Water use and soil water balance of Mediterranean tree crops assessed with the SIMDualKc model in orchards of southern Portugal

Orchards consist of complex agricultural systems, with a variety of characteristics (planting density, tree height, training system, canopy cover, irrigation method, interrow management) influencing crop evapotranspiration (ETc). Thus, irrigation water management requires finding crop coefficients (Kc) that represent the characteristics of local orchards, evidencing the need for site specific data. The main objective of this study was to derive the Kc of almond, olive, citrus, and pomegranate orchards in Alentejo, southern Portugal, wherein they became dominant over the last decade. Monitoring was carried out in nine orchards, which management decisions were performed by the farmers. The ETc was estimated from the soil water balance computed for each orchard using the FAO56 dual-Kc approach with the SIMDualKc model. The model successfully simulated the soil water contents measured in the various fields along two growing seasons, with root mean square error values lower than 0.005 m3 m−3 and modeling efficiencies from 0.363 to 0.782. The estimated basal crop coefficients (Kcb) for the initial, mid- and end-seasons were respectively 0.22, 0.58, and 0.50 for almond; 0.32–0.33, 0.35–0.36, and 0.33–0.34 for olive; 0.40, 0.40–41, and 0.40–0.41 for citrus; and 0.24, 0.60, and 0.52 for pomegranate. Small variations in olive and citrus Kcb values were found to be related to differences in the fraction of the ground covered by trees’ canopies and tree height. The single Kc values, which included the component relative to soil evaporation, were also estimated. Furthermore, evaluation of the soil water balance in the nine case studies showed salinity effects in one almond orchard, mild irrigation water deficits in olive systems, and large non-consumptive water use in citrus and pomegranate orchards. These results evidence the need for better management of orchards irrigation water in the region, and the current study provides for reliable information on the Kc of tree crops to support improving the management of local orchard systems and the preservation of soil and water resources. Aimed at these resources and the sustainability of their use, simulated alternative irrigation schedules were performed, which identified possible water savings of 20 mm in case of olives, up to 855 mm for citrus.