Three years of monitoring evapotranspiration components and crop and stress coefficients in a deficit irrigated intensive olive orchard

A long-term experiment was conducted to study water use of olive trees in a six-year-old, deficit irrigated, intensive olive orchard ('Arbequina') in one of the driest regions of Southern Portugal. Woody agricultural crops are regularly cultivated with some water stress to maintain an equilibrium between the vegetative and reproductive cycles, to improve production quality and, when irrigated, to save water. To achieve a precise irrigation scheduling it is necessary to quantify water use reduction due to water stress. This study reports results from spring 2010 to autumn 2012, and encompasses a hydrological drought occurred in 2012. A long-term seasonal time series of transpiration (Tr), was obtained by combining data on evapotranspiration (ET) measured with the eddy covariance method, soil evaporation measured with microlysimeters and sap flow measured with a heat dissipation method. For the years 2010 and 2011, with normal precipitation, Tr varied between 2 and 4 mm/day, in the summer. In 2012, due to the winter drought, soil water content did not reach field capacity during the wet season and an important reduction in Tr was observed ranging from 1 to 2 mm/day during summer. Predawn leaf water potential (Ψpd) was selected as plant water status indicator, because these olive trees showed near-isohydric behaviour. A function relating Ψpd to the correspondent stress coefficient (Ks) was used to decompose Tr/ETo into the basal crop coefficient (Kcb) and the Ks. The first, during the summer period, oscillated around 0.4 for years with precipitation close to average, while Ks estimated from Ψpd decreased between 1.0 in early June to about 0.83 before first autumn rain. However, these Ks values did not explain the important reduction observed in Tr during the 2012 severe drought. Measured Kcb values were compared to the ones modelled using approaches based on density factor. The derived Kcb values for summer were lower than those observed.