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.