One approach to measure potential Evapotranspiration (ETo) is using the Heat Balance (Bowen Ratio-BR) method. In this study ET of irrigated short grass estimated by BR was compared and contrasted with those measurements of three micro-lysimeter and a drainage-system. The study covered a period of six months from May to October of 2005. The grass field was irrigated regularly to provide sufficient water for potential evapotranspiration. In the Heat Balance method temperature and humidity sensors were installed 0.5 and 2.5 meter height above grass in order to measure the pertaining gradients. Three weighting micro-lysimeters were located in different parts of the field. The daily ETo estimated by BR method (5.3 mm/day) agreed closely with weighting micro-lysimeter values (5 mm/day), however, the lysimeter with drainage overestimated ETo by 20% that is why latter method is not suggested to be used for daily measurements. The Bowen Ratio values ( ) usually were between zero and 1 during daytime. It had a rapid increase in the morning and reaching its maximum values between 7:00-9:00 and afterwards it decreased in the fast trend.

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