Abstract
Abstract. Rapid flow processes in connected preferential flow paths are widely accepted to play a key role in the rainfall–runoff response at the hillslope scale, but a quantitative description of these processes is still a major challenge in hydrological research. This paper investigates the approach of incorporating preferential flow paths explicitly in a process-based model for modelling water flow and solute transport at a steep forested hillslope. We conceptualise preferential flow paths as spatially explicit structures with high conductivity and low retention capacity, and evaluate simulations with different combinations of vertical and lateral flow paths in conjunction with variable or constant soil depths against measured discharge and tracer breakthrough. Out of 122 tested realisations, six set-ups fulfilled our selection criteria for the water flow simulation. These set-ups successfully simulated infiltration, vertical and lateral subsurface flow in structures, and allowed predicting the magnitude, dynamics and water balance of the hydrological response of the hillslope during successive periods of steady-state sprinkling on selected plots and intermittent rainfall on the entire hillslope area. The number of equifinal model set-ups was further reduced by the results of solute transport simulations. Two of the six acceptable model set-ups matched the shape of the observed breakthrough curve well, indicating that macrodispersion induced by preferential flow was captured well by the topology of the preferential flow network. The configurations of successful model set-ups suggest that preferential flow related to connected vertical and lateral flow paths is a first-order control on the hydrology of the study hillslope, whereas spatial variability of soil depth is secondary especially when lateral flow paths are present. Virtual experiments for investigating hillslope controls on subsurface processes should thus consider the effect of distinctive flow paths within the soil mantle. The explicit representation of flow paths in a hydrological process model was found to be a suitable approach for this purpose.
Highlights
Understanding how the internal architecture of hillslopes controls subsurface flow and transport processes and predicting this interplay with models “that work for the right reasons” are still unsolved problems in hillslope hydrology, and of considerable importance for hydrological predictions at larger scales.Structures and patterns play a key role in the organisation of hydrological processes across scales (Vogel and Roth, 2003; Schulz et al, 2006; McDonnell et al, 2007)
In the context of soil hydrology, it is well known that structural features like pipes and macropores generated by plant roots and animals, or soil cracks from desiccation, offer much less resistance to gravity-driven flow than the surrounding soil matrix, and allow rapid flow and transport rates, which has led to the term “preferential flow” (Beven and Germann, 1982; Flury et al, 1994)
The results show that there is a difference in modelled hillslope response between these two bedrock configurations, but the set-ups with constant soil depth performed better
Summary
Understanding how the internal architecture of hillslopes controls subsurface flow and transport processes and predicting this interplay with models “that work for the right reasons” are still unsolved problems in hillslope hydrology, and of considerable importance for hydrological predictions at larger scales.Structures and patterns play a key role in the organisation of hydrological processes across scales (Vogel and Roth, 2003; Schulz et al, 2006; McDonnell et al, 2007). Connected networks of preferential flow paths facilitate rapid vertical and lateral transport of water and solutes in the subsurface over considerable distances (Sidle et al, 2001; Anderson et al, 2009a; Wienhöfer et al, 2009a; Baram et al, 2012). This occurrence of preferential flow is bound to the existence of distinctive void structures (Sanders et al, 2012), the actual preferential
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