Estimation of reference evapotranspiration in the data condition of deficiency and assessment of Valiantzas methods for the coastlines in the north and south of Iran

Evapotranspiration (ET) is the most important parameter in climatic and hydrological studies and that of the irrigation management and planning. Estimation of reference evapotranspiration (ETo) in a simple way is of great interest, especially in developing countries, where meteorological data required for standard method of FAO Penman- Monteith (PMF-56) is incomplete or not available. Also, despite of the large amount of agricultural development in the coastal of Iran, no comprehensive research in calculating evapotranspiration on the northern and southern coastlines has not been conducted. The aims of this study were: 1. assessment of reference evapotranspiration using five methods required low data including: Hargreaves-Samani, Priestley Taylor, Makkink, Turc. 2. Evaluation of the two models of the newest methods to estimate ETo by Valiantzas along with PMF-56 method, for evaluating the methods described. For this study meteorological data from 14 synoptic stations including 8 stations located in the Southern coastline and 6 stations located in the northern coastline of Iran, were selected. The statistical results have shown that among ETo estimation methods in terms of lack of data Priestley- Taylor method because of the higher temperature and humidity for the station located in the southern coastline, and Turc method due to temperate and humid climate for stations located in northern coastline has the best performance compared to PMF-56 method. Among the methods developed by Valiantzas, method of Valiantzas 2 has the highest correspondence compared to PMF-56 method.