presenting an empirical model for determining the sugar beet evapotranspiration by gdd parameter (case study: torbat-jam, iran)

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 blanycriddle, 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 nashsutcliffe, 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 nashsutcliffe coefficient were 0.683, 1.117 and 0.99, for equation 4, respectively.