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

M H Mueller,R Weingartner,C Alewell

doi:10.5194/hess-17-1661-2013

Abstract

Abstract. The mean transit time (MTT) of water in a catchment gives information about storage, flow paths, sources of water and thus also about retention and release of solutes in a catchment. To our knowledge there are only a few catchment studies on the influence of vegetation cover changes on base flow MTTs. The main changes in vegetation cover in the Swiss Alps are massive shrub encroachment and forest expansion into formerly open habitats. Four small and relatively steep headwater catchments in the Swiss Alps (Ursern Valley) were investigated to relate different vegetation cover to water transit times. Time series of water stable isotopes were used to calculate MTTs. The high temporal variation of the stable isotope signals in precipitation was strongly dampened in stream base flow samples. MTTs of the four catchments were 70 to 102 weeks. The strong dampening of the stable isotope input signal as well as stream water geochemistry points to deeper flow paths and mixing of waters of different ages at the catchments' outlets. MTTs were neither related to topographic indices nor vegetation cover. The major part of the quickly infiltrating precipitation likely percolates through fractured and partially karstified deeper rock zones, which increases the control of bedrock flow paths on MTT. Snow accumulation and the timing of its melt play an important role for stable isotope dynamics during spring and early summer. We conclude that, in mountainous headwater catchments with relatively shallow soil layers, the hydrogeological and geochemical patterns (i.e. geochemistry, porosity and hydraulic conductivity of rocks) and snow dynamics influence storage, mixing and release of water in a stronger way than vegetation cover or topography do.

Highlights

The time of water traveling through a catchment gives information about storage, flow paths, sources of water and about retention and release of solutes in a catchment (McDonnell et al, 2010; McGuire et al, 2005)
In contrast to McGuire et al (2005) for example, we found no relation of mean transit time (MTT) with mean catchment slope or the topographic wetness index (TWI)
We investigated the influence of differing percentage of shrub cover on MTTs in four mountainous micro catchments

Summary

Introduction

The time of water traveling through a catchment gives information about storage, flow paths, sources of water and about retention and release of solutes in a catchment (McDonnell et al, 2010; McGuire et al, 2005). The mean transit time (MTT) of water can be defined as the mean time that elapses from the input of water to a system until the output of that water (Eriksson, 1971; McDonnell et al, 2010) It can be calculated via stable isotopes of the water molecule (McGuire and McDonnell, 2006). In regions with seasonally varying air temperatures, the stable isotope signature in precipitation varies seasonally (Dansgaard, 1964). This variability can be observed in stream flow samples but often is delayed and/or dampened, depending on the MTT and transport properties within the aquifer (McGuire and McDonnell, 2006)

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