SIRIAS: a simulation model for sprinkler irrigation - II. Calibration and validation of the model

The effects of two levels of water stress on fruit yield, quality and relat ive economic return were investigated in French prune over a 3-year period. Water stress was achieved by allowing a progressive decrease in midday ste m water potential (midday Psi (stem)) over the season. The control treatmen t (T1) received approximately 100% of the estimated seasonal crop water req uirements, but did exhibit a decline in midday Psi (stem) during the season to below that expected for fully irrigated prune trees (-1.0 MPa). For the moderate and severe stress treatments (T2 and T3 respectively), irrigation was managed to reach a target midday Psi (stem) of approximately -1.5 and -2.0 MPa, respectively, by harvest. Treatments T2 and T3 received an averag e of 57% and 38% of estimated seasonal crop water requirements. respectivel y, over the 3 years of the study but this did not negatively impact fruit d ry mass accumulation or fruit quality. Over a 3-year period, the stress tre atments had less dry mass of fruit dropped, fewer fruit dropped, improved ( lower) fruit hydration ratios, less fruit side-cracking and decreased veget ative growth compared with the control. There were no significant effects o f either of the stress treatments on flowering, dry fruit yields or relativ e economic return for the 3-year average data. However, a significantly low er relative economic return for T3 during the third year of the study sugge sts that the cumulative effect of this level of stress may be detrimental t o long-term prune productivity. Allowing a progressive decline in midday Ps i (stem) to approximately -1.5 MPa by harvest appears to be an effective wa y to reduce irrigation and maintain economic returns in French prune.