The thesis has been performed for the Rietholzbach, a research catchment in north-eastern Switzerland. It focusses on the presence of hysteresis in the relation between catchment storage and discharge, and so the coupling in the saturated and unsaturated zone, investigated with the help of a Richards’ equation based model.
Uniqueness of catchment-scale storage-discharge relations in a Swiss pre-alpine catchment investigated with a Richards' equation model
Supervisors: Ryan Teuling and Martine van der Ploeg
In 2009, Kirchner published a simple approach to catchment hydrology, in which the catchments are represented by a simple dynamical system. In 2010, this method was applied by Teuling et al. to the Rietholzbach catchment. This is a research catchment in north-eastern Switzerland which is heavily instrumented by the ETH Zürich. Kirchner’s method assumes no hysteresis in the relation between catchment storage and discharge, and so a full coupling between saturated and unsaturated zone. This thesis focusses on the presence of this coupling by investigating these phenomena for the Rietholzbach with a Richards’ equation based model.
During the research, numerical problems occurred with many soils in the 2D-model, forcing the use of soil physical properties typical for coarse sand for the selected cross section, which results in a low field capacity and small water storage. In addition, a 1D-model was run with a peaty clay soil properties and a scaled upper layer. However a 1D-model only simulates one specific profile, so it is not representative for the whole catchment. This is consistent with the results, which show that modelled fluxes at different depths do not exactly match with the discharge observations. The 1D-model results (peaty clay as well as coarse sand) are much better in simulating the discharge dynamics than the 2D-model with coarse sand, due to the longer travel paths in 2D and the properties of coarse sand.
For the peaty clay as well as the coarse sand in the 1D-model, hysteresis effects are visible. The relation for coarse sand is stronger than the peaty clay, most probably because of a higher conductivity and less storage processes. When a slope is introduced or another saturated conductivity in the 1D-model is used, hysteresis effects do not significantly change. In the 2D-model with coarse sand, we also see hysteresis effects. The relation between saturated and unsaturated zone storage is shown to exist for the peaty clay as well as the coarse sand. This relation is much stronger for coarse sand than for the peaty clay, due to the higher conductivity and the fact that a coarse sand dries out faster.