Evaluation of reference evapotranspiration models for warm arid climate (Case Study: Zahedan station)

Background and objectives: Evapotranspiration (ET) is an important parameter for climatological and hydrological studies, as well as for irrigation planning and management. Evaluation of simple reference evapotranspiration (ETo) methods has received considerable attention in developing countries where the weather data needed to estimate ETo by the Penman–Monteith FAO 56 (PMF-56) model are often incomplete and/or not available. So the goal of this study, was to Evaluation and Comparison 30 different methods according to the standard method of reference evapotranspiration PMF-56 equation and determine the best equation, as a Replace to PMF-56, in a warm arid climate is the city and station of Zahedan.

Materials and methods: In this study, 30 commonly used ETo equations that belonged to four groups: (1) pan evaporation-based methods, (2) temperature-based methods,(3) radiation-based methods, and (4) mass transfer-based methods were evaluated against the PMF-56 standard model; and the best and worst equations of each category were determined from the Zahedan station located in a warm arid climate.The assessed methods were: Cuenca, Allen and Pruitt, Snyder, Modified Snyder, Pereira, Orang, Raghuwanshi and Wallender, FAO-56 (pan evaporation-based), Schendel, four new types of Hargreaves equation reported by Droogers and Allen and Trajkovic, Blaney–Criddle (temperature-based), Makking, Turk, Jensen–Haise, Priestley- Taylor, FAO-24 (radiation-based), Dalton, Trabert, Meyer, Rohwer, Penman, Albrecht, Romanenko, Brockamp and Wenner, WMO and Mahringer (mass transfer-based).

Results: Initially, in each group the monthly mean values ETo (using 30 model) were determined and compared their relative performance with respect to PMF-56 ETo estimates in the study.
The ETo calculated by the Snyder equation (R2=0.85، NRMSE=0.08، PE=8.30), Hargreaves- 4 (R2=0.93، NRMSE=0.04، PE=8.33), Turk 24 (R2=0.96، NRMSE=0.03، PE=6.52) and Dalton (R2=0.81، NRMSE=0.18 PE=12.89), pan evaporation-based, temperature-based, radiation-based and mass transfer-based respectively best matched the ETo estimates by the PMF-56 equation with the lowest errors rates.

Conclusion: In general, the comparative results showed that the mass transfer-based equations had the worst performances, while the radiation-based and temperature-based models (as Turk, Jensen–Haise, Hargreaves- 4 and Blaney–Criddle) were the best-suited equations for estimating ETo in this Warm arid climate (Zahedan). Considering the unavailability of full weather data for applying the PMF-56 model for estimation of ETo in many regions of the world and in different parts of the Iran, especially in developing countries, the results will be useful for choosing the simpler ETo methods in Warm arid climates. It is better to achieve real results for other places with similar climatic conditions to be assessed.