Estimation of Reference Evapotranspiration Using Full and Limited Data (Case Study: Tabriz and Urmia Synoptic Stations)

One of the essential prerequisites for improving water consumption management in a farm is the accurate estimation of the crop water consumptive use. Accurate estimation of the consumed water by the crop evapotranspiration is one of the important factors in planning for agriculture development and increasing agricultural production. The FAO Penman– Monteith (FPM) method has been proposed as a universal standard method to estimate the reference crop evapotranspiration (ETo) and to evaluate the other procedures of EToestimation, which needs solar radiation, wind speed, air temperature and relative humidity data. Unfortunately, there are no facilities for measurement of some weather parameters at some weather stations. Therefore, identifying the correctness of the ETo estimation methods by using the estimated climatic data is very important. Based on this, the objective of the present study was to evaluate the performance of the FPM method for estimating ETo when solar radiation (Rs), vapor pressure (ed) and relative humidity (RH) data were not available under climatological conditions of the northwest of Iran. Data of Urmia and Tabriz stations were used to compare the estimated ETo by the FPM method for the complete and limited data sets. The results showed that when RH and ed data were missing, the FPM method was still a very good option for estimating ETo in Urmia and Tabriz stations, with RMSE value smaller than 0.4 mm day-1. When Rs was missing, the FPM method was not good enough for estimating ETo, and RMSE increased to 0.7 mm day-1. The results also showed that solar radiation was the most effective parameter on ETo estimation and both vapor pressure and relative humidity had the same effect on ETo estimation.