Evaluation of Different Reference Evapotranspiration Methods and their Zonings in Iran

Introduction
Iran is characterized as arid and semi-arid climates due to low annual precipitation, high frequencies of flood and high rate of evapotranspiration. Evapotranspiration as one of the main components of hydrologic cycle is affected by atmospheric conditions and plant physiology. The complex of weather factors and plant physiology has encouraged the researchers to use numerous approaches for estimation reference evapotranspiration rate (ET0). Although lysimeters are the only direct methods for measuring ET0, the high costs of installation and maintenance are among the main problems for its popular use. Nevertheless, lysimeters are the most reliable tools for ralidation outputs of the ET0 models. The main objective of the present study is to assess various reference evapotranspiration models in order to provide Iso-ET0 maps as useful tools for water resources managers.

Materials and Methods
This study contains the UNESCO approach to classify the climates of 91 weather sites. The authors have also used lysimeyer data for selecting the most accurate ET0 model. All 91 sites of case-studies were classified in six different groups accordingly and the ET0 values were determined for 13 different ET0 models by using Ref-ET software. Computation ET0 methods include seven combination Penman type methods, two temperature-base ones, three radiation-temperature ones and at last one radiation method. In the next stage, among the 13 ET0 models, the best performance ET0 model was selected for each climate type according to the lysimeter data. The selection of the best ET0 method was done by different statistics such as coefficient of determination (R2), root mean square error (RMSE), and mean bias error (MBE). The authors used geo-statistical Kriging approach to prepare the estimated ET0 maps. The Iso-ET0 maps were plotted by means of geographical information of the weather site, the GIS and Surfer tools.

Results and Discussion
The results highlighted that among the 91 weather sites and 9 lysimeter stations, in 55.6 % cases, Penman type methods perform the most accurate ET0 estimates for most parts of Iran (Rahimi Khoob, 2008; Dehghani Sanij et al, 2007; Jensen et al., 1990). In 22 percent of the stations Blaney-Cridle and Hargrive Samani two methods were appropriate. The comparison of model results with those of lysimeter data showed reasonable deviations (RMSE=1.1 mmday-1, MBE= -0.23 mmday-1, R2=0.85) suggesting the accuracy of the each selected ET0 model. The geostatistic Kriging Iso-ET0 maps indicated that 23% of the country (mainly northern parts) experiences mean ET0 values of less that 4.48 mm per day. For the rest of the country (77%), the mean daily ET0 was more than 10.7 mm. It seems that the low rates of evapotranspiration in the northern elevated sites is a result of higher precipitation (higher humidity) and relatively lower temperature in this region. In arid and semi-arid sites, the irregularity of precipitation time series and dramatic changes in precipitation variability are the main reasons of higher evapotranspiration a water resources deficit in these regions. According to the extent of many countries, and the limited number of stations Laysymtry, estimated evapotranspiration, water management in the agricultural sector is required. Therefore, having the best model to estimate evapotranspiration in different areas and its interpolation with the most appropriate method helps a lot more accurate estimation in areas without measuring.

Conclusions
In consistence with the results addressed by Farshi and Shariati (1999) and Dehghani Sanij et al. (2004), our model results showed that Penman type models are the most reliable ET0 models for estimation of reference evapotranspiration in most parts of Iran. The Kriging-GIS maps of ET0 values showed that 23% of the country mainly located in high altitude of north Alborz mountain range experience low evapotranspiration. In contrary, about 77% of Iran show high evapotranspiration rates. The existence of Alborz mountains, north forests and high humidity are the major reasons for lower evapotranspiration rate in the northern areas (Rahimi Khoob and Khoshkam, 2003, 55). In comparison with the ET0 of the north, geographical maps of Iso-ET0 values showed higher potential of ET0 in southern parts of the country. Overall, water is a valuable commodity for Iran, especially in the areas with high potential ET0, in agriculture and consumer other sectors. Watershed management plans as well as plans for surface water storage, given the lack of precipitation and high average ET0, is essential.