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).