Stochastic soil water dynamics of phreatophyte vegetation with dimorphic root systems

As the direct uptake of deep groundwater by vegetation may be essential in semiarid regions, we incorporated this process in stochastic root zone water balance models. The direct water uptake by vegetation via deep tap roots is simulated using one additional empirical parameter. This is considered for the case of feedback with root zone saturation and without such feedback. The model that accounts for feedback between shallow root zone saturation and groundwater uptake by deep roots takes up less water if the shallow root zone is wet. The behavior of the models demonstrates that for certain combinations of climate and groundwater depths this feedback becomes important in determining differences in total evapotranspiration (ET). This feedback mechanism also captures hydraulic redistribution processes. The range of relative contributions of groundwater to ET predicted by the models was similar to values derived in isotope studies