of water, needed for crops as well as for the planning of water resources management. Several techniques, methods, and equations have been used for computing ETo. Thus, required weather data sets are the main challenge for evaluating this factor. FAO Penman-Monteith is the most popular technique to determine the ETo. The FAO ۵۶-PM equation requires accurate weather data like air temperature, humidity, solar radiations, and wind speed. Unfortunately, not all these data are possible to reach easily on the station's side. Therefore, FAO ۵۶ recommended another equation namely Hargreaves-Samani (HS) equation when sufficient weather data may not be available to estimate ETo by FAO۵۶-PM. In the context of this, this study aimed to estimate ETo using the HS equation. For this purpose, historical annual, seasonal, and monthly temperature and wind data were collected from ۱۹۸۱ to ۲۰۲۰ using ‘The Prediction of Worldwide Energy Resources (POWER)’ web portal. It is concluded that the HS method in conjunction with the POWER datasets and spatially mapping with IDW interpolation gave reliable and accurate results of ETo. This technique gives an idea of water losses in a district and demonstrates a trend of historical annual, seasonal, and monthly ETo.

Reference evapotranspiration, IDW interpolation, NASA-POWER, Hargreaves-Samani