Abstract/Summary

This thesis identifies, quantifies and models water fluxes within the Dauges National Nature Reserve, an acidic valley mire in the French Massif Central. A range of techniques were used to investigate the nature and geometry of granite weathering formations and of peat deposits. Rainfall, reference evapotranspiration, stream discharge, stream stage, groundwater table depths and piezometric heads were monitored over a three-year period. The distributed, physics-based hydrological model MIKE SHE / MIKE 11 was used to model water flow within the mire and its catchment. It was shown that the mire is mostly fed by groundwater flowing within the densely fissured granite zone and upwelling through the peat deposits. Upwelling to the peat layer and seepage to overland flow were highest along the mire boundaries. However hydrological functioning differs from this general conceptual model in some locations due to the high variability of the peat hydraulic characteristics, the presence of highly permeable alluvial deposits or past human interference including drainage. The equivalent porous medium approach used to model groundwater flow within the fissured granite zone gave satisfactory results: the model was able to reproduce discharge at several locations within the high-relief catchment and groundwater table depth in most monitoring points. Sensitivity analyses showed that the specific yield and horizontal hydraulic conductivity of the fissured zone are the parameters to which simulated stream discharge and groundwater table depth, including in peat, are most sensitive. The model was forced with new vegetation parameters to assess the potential impacts of changes in catchment landuse on the mire hydrological conditions. Replacement of the broadleaf woodlands that currently cover most of the catchment with conifer plantations would lead to a substantial reduction in surface and groundwater inflows to the mire and to a substantial drop in summer groundwater table depths, particularly along the mire margins.