Transpiration of young almond trees in relation to intercepted radiation

Increased water scarcity demands more efficient use of water in the agricultural sector which is the primary consumer of water. Precise determination of irrigation requirements based on specific crop parameters is needed for accurate water applications. We conducted a 4-year study on almond evapotranspiration using a large weighing lysimeter. Tree canopies changed from 3 to 48 % ground cover during the course of the study. Sap flow measurements made on the lysimeter tree provided a continuous record of tree transpiration. We propose to use the daily fraction of photosynthetically active radiation intercepted by the canopy (fIR(d)) as a predictor of almond orchard maximum transpiration. The transpiration coefficient (T/ET (o) or K (T) ) was related to the fIR(d) of the last two years, and the ratio between fIR(d) and K (T) stayed more or less constant around a value of 1.2. Such value extrapolated to the size of a mature orchard with 85 % intercepted radiation gives a K (T) of around 1.0, a number above the standard recommendations, but fully compatible with the maximum K (c) values of 1.1-1.15 recently reported.