Importance of vegetation, topography and flow paths for water transit times of base flow in alpine headwater catchments

摘要

The mean transit time (MTT) of water in a catchment gives information aboutstorage, flow paths, sources of water and thus also about retention andrelease of solutes in a catchment. To our knowledge there are only a fewcatchment studies on the influence of vegetation cover changes on base flowMTTs. The main changes in vegetation cover in the Swiss Alps are massiveshrub encroachment and forest expansion into formerly open habitats. Foursmall and relatively steep headwater catchments in the Swiss Alps (UrsernValley) were investigated to relate different vegetation cover to watertransit times.Time series of water stable isotopes were used to calculate MTTs. The hightemporal variation of the stable isotope signals in precipitation wasstrongly dampened in stream base flow samples. MTTs of the four catchmentswere 70 to 102 weeks. The strong dampening of the stable isotope inputsignal as well as stream water geochemistry points to deeper flow paths andmixing of waters of different ages at the catchments' outlets. MTTs wereneither related to topographic indices nor vegetation cover. The major partof the quickly infiltrating precipitation likely percolates throughfractured and partially karstified deeper rock zones, which increases thecontrol of bedrock flow paths on MTT. Snow accumulation and the timing ofits melt play an important role for stable isotope dynamics during springand early summer.We conclude that, in mountainous headwater catchments with relatively shallowsoil layers, the hydrogeological and geochemical patterns (i.e. geochemistry,porosity and hydraulic conductivity of rocks) and snow dynamics influencestorage, mixing and release of water in a stronger way than vegetation coveror topography do.