VeWa: Assessing Vegetation Effects on Water Flows and Mixing in Northern Mountain Environments using Stable Isotopes and Conceptual Runoff Models

Abstract:

The lack of comprehensive tracer data sets still hinders the development of a generalized understanding of how northern headwaters function hydrologically. As part of the ERC funded “VeWa” project, we combined a conceptual rainfall-runoff model and input-output relationships of stable isotopes to understand ecohydrological influences on hydrological partitioning in in six high-latitude experimental catchments located in the UK, USA, Sweden and Canada. We used stable isotope records from precipitation and stream flow to examine the effects of soils and landcover. A meta-analysis was carried out using the HBV-model to estimate the main storage changes characterising annual water balances. Annual snowpack storage importance was ranked differently across the sites, and the subsequent rate and longevity of melt was reflected in calibrated parameters that determine partitioning of waters between more rapid and slower flowpaths and associated variations in soil and groundwater storage. Variability of stream water isotopic composition depends on: (i) rate and duration of spring snowmelt; (ii) significance of summer/autumn rainfall; (iii) relative importance of near-surface and deeper flowpaths in routing water to the stream. Flowpath partitioning also regulates influences of summer evaporation on drainage waters. Deviations of isotope data from the Global Meteoric Water Line showed subtle effects of internal catchment processes on isotopic fractionation most likely through evaporation. After accounting for climate, evaporative fractionation is strongest at sites where lakes and near-surface runoff processes in wet riparian soils can mobilize isotopically-enriched water during summer and autumn. Given close soil-vegetation coupling, this may result in spatial variability in soil water isotope pools available for plant uptake. We argue that stable isotope studies are crucial in addressing the many open questions on hydrological functioning of northern environments.