1. Introduction
The temperature of water plays a critical role in the stability of
various habitats including those of freshwater fish (Ebersole, Liss, &
Frissell, 2001). The major energy fluxes that determine stream
temperatures are net shortwave and longwave radiation, sensible and
latent heat transfer, bed heat conduction, advection by groundwater, and
hyporheic exchange (Moore, Sutherland, Gomi, & Dhakal, 2005). These can
be classified as exchanges occurring at either the air/water interface
or the streambed/water interface (Caissie, 2006). Energy-balance studies
have found that net streambed heat fluxes make up a greater fraction of
the total heat budget for smaller streams. In a catchment in New
Brunswick, Canada, the streambed heat flux accounted for 23 % of the
total heat flux for a smaller stream compared to 12 % for a larger
stream (Hebert, Caissie, Satish, & El-Jabi, 2011). In a headwater
stream, in British Columbia, hyporheic exchange accounted for up to 25
% that of net radiation during the period of the day with the greatest
warming rate (Moore et al., 2005a). However, the streambed heat fluxes
vary both spatially and temporally within the channel (Caissie & Luce,
2017). Groundwater discharge results in summer stream cooling and winter
stream heating, acting as a buffer from thermal extremes (Hayashi &
Rosenberry, 2002). Within the stream channel, patterns of seepage flux
are determined by channel profile and morphology, such as pool and
riffle sequences and cross-channel gradients (Gariglio, Tonina, & Luce,
2013).
There is growing concern that climate change will result in stream
warming due to rising air temperatures, decreased shading in forested
areas due to wildfires, and changes in streamflow (Leach & Moore,
2010). Long-term monitoring data have already shown stream temperature
increases over the last 30 years across the northwestern United States
(Isaak, Wollrab, Horan, & Chandler, 2012). In mountain regions, changes
to flow regimes due to changing snowmelt dynamics (Bavay, Grünewald, &
Lehning, 2013), and rising groundwater temperatures must also be
considered (Kurylyk, MacQuarrie, & McKenzie, 2014). Previous work has
revealed that groundwater, as distinguished from recent seasonal
precipitation and meltwater inputs, makes up an important fraction of
mountain headwater discharge (Hood & Hayashi, 2015). Estimates vary
across catchments and seasons; up to 60 % of streamflow, during early
snowmelt, was attributed to groundwater in a catchment in the Colorado
Rocky Mountains (Liu, Williams, & Caine, 2004). In the Canadian
Rockies, a rock glacier spring contributed up to 50 % of streamflow
during summer baseflow periods and up to 100 % of streamflow over
winter (Harrington, Mozil, Hayashi, & Bentley, 2018). The relative
position and size of alpine aquifers, including rock glacier; moraine;
talus; and alpine meadow, determine the groundwater storage-discharge
characteristics in these catchments (Hayashi, 2020).
There have been few studies on the effects of groundwater discharge on
the temporal and spatial variability in alpine thermal regimes.
Consequently, the impact of groundwater on ecosystems in these settings
is poorly understood (Brown, Milner, & Hannah, 2007). The few existing
studies characterize the different source water contributions including
snowmelt, glacial icemelt and groundwater (Brown, Hannah, & Milner,
2005). Groundwater-fed alpine streams exhibit greater streamflow
permanency (Brown, Hannah, & Milner, 2006) and dampened diurnal
temperature variations (Constantz, 1998). Discrete groundwater
discharge, such as from a rock glacier spring, can mitigate the warming
effect of lake outflows, serving as a thermal refuge under a warming
climate (Harrington, Hayashi, & Kurylyk, 2017).
Direct precipitation onto the stream surface is an advective component
of stream energy balance (Webb & Zhang, 1997). However, most studies do
not include this flux, (e.g., Garner, Malcolm, Sadler, Millar, &
Hannah, 2015; Harrington et al., 2017; Moore, Spittlehouse, & Story,
2005) citing a study showing it to be negligible, even during heavy rain
(Evans, McGregor, & Petts, 1998). However, there are evidences
indicating that stream temperatures do respond to precipitation events.
Brown and Hannah (2007) attribute observed temperature responses in an
alpine stream to ‘advected energy inputs, primarily from surface and
near-surface hillslope pathways and by groundwater, rather than direct
heat flux by falling precipitation’. The idea that precipitation results
in stream cooling due to the corresponding flow generation processes is
repeated elsewhere, where direct precipitation accounted for less than
1.2% of the daily total heat flux and could not account for observed
cooling (Hebert et al., 2011). However, these studies did not consider
the effects of solid precipitation which could have a larger impact on
the energy balance. To our knowledge, Leach and Moore (2017) were the
first to demonstrate the role of channel-intercepted snowfall on stream
temperatures. Little work has been done to date to quantify the
processes that lead to observed stream temperature cooling during
precipitation events.
The aim of this study is to improve and quantify our understanding of
the processes controlling the thermal regimes of groundwater-fed
headwater streams in mountain regions. The three main study objectives
are:
- Determine the spatial and temporal variability in groundwater
discharge temperature to a sub-alpine stream and develop a conceptual
model for the system.
- Quantify the solid precipitation induced stream cooling processes to
improve stream temperature models.
- Characterize the downstream changes in the first-order stream channel
by quantifying transient storage and increases in discharge.