Year of selection 2012
Institution Albert-Ludwigs-Universität Freiburg
Hydrological dynamics can be characterized in terms of sources, flowpaths and residence time of water and solutes. Besides climate, geology, topography and soils, vegetation is known to be a principal, but in both space and time highly variable control of these dynamics. As a consequence, not only the hydrograph response, but also the residence and transit times are expected to differ among contrasting vegetation covers. This hypothesis seems to be plausible because unsaturated zone dynamics were identified to strongly control time variant transit time distributions, but was only rarely tested. In order to identify the effect of different vegetation covers on hydrological dynamics, other controlling variables must be eliminated. That is, the study sites should receive the same atmospheric inputs and feature the same geology, topography etc. Additionally the experimental and measurement setups must be the same. As these criteria are difficult to meet, systematic site intercomparison studies of water sources, flowpaths and water ages in respect to vegetation are missing. Additionally, there are some fundamental methodological problems in the commonly applied linear transfer models to estimate travel time distributions from tracer data. Time invariant travel time distributions are not valid under dynamic storage conditions and it is unknown to which extent the vegetation itself alters the tracer time signal in streamflow. Therefore a different approach should be chosen, e.g. the use of physically-based, spatially distributed rainfall-runoff models with a solute transport routine. The proposed study aims to explore the effect of vegetation cover on hillslope hydrological dynamics and will be the first with a focus on storage and transit time dynamics within a model framework. The study will be based on an existing data set. At three adjacent trenched hillslopes with similar characteristics but contrasting vegetation (grassland, coniferous forest, mixed forest), transient water table dynamics are observed in high spatial and temporal resolution (30 wells per hillslope). The measurements also include continuous trenchflow and meteorological data and throughfall patterns for several events. The hydrologic model HillVi will be calibrated against the distributed well and flow data. To back up the model structure and reduce its uncertainty, a conservative salt tracer will be applied at the hillslopes and detected in the trenchflow. The calibrated parameter ranges will be compared and linked with known vegetation effects. The solute transport routine will be supplemented with a time accounting scheme to obtain time variant residence time distributions of the unsaturated and saturated zone and transit time distributions of subsurface outflow and percolation from the hillslopes for each time step. From this, also pre-event/event water contributions can be easily calculated and compared.
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