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; 6–9 years 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 (KT = T/ETo). Average mid-season KT 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 KT values standardized by GC were supported by the independent estimations of KT/GC obtained in the plots using the water balance, except in 2016, when a very high KT value was observed in the lysimeter tree. There were significant fluctuations in daily KT during mid-season, which we found to be related to wind speed. Furthermore, the exceptionally high KT/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 KT values measured here, we propose that the mid-season KT of fully mature almond orchards, with a GC of 75%, should be around—between 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.