Seasonal changes of maximum daily shrinkage reference equations for irrigation scheduling in olive trees: Influence of fruit load

Maximum daily shrinkage (MDS) is the parameter of daily cycle of trunk diameter most widely suggested in irrigation scheduling for several fruit trees. However, as in other plant-measured approaches, the irrigation decision may be difficult due to the influence of the environment on the values obtained. Reference equations of MDS have been established in order to avoid the effects of environmental conditions. Such equations are usually related to simple meteorological data, in order to easily estimate MDS values in full-irrigated conditions. This paper studies the influence of fruit load and the inter-annual variations on the reference equation of MDS in olive trees. These reference equations were calculated during 4 seasons in a full-irrigated orchard and the equations were validated with the data from a different season. The MDS values were related to vapour pressure deficit (VPD) and temperature taken near the experimental orchard. In addition, meteorological data were considered as mean daily or as midday values; only for temperature was the maximum daily value also used. The validation of the equations was made using the fits with all the meteorological data considered (midday and mean daily of VPD and temperature). In addition, two different fits were used in each meteorological data—one according to fruit load and other with the complete pool of data. The equations fit were significantly different each season in all the meteorological data considered. However, seasons with similar fruit load were more similar to each other. In both meteorological data considered (VPD and temperature) the midday values improved the fit in respect to mean daily values. The equations obtained with maximum daily temperature were similar in accuracy to the one of midday. The reference equations in which temperature was used obtained a better fit that the ones calculated with VPD. No significant differences were found in the validation when equations according to fruit load or the complete pool data were compared. The limitations and usefulness of these reference equations are also discussed.