Monitoring Crop Evapotranspiration and Transpiration/Evaporation Partitioning in a Drip-Irrigated Young Almond Orchard Applying a Two-Source Surface Energy Balance Model

Encouraged by the necessity to better understand the water use in this woody crop, a study was carried out in a commercial drip-irrigated young almond orchard to quantify and monitor the crop evapotranspiration (ETc) and its partitioning into tree canopy transpiration (T) and soil evaporation (E), to list and analyze single and dual crop coefficients, and to extract relationships between them and the vegetation fractional cover (fc) and remote-sensing-derived vegetation indices (VIs). A Simplified Two-Source Energy Balance (STSEB) model was applied, and the results were compared to ground measurements from a flux tower. This study comprises three consecutive growing seasons from 2017 to 2019, corresponding to Years 2 to 4 after planting. Uncertainties lower than 50 W m−2 were obtained for all terms of the energy balance equation on an instantaneous scale, with average estimation errors of 0.06 mm h−1 and 0.6 mm d−1, for hourly and daily ETc, respectively. Water use for our young almond orchard resulted in average mid-season crop coefficient (Kc mid) values of 0.30, 0.33, and 0.45 for the 2017, 2018, and 2019 growing seasons, corresponding to fc mean values of 0.21, 0.35, and 0.39, respectively. Average daily evapotranspiration for the same periods resulted in 1.7, 2.1, and 3.2 mm d−1. The results entail the possibility of predicting the water use of any age almond orchards by monitoring its biophysical parameters.