Defining irrigation volumes and timing under slight or moderate water stress conditions requires to monitor the water status in the soil–crop system and identify indicators to guide irrigation scheduling. Leaf water potential (LWP) is a commonly used variable to describe crop water status and, when measured at pre-dawn or mid-day, is an indicator of any instantaneous crop water stress condition. Even if leaf water potential measurements are considered one of the most reliable methods for direct determinations of crop water status, these determinations are destructive and time consuming and require skillful operators. Agro-hydrological models can be considered an easy-to-use tool for indirect determination of soil and crop water status, as well as to estimate other parameters related to crop development. However, the application of these models requires the preliminary calibration and validation of the algorithms used for the different processes occurring in the Soil-Plant-Atmosphere continuum. Therefore, a reasonable use of the models to predict crop water stress conditions should be based on site-specific experimental investigations and requires an improvement of sub-models related to the specific crop response to changes in soil water deficit. Several models have been proposed to quantify the water stress coefficient as linear or nonlinear functions of soil water status, expressed in terms of matric potential or soil water depletion. For a certain crop, the water stress function is applied once it is known its shape and the thresholds values identifying the soil water status beyond which crop water stress occurs and the level of maximum stress. This review aims to demonstrate how the use of agro-hydrological models is able to predict the water stress dynamics of two important Mediterranean tree crops (olive and citrus), which have a different eco-physiological water stress response to soil water deficit conditions. After examining the eco-physiological response of both crops to soil water deficit, we assessed the potential of FAO-56 agro-hydrological model to identify crop water stress under different irrigation management strategies. The proposed the water stress functions are the result of recent experiments based on long term series of field measurements of soil (volumetric water content) and plant (xylem water potential and transpiration flows) water status of olive and citrus trees. Experiments carried out during three years in a Sicilian olive grove and a Valencian citrus orchard allowed to identify the specific crop water stress response to soil water deficit conditions and confirm for citrus the original schematization proposed in FAO-56 paper. Moreover, after evaluating the similarity between the measured midday stem water potential using the simulated crop water stress coefficient, the fairly good performance of FAO-56 agro-hydrological model to predict soil water content was proven. The obtained results evidenced that the crop water stress coefficient estimated by the model is used as a suitable indicator to replace the tedious and time-consuming field measurements of midday stem water potential. On the other side, when specific crop water stress function are implemented, the FAO-56 agro-hydrological model can predict soil water content and the crop water requirement, even when water saving strategies are used.
Keywords: Agro-hydrological model, water stress function, olive, citrus