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Give and take: how much water does soil retain for trees?

The project aims to investigate the role of soils in the water balance of Swiss forest ecosystems. Soil moisture conditions directly influence water and nutrient availability to trees. Yet, it has not been quantified to what extent the water storage capabilities of soils help to mitigate adverse effects of drought periods on forest trees on plot or regional scales. Efforts to assess the broad-scale impacts of droughts on forest health often lack the effect of soil properties.

Within this project, an inventory of soil water availability to trees in Swiss forest soils is established. This is done by means of model-based analysis of the water balances at 16 long-term monitoring sites in Switzerland (Figure 1). These sites represent various geographical locations, soil and vegetation types. Not only volumetric soil moisture θ, but also its availability to vegetation – which is expressed through the soil water potential ψ – is included in the analysis. These continuous, long-term observations are complemented with periodic measurements of stable isotopic compositions δ of soil and vegetation water to constrain modelled water fluxes. An illustration of a possible system definition is shown in Figure 2.

Different structural models to describe the observed water balances at the monitoring sites are compared. This approach is expected to lead to well-constrained models by means of Bayesian parameter estimation. The improvement of parameter uncertainty due to inclusion of isotopic observations δ is assessed. Further, the approach allows quantifying the importance of specific soil water processes in the unsaturated zone needed for reproduction of the observations. Lastly, the analysis can be extended to include feedback effects with vegetation or to identify controls on model parameters such as soil or vegetation characteristics.