4.6 Isotope hydrology
To understand the influence of instream hydraulics on salmon under current and expected future climatic conditions, the role of catchment hydrology as the main driver of flow variability must be recognised. Short-term, seasonal and inter-annual flow variability depends on hydroclimate and its effect on the inputs of water as precipitation, how this water is stored in, and moves through the catchment, and how it exits as streamflow or evapotranspiration (Soulsby et al., 2016a).
Monitoring the hydrology of a large, complex and remote catchment like the Girnock is logistically challenging. Over a large heterogeneous area, it is unclear how the complexities of climate and landscape characteristics interact to govern the movement and storage of water as much depends on the subterranean landscape. In 2003, weekly samples of rainfall and streamflow were taken from several sites around the catchment over a year and analysed for stable water isotopes to gain a preliminary understanding of the integrated effects of these processes (Tetzlaff et al., 2007). This followed previous successful Scottish studies that used isotopes to understand streamflow generation processes in larger catchments (Soulsby et al., 2000; 2006). Isotopes of hydrogen and oxygen in rainfall vary seasonally and on a day-to-day basis, reflecting air mass sources and energy available to evaporate heavier isotopes (Fig. 5). Isotopic variation can be traced in the streamflow response and used to infer water travel times through the catchment and the ages of streamflow (Tetzlaff et al., 2007). This sampling evolved into a >10 year programme of daily sampling of isotopes in precipitation and streamflow which is internationally unique.
Resulting data show that isotopes in the stream follow the seasonality and short-term variations in precipitation inputs in a highly damped and lagged way (Fig. 5). Spatial variation in the damping and lagging of the rainfall signal depends primarily on distribution of soil and drift which control the dominant streamflow generation processes (see travel time distributions in Fig. 6) (Soulsby et al., 2007). A constant groundwater isotope signature dominates base flows, with groundwater discharging directly into the stream through the hyporheic zone or by exfiltration from hillslopes, via springs and seeps, around the edge of the peatlands in the valley bottoms (Scheliga et al., 2016). High flows are dominated by overland flow/shallow subsurface flow from peaty soils, though rainfall events largely displace water already stored in the catchment (Scheliga et al., 2019). The riparian peatlands act as “isostats” at high flows, damping the rainfall isotope signal and the component of “new” rain water in storm runoff is usually <10% (Tetzlaff et al., 2014). The isotope studies have shown that the mean stream water age is around 1.5 years; ranging from a few weeks at high flows to over 3 years low flows (Soulsby et al., 2015; Benettin et al., 2017). However inter-annual variability in mean stream water age between drier and wetter years can be large, ranging between 2 years and <1 year, respectively (Birkel et al., 2015).