4.7 Catchment hydrology
Initial isotope studies resulted in hypotheses about catchment
hydrological function that were tested via direct monitoring of
hillslope hydrology (Fig. 6), also using tracers, to assess soil
moisture dynamics in the catchment, groundwater responses and generation
of overland flow in storm events (Tetzlaff et al., 2014). Most years,
the Girnock catchment follows a clear annual water balance; in winter,
higher precipitation inputs, coupled with low evapotranspiration, result
in high runoff coefficients (>60%), whilst in drier
summers the runoff coefficient is <40% and evapotranspiration
is the dominant water flux out of the catchment (Kuppel et al., 2020).
Throughout the year, groundwater mostly stored in various glacial drift
deposits sustains baseflows, and the Girnock and its major tributaries
are perennial streams. This isotopic evidence shows the groundwater
mainly recharges from winter precipitation, through the more freely
draining podzolic soils. Monitoring isotopes in soils and groundwater
along a hillslope transect showed that the hillslopes act as
advective-dispersive flow systems (Tetzlaff et al., 2014). However,
connectivity between hillslopes and riparian wetlands during rainfall
events controls the non -linearity of the catchment’s storm runoff
response which is dominated by saturation overland flow (Birkel et al.,
2009; Ala aho et al., 2017).