Water requirements of mature almond trees in response to atmospheric demand

Accurate methods to determine irrigation requirements are necessary for the efficient use of water in agriculture. We conducted measurements of transpiration (T) of one almond tree placed in a large weighing lysimeter and instrumented with sap-flow probes for three seasons (2014-2016; 6-9years after transplanting). We also conducted independent T estimations by the water balance method in four plots. Transpiration was related to reference evapotranspiration (ETo) to obtain the coefficient of transpiration (K-T=T/ETo). Average mid-season K-T of the lysimeter tree was 0.55, 0.68 and 0.91 in 2014, 2015, and 2016, respectively, and maximum ground cover (GC) was 55, 59 and 55% for the same years. These K-T values standardized by GC were supported by the independent estimations of K-T/GC obtained in the plots using the water balance, except in 2016, when a very high K-T value was observed in the lysimeter tree. There were significant fluctuations in daily K-T during mid-season, which we found to be related to wind speed. Furthermore, the exceptionally high K-T/GC relationship of 2016 was apparently related to the very high crop load in the lysimeter of that year (75% more than the 2 other normal years). Hourly bulk canopy conductance values were derived from lysimeter T records to confirm the high transpiration rates prior to harvest during 2016. From the K-T values measured here, we propose that the mid-season K-T of fully mature almond orchards, with a GC of 75%, should be aroundbetween 0.9 and 1.05. An estimation of evaporation from soil, which is dependent on the method of irrigation, needs to be added to obtain the net water requirements.