Hillslope hydrological modeling : the role of bedrock geometry and hillslope-stream interaction

Keywords: Hillslope hydrology, hydrological modeling, bedrock geometry, boundary condition, numerical solution. This thesis focuses on hillslope subsurface flow as a dominant control on the hydrological processes defining the catchment response to rainfall. Due to the difficulties associated with the three-dimensional equations, both in complexity and regarding the computational demand, focus is on 1D physically-based hillslope groundwater flow models. The main core of this research is to study how the simulated hillslope response, and specifically runoff generation, is affected by different simplifying assumptions concerning hillslope geometry and boundary conditions. Several generalizations have been made to incorporate the complexity of bedrock geometry in models of the hillslope hydrological response. In order to handle and compare these different models, a general and flexible numerical algorithm has been developed. Through the proposed numerical scheme several applications of these models are performed: (a) to investigate the role of the hillslope lower boundary condition on the groundwater response, (b) to study the role of bedrock geometry on the hillslope hydrological response, and (c) to approximate the hillslope response using the quasi-steady state approximation. Evaluation of the numerical scheme shows that it is flexible and capable to handle all these different models, which have been applied for different purposes, and that the mass balance is preserved. Evaluation of the developed models leads to the following general conclusions: (a) A head-discharge relationship as the hillslope lower boundary condition is a proper approach to model the coupled hillslope-stream system; (b) The recession characteristics of hillslopes depend on their bedrock profiles and local minima and maxima in the curved bedrock profile influence the groundwater head for small rainfall intensities; (c) The quasi-steady state approximation proves to be a simple but adequate method (both as a model and regarding computation time) to simulate the groundwater dynamics of hillslopes. In summary, this thesis aims to understand theoretically how hydrological processes (subsurface flow and water table dynamics) react to bedrock complexity and the hillslope lower boundary condition. The presented numerical scheme can be applied in many investigations of hillslope hydrology because of its generality and flexibility.