4.8. Terrestrial ecohydrology and greenwater fluxes
Recent research has seen increased focus on the ecohydrology of the dominant vegetation communities in partitioning rainfall reaching the land surface (Soulsby et al., 2017b). This has involved direct monitoring of canopy-water interactions and transpiration, as well as coupling energy and water budgets in ecohydrological models (Wang et al., 2017). Evapotranspiration is the main loss of water from the catchment in most summers (between May and August) and contrasting vegetation communities have different effects on ecohydrological partitioning. Evapotranspiration losses are ~20-30% higher from Scots Pine than heather and Sphagnum covered areas, mainly as a result of higher interception losses from the forest canopy, higher transpiration losses and higher soil evaporation (Wang et al., 2018; Kuppel et al., 2020). Isotopic ecohydrology studies also show the xylem isotopes in pine trees and heather stems, can be largely explained by the isotopic composition of water in the near surface soils, though internal storage and mixing in the trees appear to contribute to a more complex picture (Tetzlaff et al., 2021). In contrast to streamflow, which tends to draw on older (>3 years) groundwater, evapotranspiration fluxes recycle much younger soil water. Soil evaporation is predominantly < a few weeks old and transpired waters tend to be a few months old (Kuppel et al., 2018). Transpired water is older from trees because water ages increase from ~weeks/months in shallow horizons to ~9 months at depth, with tree roots being able to access deeper water (Smith et al., 2020a).
New insights into terrestrial ecohydrology have informed the evolution of catchment models used in the Girnock; which have uniquely also used isotopes and other tracers to improve model realism. These started with lumped conceptual models (Tetzlaff et al., 2008; Birkel et al., 2010, 2011a,b), which were then spatially distributed (van Huijgevoort et al., 2016a,b) and have included complex physically-based models (Ala aho et al., 2017). Most recent has been the development of EcH2O-iso, a new physically-based, spatially distributed ecohydrological model that includes an isotope mass balance module to help test and constrain process representation (Kuppel et al., 2018). Such robust tracer-aided models can then be used for predicting effects of environmental change in the Girnock, such as climate-driven changes where drier summers and warmer winters are likely to decrease and increase respective seasonal flows (Capell et al., 2013, 2014). Very recent work has examined the potential impacts of re-forestation on the catchment water balance, showing limited impacts on high flows, but potentially reduced groundwater recharge and diminished low flows (Neill et al., 2021). This is significant, given increased momentum behind re-forestation for re-wilding, nature-based solutions for flood management, carbon capture storage and biofuel production (Soulsby et al., 2017a).
Importantly, the tracer-aided modelling developed in the Girnock has been successfully applied in other studies in catchments as widespread as Germany (Smith et al., 2021), China (Zhang et al., 2019), Costa Rica (Correa et al., 2020), Sweden (Smith et al., 2020b), USA (Ala aho, et al, 2017) and Canada (Piovana et al., 2019). Again, this shows how high quality data and innovation at a long-term sites can leverage tools that can be applied elsewhere.