ABSTRACT

Estimation of reference evapotranspiration (ETo) is important for several hydrological and agricultural sciences, like water resources management, crop-water requirements, irrigation scheduling, and land use planning [21, 26]. Though, a good amount of research is done by the scientists to estimate the ETo accurately, especially for the sensitive fields like irrigation scheduling [9, 25]. Due to its comprehensive theoretical base, the Penman-Monteith method [PM] method is recommended by the United Nations Food and Agricultural Organization (FAO) as the sole method to calculate (ETo) and for evaluating other ETo calculation methods as well [3, 19]. The FAO approach to calculate ETo using the PM method was published in the FAO irrigation and drainage paper number 56 [FAO-56]. The objectives of this chapter were: (1) To investigate whether the ETo values calculated by PM method are affected significantly by Allen [2] temperature adjustments or not, and (2) To adjust the Hargreaves equation’s parameters to use it in prediction of the actual ETo values from nonstandard weather stations, for all zones of the Kingdom of Saudi Arabia. The “FAO-56 PM” method is shown in Eq. (1) and requires measurements of air temperature, relative humidity, solar radiation, and wind speed.