Dynamics of Water Flow in a Forest Soil: Visualization and Modelling

Soil water plays an important role in the terrestrial water and energy cycles. Its movement follows the gradient of the soil water potential and is most frequently described by the Richards equation. In this chapter, we focus on water fluxes in the vadose zone and model them with Water Heat and Nitrogen Simulation Model (WHNSIM) that solves the Richards equation numerically. We characterize the temporal dynamics of soil matric potentials measured at Coulissenhieb II and compare their complexity with modelled matric potential. Additionally, we summarize our previous studies on preferential flow—a common phenomenon in forest soils that cannot be modelled adequately by the Richards equation. The model WHNSIM reproduced the overall level of matric potentials in all depths. However, while it captured the complexity of the measurements in the upper soil, the matrix potentials in 90 cm depth were less complex indicating a more regular and damped signal. This result suggests that WHNSIM misses some important processes at least in the deeper soil. The soil water fluxes at Coulissenhieb II have a clear seasonal pattern with large fluxes occurring in spring during snow melt and small ones during dryer periods in summer. We could identify preferential flow in dye tracer experiments at the profile scale and attribute it mainly to macropore flow along root channels. Yet the identification and quantification of preferential pathways at the catchment scale remains challenging.

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