Talk, Hydrogeosphere useres in Europe, Liège, Belgium: 2010-03-25 - 2010-03-26
Abstract:
In mountainous catchments wetlands often make up large fractions of the total catchment area with potential implications for runoff generation and nutrient export. Wetland surfaces are typically characterized by a distinct micro-topography (hollows and hummocks). The effects of such micro-topography on surface-subsurface exchange and runoff generation for a 10 by 20 m synthetic section of a riparian wetland are investigated in a modeling study. A reference model with a planar surface is run for comparison. The structure of the micro-topography replicates that of a riparian wetland in a small mountainous catchment in South-East Germany (Lehstenbach) and is created using geostatistical simulation. Flow is modeled with the fully integrated surfacesubsurface code HydroGeoSphere. Simulation results show that the specific structure of the wetland surface results in distinct shifts between surface and subsurface flow dominance. Surface depressions fill and start to drain via connected channel networks, when groundwater levels intersect the land surface. These networks expand and shrink when the shallow water table fluctuates around the mean surface elevation under variable rainfall inputs. The microtopography efficiently buffers rainfall inputs and produces a hydrograph that is characterized by subsurface drainage during most of the year and only temporally shifts to surface flow dominance (> 80% of total discharge) during intense rainstorms. In contrast the hydrograph in the planar reference model is much “flashier” and more influenced by surface runoff. A non-linear, hysteretic relationship between groundwater level and discharge observed at the study site could be reproduced with the micro-topography model. Hysteresis was also observed in the relationship between surface water storage and discharge, but over a relatively narrow range of surface water storage values. Therefore surface water storage was found to be a better predictor for the occurrence of surface runoff than groundwater levels. The dynamic exchange between the surface and subsurface domains and the expected broader range of subsurface residence times in the micro-topography model may provide new clues to better understand the volumetric composition (event versus pre-event water) and variable chemical signature of stream water commonly observed in wetland and small catchment runoff.