Comparing Wheat Evapotranspiration Estimated by Bowen Ratio Energy Balance (BREB) Method with Result of Lysimeter

Evapotranspiration is one of the important elements of the hydrologic cycle in agricultural projects. Energy balance (the bowen ratio) is a method for estimating evapotranspiration of plant which is based on measurements of temperature and humidity gradients in two different heights of a plant. An experiment was conducted in agriculture faculty of Ferdowsi university of Mashhad by using three Lysimeter to estimate evapotranspiration of Gascogne wheat and the resulting were compared with direct method. Required data for measuring the amount of evapotranspiration using energy balance method was obtained throughout plant phenology with one hour intervals using energy balance (model 5200 – DIK) estimation device of evapotranspiration was daily calculated. The rate of daily evapotranspiration that obtained by using energy balance method amounted to 2. 4 mm which is in a high correlation (0. 98) with the Laysimeter result that was 2. 4 mm. The range of Bowen ratio changes was between -1. 5 to 1. 9 during the day which the negative values occurs after sunset that is the sensible heat flux begins to decrease. The value of Boven ratio gradually increase so that it''s maximum value between 8 AM to 9 AM، and then followed a decreasing trend until the afternoon.