Islam et al.: Evaluation of mass transfer evapotranspiration models under semiarid conditions using MCDM approach - 6355 - APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 18(5):6355-6375. http://www.aloki.hu ● ISSN 1589 1623 (Print) ● ISSN 1785 0037 (Online) DOI: http://dx.doi.org/10.15666/aeer/1805_63556375 © 2020, ALÖKI Kft., Budapest, Hungary EVALUATION OF MASS TRANSFER EVAPOTRANSPIRATION MODELS UNDER SEMIARID CONDITIONS USING MCDM APPROACH ISLAM, S. 1* – ABDULLAH, R. A. B. 1 – TIRTH, V. 2 – SHAHID, S. 1 – ALGARNI, S. 2 – HIROL, H. 1 1 Department of Civil Engineering, University Teknologi Malaysia, P.O. Box 81310 Johor Bahru, Johor, Malaysia 2 Department of Mechanical Engineering, College of Engineering, King Khalid University, Abha 61413 Asir, Kingdom of Saudi Arabia *Corresponding author e-mail: isaiful2@graduate.utm.my; phone: +966-59-521-9933; fax: +966-17-241-8816 (Received 21 st Mar 2020; accepted 11 th Aug 2020) Abstract. The selection of suitable reference evapotranspiration (ETo) models in case of climatic data scarcity is a challenging task as it plays a pivotal role in agriculture and water resource management. Therefore, the research work deals with selecting the appropriate mass transfer reference evapotranspiration model using multi criteria decision technique (MCDM) in a semi-arid region of the southern part of Kingdom of Saudi Arabia i.e., Abha. The ten mass transfer methods with ten criteria (statistical indices) using available weather parameters from 1980 to 2018 have been illustrated in this study. Models were calibrated (1980-2006) and validated for the period (2007-2018). The objective weight was computed by criteria importance through inter criteria correlation (CRITIC) method and performance score by weighted sum model (WSM), weighted product model (WPM), weighted aggregates sum product assessment (WASPAS) and evaluation based on distance from average solution (EDAS) methods which in turn rank the evapotranspiration method. The rankings obtained from MCDM techniques were validated with ranking by GPI method using spearman ranking coefficient. The result from MCDM shows that Saif model is the best model and that also GPI yielded same result. The methodology applied in this study can be adopted in any other region which in turns proved to be beneficial for crop cultivators, crop advisors, researchers, and water resource management. Keywords: water management, CRITIC, WSM, WPM, WASPAS, EDAS Introduction The reliable estimation of reference evapotranspiration (ETo) is a crucial part of regional water resources planning, net irrigation requirement, agriculture water requirements and to model the climate change effect (Pandey et al., 2016). The ETo can be measured directly by lysimeter, but the procedure is time consuming and expensive (Mehdizadeh, 2018). The ETo could be indirectly calculated using a site-specific energy balance or empirical models that typically includes meteorological data, altitude, and latitude. Of the indirect methods, FAO56-PM is the most effective method for the precise estimation of ETo (Allen et al., 1998; Berti et al., 2014). However, it demands high and reliable data quality which is difficult to achieve (Valiantzas, 2013). The precise quantification of the ETo forecasts is dependent on meteorological input data (Allen, 2008). The lack of climate data leads to the need for a simple empirical equation requiring less climate parameters, such as mass transfer, radiation and temperature methods (Sentelhas et al., 2010). Researchers in different regions such as India, Bosnia, Africa, China and Saudi Arabia have developed and applied many empirical equations (Pandey and Pandey, 2018; Cadro et al., 2017; Djaman et al., 2017; Lang et al., 2017;