Water Resour Manage (2009) 23:3057–3067 DOI 10.1007/s11269-009-9423-4 Evaluation of Reference Evapotranspiration Equations Under Humid Conditions Slavisa Trajkovic · Srdjan Kolakovic Received: 13 January 2008 / Accepted: 2 February 2009 / Published online: 24 February 2009 © Springer Science + Business Media B.V. 2009 Abstract Five reference evapotranspiration (ET 0 ) equations are evaluated using data from seven humid locations. The equations evaluated include Hargreaves, Thornthwaite, Turc, Priestley–Taylor, and Jensen–Haise. The objective of this study is to evaluate ET 0 estimated by these equations against the corresponding values estimated using the standardized FAO-56 Penman–Monteith (PM) equation. For each location, ET 0 estimates by the all equations were statistically compared with FAO-56 PM ET 0 estimates. The Turc equation yielded the smallest root-mean- square-difference (RMSD) values at the all locations except Novi Sad, Serbia. The final ranking of equations was based on the weighted RMSD. The Turc equation has the lowest weighted RMSD and ranking first, and other equations ranked in decreas- ing order are: Priestley–Taylor, Jensen–Haise, Thornthwaite, and Hargreaves. The Turc equation gives the reliable calculation at all humid locations and it has proven to be the most adjustable to the local climatic conditions. The results obtained from this study, indicate very clearly that the Turc equation is most suitable for estimating reference evapotranspiration at humid locations when weather data are insufficient to apply the FAO-56 PM equation. Keywords Reference evapotranspiration · FAO-56 Penman–Monteith equation · Humid climate · Radiation-based equations · Temperature-based equations S. Trajkovic (B ) Faculty of Civil Engineering, University of Nis, A. Medvedeva 14, 18000 Nis, Serbia e-mail: slavisa@gaf.ni.ac.rs S. Kolakovic Faculty of Technical Sciences, Trg D. Obradovica 6, 21000 Novi Sad, Serbia