University of Birjand Water Harvesting Research 2476-6976 1 1 2016 09 01 Presenting an empirical model for determining the sugar beet evapotranspiration by GDD parameter (Case study: Torbat-Jam, Iran) 22 31 374 10.22077/jwhr.2017.374 EN Fateme Jan Nesar Graduated student of Department of Water Engineering, Faculty of Agriculture, University of Birjand, Birjand, Iran. Abbas Khashei-Siuki m Seyed Reza Hashemi Assistant Professor of Department of Water Engineering, Faculty of Agriculture, University of Birjand, Birjand, Iran. Shahla Moradi Kashkooli M.Sc. Student of Irrigation and Drainage, Department of Water Engineering, Faculty of Agriculture, University of Birjand, Birjand, Iran. Journal Article 2016 09 14 So far, various methods have been presented for determining reference crop evapotranspiration in different parts of the world. The most popular and prestigious of them are probably the combined methods of Penman family, modified Blany-Criddle, Hargreaves - Samani and thornthwaite. Due to lack of lysimeter data in many parts of Iran, presenting an equation according to the regional condition with high precision is very paramount. The purpose of this paper is to present a method with higher precision for determining the evapotranspiration of sugar beet by using the meteorology and GDD (Growing Degree Day) parameters in Torbat_Jam, Iran. Determined methods were evaluated with FAO- Penman- Monteith method as a standard model. RMSE, R2 and Nash-Sutcliffe, and NS indexes were used for comparing fitness indexes. Results showed that equation 4 is appropriate for calculating crop water requirements, also R2, RMSE, and Nash-Sutcliffe coefficient were 0.683, 1.117 and 0.99, for equation 4, respectively. https://jwhr.birjand.ac.ir/article_374_108d181765838e98870f8bf2182a8cee.pdf