The Determination of Potential Evapo-Transpiration by the use of Drainage Lusimeters

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El Nadi, Abdel Mohsin Hassan
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The aim of this work was to find a correlation between water use of a Lucerne crop when soil moisture is not limiting, and some easily measured or calculated quantity which is influenced by the weather parameters that determine the rate of water loss from plants. After a series of modifications, a reliable type of a drainage lysimeter was developed as a technique employed in the investigation. Before and after the crop was completely shading the ground, the rate of water loss was influenced by increase in crop surface as affected by crop height, leaf development and density of cover. Increase in crop surface appeared to have increased advective effects which, although of considerable importance in arid, tropical zones, yet were not taken into account in any of the formulae so far derived to estimate the rate of water loss from plants. This led to an under-estimation of the calculated rate of water loss using Penmans relation, in comparison with the measured rates of water loss from both dish evaporimeters at about crop height, and Lucerne more then about 10-12 cm, in height. Correlations between the rate of water loss from dish evaporimeters adjacent to the crop were possible, but not easy. Plant species was also an important variable in determining potential evapo-transpiration rates; wheat representing about 80% that of Lucerne. Potential evapo-transpiration rates of plants grown in artificial soil mixture or natural field soil were not influenced by the soil type.
Potential Evapo, Transpiration, Drainage Lusimeters