Normalization of the crop water stress index to assess the within-field spatial variability of water stress sensitivity

This paper presents a novel methodology for identifying homogeneous areas within high-frequency drip-irrigated orchards and for defining the most sensitive and resistant areas of the field to water stress. The methodology proposed here is based on the assessment of water status at the tree level during mild water stress using remote sensing derived indicators which provide valuable information about the spatial distribution of the response to water stress within an orchard. The areas more resistant to water stress will maintain a good water status, while those prone to water stress will develop initial symptoms of water deficit. The study was performed over three different peach orchards that were evaluated from 2 to 3 years. Water status was monitored using high-resolution thermal imagery acquired before and after the onset of water stress. The Thermal Sensitivity Index (TSI), derived from the difference of the CWSI and the cumulated reference evapotranspiration between the two dates, demonstrated to be well related to the increase of stem water potential. The spatial distribution of TSI enables the identification of sensitive areas within a peach orchard, a first step for establishing precision drip irrigation programs.