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).