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Ve-Wa:Vegetation effects on water flow and mixing in high-latitude ecosystems–Capability of headwater catchments to mediate potential climate change

Final Report Summary - VEWA (Ve-Wa:Vegetation effects on water flow and mixing in high-latitude ecosystems–Capability of headwater catchments to mediate potential climate change)

Understanding water cycling at the Earth’s surface, and interactions between soil, water and vegetation, is of crucial importance for sustainable management of land and water resources. Given increased global concerns over climate change, it is essential that we quantify these interactions which control the ways in which the landscape partitions, stores and releases water. This is needed to secure water supplies to sustain crop growth, recharge groundwater reservoirs and maintain river flows in an era when the climate is rapidly changing. River basins in high-latitude boreal regions of the Northern hemisphere are particularly sensitive to climate change as the annual water balance critically depends on snow. These catchments are key sites for monitoring climate change, because recent data and long-term climate projections indicate significant warming for large areas of the North which will affect the patterns and amounts of snowfall and release of water during snowmelt. In these areas, vegetation plays a crucial role in patterns of snow accumulation and melt, and the ways in which water is subsequently partitioned into “green” (evaporation and transpiration) and “blue” (groundwater recharge and runoff) water fluxes. The implications of anticipated climate change effects for water resources and freshwater ecosystems means we need to understand the ecohydrology of vegetation more quantitatively.

To address these issues, the VeWa project used stable isotopes of hydrogen and oxygen atoms in water molecules as tracers to understand how vegetation affects water partitioning into “green” and “blue” water fluxes in boreal landscapes. The project focused on seven long-term sites in Scotland, Sweden, Canada, northern Germany and the USA. In terms of climate, topography, soil and vegetation, these are broadly representative of the wider North. In addition to long-term hydrological data; at each site, typical soil-vegetation types were monitored for isotopic composition in precipitation, soil water, plant water and stream water. We used the resulting data to develop new tracer-aided ecohydrological models. Firstly, we developed the Spatially Distributed Tracer-Aided Rainfall-Runoff model (STARR) to simulate fluxes, storage and mixing of water and tracers, as well as estimating water ages at the different VeWa sites and to show the utility of isotopes in a modelling framework in snow-influenced catchments. Secondly, we developed EcH2O-iso, a model, which is spatially distributed and process-based, combining modules which integrates (a) energy balance, (b) water balance and (c) vegetation dynamics. Crucially, this allows the model to explicitly quantify influence of vegetation on water flux and storage dynamics across spatial and temporal scales in cold regions. It can also track isotope transformations in the landscape which can help model calibration and testing. Modelling highlighted the effectiveness of using multiple data sources to condition ecohydrological models, including discharge, isotopes, soil moisture, soil temperature, transpiration and biomass production.

VeWa was extremely successful with 49 peer-reviewed publications and presentations at 17 international conferences. The isotope data provided quantitative understanding of how soil-vegetation water linkages vary at northern sites. This provided the first detailed basis for isotope-driven refinement and testing of ecohydrological models for a more robust basis for assessing the role of vegetation in scenario analysis for future land use and climate change. This is valuable to other interdisciplinary researchers in ecohydrology, as well as land use planners, environmental managers and land owners seeking an evidence base to understand the trade-offs between different land uses, vegetation productivity and water availability in northern regions. Findings help to provide a rational basis for land management in northern regions in an era of rapid climate change.