Water potential (¿) has been traditionally used to establish thresholds for irrigation scheduling. The underlying idea is that a change in ¿ induces a concomitant variation in transpiration (T). However, according to the catenary hypothesis, such relation is modulated by the global resistance of the system (R) (the soil-plant-atmosphere continuum, SPAC). These three terms ¿, T, and R, are not linearly related. As a consequence, the analysis of the response of one of them when the other two change is not straightforward. To assist in the study of the relations between ¿, T and R, a SPAC model with a multi-compartment solution was used. The model was previously validated for olive and almond trees under different levels of water stress, allowing the modification of R through changes in the leaf-root area relations. The simulation results show that ¿ varied at field capacity or permanent wilting point for the different LA:RA ratios. The results indicate that the ¿ threshold for irrigation obtained in one experiment may only be applied to other sites if R is the same; otherwise, the ¿-T relation will be different and hence the irrigation threshold.