Surface energy flux measurements over a drip-irrigated young almond orchard

Irrigated almond orchards are fast extending in the past few years in arid and semiarid areas of the Mediterranean basin with limited water resources. A good knowledge of the almond crop evapotranspiration (ET_c) becomes then crucial for a correct irrigation strategy. A study was conducted during May-August 2018 in a ~11 ha young almond (Prunus dulcis (Mill.) D.A. Webb) orchard in a semiarid location in southeast Spain. The objectives of this work were: a) quantify the water use of drip-irrigated young almond trees under no soil water limitations; b) determine the crop coefficient (K_c) values for young almond trees ‘Lauranne’; c) parameterize K_c as a function of fractional canopy cover and vegetation indexes obtained by remote sensing techniques. With this aim, an eddy covariance (EC) system was installed in the center of the experimental plot together with a net radiometer and a set of soil heat flux plates. Data of the different terms of the energy balance equation were stored every 15 min, and then averaged at an hourly and daily scales. A footprint analysis was first carried out to ensure the validity and representativeness of the turbulent flux measures within the field boundaries. A lack of closure in the energy balance equation around 10% was observed. This imbalance was corrected by applying both residual and Bowen ratio techniques, and recalculating sensible and latent heat fluxes. When this lack of closure was corrected following the Residual approach, seasonal almond ET_c measured was 243 mm, resulting in an average daily ET_c value of 2.29 mm d^‑1. Maximum K_c value of approximately 0.58 was reached in August, coinciding with maximum canopy cover values. Linear relationships were established between K_c and both, canopy cover and the normalized vegetation index.