Using Plant-Based Indicators to Schedule Irrigation in Olive

Results from the literature and from recent experiments were used to evaluate the usefulness of indices derived both from sap flow (SF) and trunk diameter variation (TDV) records for scheduling irrigation in olive orchards. The sensors and related equipment of both methods are robust and reliable, and suitable for operating in the field for entire irrigation seasons. Both allow for automatic data collection and transmission. They are still expensive and some training is required for proper installation, but they are easy to operate and maintain. Both SF- and TDV-derived indices may show a high tree-to-tree variability, especially in old trees, but the signal intensity is usually big enough to achieve an acceptable sensitivity. Outputs from SF records are easier to interpret than those from TDV measurements, since the latter are affected by a variety of factors, apart from water stress. This limits the suitability of TDV records for automatic irrigation control. Both methods can potentially be used to schedule low-frequency irrigation in olive orchards, and are particularly useful when a deficit irrigation strategy is applied. They may fail, however, when the irrigation strategy is designed to maintain the soil close to field capacity, especially in the case of mature trees with big root systems growing in soils with medium-to-high water retention capacities. The development of water stress according to both methods lags a few days behind the reduction or cease of water supplies, making them unacceptable for the control of high frequency irrigation. For severely stressed trees, the analysis of the TDV trend may be more useful for scheduling irrigation than the signal-intensity approach.