5.2 Effects of direct precipitation on stream temperature in cold
regions
Previous studies have concluded the direct precipitation heat flux to be
negligible (Evans et al., 1998) and observed stream cooling has been
attributed to the resulting flow generation processes (Brown & Hannah,
2007). We began this study with the aim of quantifying these subsurface
flows to improve stream temperature models. However, we found that
direct precipitation made up a large component of the energy balance
during solid precipitation events (Figure 11B) consistent with the
findings of Leach and Moore (2017). Furthermore, Hathataga Creek did not
undergo any noticeable cooling during rainfall events. This suggested
that an increase in advective inputs, from groundwater and hillslope
pathways, did not play an important role in altering stream
temperatures.
The stream energy balance was dominated by shortwave radiation during
the day and longwave radiation at night (Figure 11B). The largest
temperature residuals occurred between 14:00 and 15:00 when measured
temperatures were up to 1.5 °C warmer than modelled ones. The
uncertainties in shortwave radiation came from estimates of shading.
This is attributed to shading values that were fixed in the model and
therefore did not change throughout the day. The radiation measurements
were taken in an open meadow. For most of the day the stream was well
shaded as captured by the shading estimates. However, in mid-afternoon
the position of the sun was such that it shone along the length of the
stream and shading estimates became erroneous. In future modelling
studies, shading and view to sky coefficient should be quantified in the
field using hemispherical images (Garner, Malcolm, Sadler, & Hannah,
2017) or a densiometer (Gravelle & Link, 2007).
Uncertainties in longwave radiation came from the assumption that canopy
surface temperature equals air temperature. In reality, longwave
radiation from surrounding vegetation (Lveg, Equation
S2-5) is likely greater on sunny days when canopy temperature is greater
than air temperature (Pomeroy et al., 2009). The remaining fluxes were
relatively small in comparison. Uncertainties in the latent and sensible
heat flux came from meteorological measurements made at 3.45 m height
rather than 2 m. As a result, wind speed and the latent heat flux were
likely lower. Furthermore, measurements were made over the meadow rather
than the stream surface. It has been shown that wind speed is higher and
more variable at exposed regional weather stations compared to sheltered
microclimate sites (Benyahya, Caissie, El-Jabi, & Satish, 2010). In
this study the stream surface is located in a topographic low relative
to the meadow. Therefore, wind speed was likely lower than measured.
During the snowfall event the direct precipitation flux was ten times
greater than other energy inputs.
The energy consumption caused by the melt of solid precipitation have
important implications for stream temperatures in alpine environments
and other cold regions. Equation 2, which describes this process, should
be considered in stream energy balance models. Without this flux,
parameter estimates from calibration would be less reliable to account
for the lower stream temperatures. In other mountain regions, such as
the Vernagtbach basin in Austria, there are years when solid
precipitation accounted for up to 70 percent of total precipitation
during the ablation season. During the same period, there were up to 50
days with snowfall (Escher-Vetter & Siebers, 2007). In the Hathataga
catchment, solid precipitation accounted for 23 percent of total
precipitation from May to September 2019 with 20 days of snowfall.
This could also have important implications for the field of meteorology
where it is a challenge to determine precipitation phase. Greater than
expected temperature changes in small streams or of fluid in a
precipitation gauge (Geonor, T-200B) could be used as a proxy for
solid/liquid phase rather than using an air temperature threshold or
expensive laser-based sensors (Campbell Scientific, CS125). The
advantage of this method is that cooling associated with the latent heat
of fusion only occurs for solid precipitation, irrespective of air
temperature. This was especially relevant during summer hailstorms when
surface air temperatures were up to 10 °C but stream cooling was
observed.