Usefulness of establishing trunk diameter based reference lines for irrigation scheduling in almond trees

The suitability of trunk diameter reference baselines for irrigation scheduling of mature drip-irrigated almond trees [Prunus dulcis (Mill). D.A. Webb cv Marta] was assessed in a 3-year experiment conducted on well-watered trees in south-eastern Spain. Continuous measurements of trunk daily growth rate (DGR) and maximum daily trunk shrinkage (MDS) were carried out throughout the growing season (April-November). Day-to-day variations in MDS and DGR were related with meteorological variables. DGR was found to be weakly correlated to all the meteorological variables, while MDS showed significant relationships with most of them. The mean air vapour pressure deficit during the period 10.00-15.00 h solar time (VPDmx) was the environmental variable that best correlated with MDS. The relationships obtained were similar for all the phenological phases, except postharvest, when MDS showed a tendency towards lower values for a given atmospheric evaporative demand. The increase in trunk diameter over the 3 years also appeared to affect the relationships to some extent. As a practical procedure, it is proposed determining the baseline periodically (every 1 or 2 years) during the early stages (II-III) of the almond phenological cycle, for use as reference MDS for the subsequent growth stages, because over longer periods several factors affecting the stability of the baseline, in particular trunk diameter growth, might introduce changes in the baseline.