| Summary: | The Mediterranean basin is characterized by hot and dry summers, which are aggravated by climate change, leading to water shortages for irrigation purposes. Olive trees (<i>Olea europea</i> L.) which are the most common cultivation in the area, while tolerant to drought conditions, are often irrigated due to the fact that they suffer from water deficits with negative impacts on yield. The knowledge of the onset and water stress intensity, essentially determines the accuracy of an irrigation schedule that avoids yield reduction and waste of water. Water stress can be quantified by stress coefficient Ks, as suggested by Food and Agriculture Organization (FAO). Combinations and adjustments of the FAO method with field measured data have been used to calculate Ks more precisely and apply it on single experimental fields. The FAO method and a validation method were compared and evaluated with in situ measurements in two different olive orchards in terms of tree age and irrigation system. The results indicate that the FAO method fails to distinguish the differences between cultivars, attributed mainly to the tree age and irrigation system, rendering almost a similar slope of Ks and calculating a nearly simultaneous onset of stress, which was not confirmed by pre-dawn water potential (Ψ<sub>PD</sub>) measurements.
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