The role of soil heterogeneity on field scale evapotranspiration: 3D integrative modelling and upscaling of root water uptake

Abstract

<em>Background and Motivation</em>: Hydrological models mainly rely on empirical functions to describe root water uptake. However, in case of water limitation due to either scarce or heterogeneously distributed soil water, plants have developed strategies to adapt. One short-term strategy is the regulation of stomatal aperture either by plant hydraulics or phyto-hormones. The latter are thought to act as a kind of sensor for dry soil. It can be assumed that hormones are produced locally in single root segments as function of low root water potentials. After being transported with the xylem stream they become effective in stomatal closure. Long-term adaptation strategies are mostly related to changes in carbon allocation within the plant and result in growth adaptations. Both strategies often remain insufficiently represented in hydrological models. <br /> Methods: R-SWMS, a mechanistic soil and root water flow model that operates at the scale of a single root system was used to conduct a variety of virtual experiments. The model simulates three dimensional water flow through the soil, to, and within the roots. It was extended by a module to account for additional hormonal signalling, subsequently testing its influence in virtual split-root experiments. In a next step, direct mathematical relationships that link effective soil water potential and transpiration were derived. Considering the long-term adaptation strategies, the numerical model was modified to incorporate measured dynamic root architectures.<br /><em>Results</em>: Measured hormone concentrations in the leaves and some phenomena, like e.g. oscillations in stomatal aperture, were reproduced by the model. The direct relationships between soil water potential and transpiration showed that the stomatal behaviour depends on the underlying control and its parameterization. Experimental data, visualizing root systems over a 30 day growth period, were obtained from UFZ Halle, Germany, by CT scans. This dataset showed that plants grown under permanently limited water supply were considerably smaller with correspondingly less total water uptake compared to plants with initially unrestricted water resources. In combination with the numerical model, the flow dynamics in the soil-root system were resolved. The predicted location of root water uptake was found to be different from the measured zone of water depletion. <br /><em>Conclusion</em>: The implementation of bio-physical relationships into a mechanistic root soil model resulted in a powerful tool to identify key processes for plant water use in agricultural environments. This work provided new direct relationships between the effective soil water potential and transpiration rate in case stomata are controlled by hormones. In combination with an experimental dataset it gave new insights into water pathways within the soil-plant continuum.