Aydin: Comparison of different determining methods of reference ETo based on limited weather conditions in gap region - 10239 - APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 17(4):10239-10251. http://www.aloki.hu ● ISSN 1589 1623 (Print) ● ISSN 1785 0037 (Online) DOI: http://dx.doi.org/10.15666/aeer/1704_1023910251 © 2019, ALÖKI Kft., Budapest, Hungary COMPARISON OF DIFFERENT DETERMINING METHODS OF REFERENCE ETO BASED ON LIMITED WEATHER CONDITIONS IN GAP REGION AYDIN, Y. Department of Biosystem Engineering, Faculty of Agriculture, Siirt University, Siirt, Turkey (e-mail: yusufaydin@siirt.edu.tr; phone: +90-484-212-1111; fax: +90-484-223-1998) (Received 30 th Apr 2019; accepted 13 th Jun 2019) Abstract. Reference crop evapotranspiration (ETo) is among the most important components of the hydrologic cycle. Standard FAO-Penman-Monteith method is the most frequently used equation to estimate of ETo. However, the usability of this method is limited due to its complexity Hargreaveas- Samani (HS) and Turc methods that can be operated with limited data such as temperature and solar radiation are used as an alternative to the standard method. In the present study, the reference ETo values estimated with FAO-PM as the standard method, Hargreaveas-Samani and Turc methods were compared at semi-arid climate conditions. Based on the acquired results; highest R 2 value at the GAP region with semi-arid climate conditions was determined as 0.9872 from the monthly averages of ETo-PM and ETo-HS comparison, whereas the lowest RMSE value was also obtained from the same monthly comparison (0.08). ETo-Turc method yielded a monthly average comparison of 0.981 for the R 2 values comparison. While ETo-CAP put forth the lowest values for all three intervals. It is suggested that HS method can be used instead of Standart Penman-Monteith method (FAO-PM) under GAP region semi arid climate conditions for reference ETo estimation if Penman-Monteith can not be calculated. The Turc method is also can be a good alternative to estimate potential evapotranspiration that can be measured by Class A pan. And that ETo estimation should be made by determining the Kp value in case CAP based ETo estimation is carried out. Keywords: reference ETo, FAO-PM equation, Hargreaves-Samani equation, Turc equation, pistachio Introduction Evapotranspiration is the most important component of the hydrologic cycle and occurs with the mutual impact of evaporation and transpiration from the soil and the leaf. In this regard, it is the most important parameter for water source planning, water budget methods and irrigation software used in crop water consumption calculations. Continuity and high yield in agricultural production is possible only by the accurate and timely completion of losses due to evaporation and transpiration. For this purpose, ETo should be estimated accurately since it plays a critical role in the accurate estimation of water losses from the soil and the leaf, carrying out the plans accordingly as well as the planning of irrigation systems (Trajkovic, 2008; Çobaner et al., 2016). Evapotranspiration is generally estimated in two stages. It can be determined by estimating the evaporation gap of the air around the plant subject to certain climate parameters as well as by calculating the water consumption of the grass covering the soil surface completely which is used in estimating plant water consumption (ETc) and multiplying it by a certain coefficient specific to the crop. It is known that there are over 50 methods or models for the estimation of potential evapotranspiration subject to the data sets used (temperature, radiation, Class A Pan, mass transfer etc.), ecological conditions (arid or humid) and assumptions (Lu et al., 2005). Many researchers have tested the compliance of these methods under certain ecological conditions while also carrying out studies for putting forth the performances of difference methods under different climate conditions (Castaneda and Rao, 2005; Tabari et al., 2013).