An eddy correlation system (ECS) was used to estimate evapotranspiration (E) in a daily drip irrigated cotton field. Cotton yield, water use, and their ratio (Water use efficiency; WUE) were compared in four irrigation treatments. Three treatments were irrigated at levels of 85, 100, and 115% of E, while the fourth was irrigated according to grower's usual practice. E data were used to verify a one dimensional numerical model which simulates, in real time, the different energy fluxes existing in the soil-plant-atmosphere system. The model requires input of vegetation parameters (leaf area index, photometric properties, shading factor, root density distribution), soil parameters (texture, hydraulic and photometric properties, temporal micrometeorological data (solar radiation, wind speed, air temperature and humidity) measured above the field, and irrigation quantities. The verification study was carried out during a cotton growing season in Hula Valley, Northern Israel. Results show that E rates are strongly affected by the intensity and arrival time of the inland penetrating Mediterranean sea breeze. WUE in the treatment which was irrigated according to the ECS was highest. Accurate estimations were also made by the model.
The practical decision on irrigation timing and amount in actual farm practice rests largely on the subjective judgement of the grower, based on empirical knowledge. However, the ever increasing scarcity of water on one hand, and the introduction of irrigation control by computers on the other hand, present the need and the opportunity to improve the validity of these decisions. The precision of execution will also be improved by the development of practical management methods using objective criteria based on the results of scientific research. There are presently no convenient procedures for evaluating actual evapotranspiration (ETa) with the precision needed for management of intensive irrigation. Methods based on