On stochastic modelling of groundwater uptake in semi-arid water-limited systems: root density and seasonality effects

In recent ecohydrological modelling, a common basis has been found in methodology and axiomas, such as the minimalist, systems analysis approach and a piecewise linear root zone water loss function. In this paper, we consider the loss function for root zones in contact with ground water through capillary upflow and including deep root systems. The basic assumption of a piecewise linear behaviour is not well supported, and we highlight how this generates a series of new research questions, related to important knowledge gaps at the interface of physics, plant physiology and ecology. We develop analytical solutions for the probability density functions for soil saturation and evapotranspiration for simple ecohydrological box models that are based on different vegetation groundwater feedbacks, for which numerical results have been presented earlier. In the case of intermediate groundwater levels and wetter climates, a feedback model that regulates groundwater uptake in response to root zone wetness regulates the transpiration most realistically. In semi-arid regions, root zone salinization may occur as a result of groundwater quality and ineffective root zone leaching. How to combine the adverse effects of drought and osmotic stress in models is subject to debate, and several alternatives are presented. Seasonality is another unexplored area, and we highlight how this affects root zone salinization. Finally, a further feedback between upper root zone density and transpiration regulation in the loss function highlights dependence of deep rooted vegetation on groundwater. Experimental data are needed as the availability at this time is limiting further development of models