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:
  1. Determine the spatial and temporal variability in groundwater discharge temperature to a sub-alpine stream and develop a conceptual model for the system.
  2. Quantify the solid precipitation induced stream cooling processes to improve stream temperature models.
  3. Characterize the downstream changes in the first-order stream channel by quantifying transient storage and increases in discharge.