4.4 Stream temperature
An example period (August 14–21, 2019) was chosen that included both a
rain and snowfall event, and captured daily stream temperature
variability, to model stream temperatures over a range of meteorological
conditions (Figure 10). The first precipitation event (PE1) was a
rainfall event and occurred on August 16 between 13:15 and 17:00 with a
maximum 15-minute intensity of 6.7 mm h-1 at 14:45 and
a total of 8.9 mm. The second event (PE2) was a snowfall event and
occurred between 22:45 on August 16 and 03:30 on August 17 with a
maximum intensity was 6.6 mm h-1 at 02:00 and a total
of 13.8 mm (Figure 10B).
During the six-day period, stream temperatures at 0 m (T1) remained
between 5.0 and 5.5 °C, whereas temperature variability became more
pronounced downstream (Figure 10C). T1 was located on the eastern side
of the creek at GS1 which meant that the temperature was more
representative of S1 (Figure 5A). For this reason, T2 was used as the
boundary condition in the HFLUX model (see below). Water temperatures
reached a maximum of 6.9 °C at 246 m (T4) and 9.6 °C at 809 m (T10) at
15:00 on August 19, coinciding with a maximum air temperature for the
period of 17.8 °C. A minimum air temperature of -1.4 °C was attained at
07:15 on August 17. This low did not correspond with stream temperature
lows, which occurred at 02:00 on August 17, during the snowfall event.
At this time water temperature was 2.9 °C at 246 m and 1.1 °C at 809 m
downstream. This meant a temperature drop during the snowfall event that
started at 22:45 of 2 °C and 3.5 °C, at T4 and T10 respectively (Figure
10C). During the rainfall event stream temperatures only fell by 0.5 °C
at 246 m and 0.8 °C at 809 m.
Groundwater levels in P2, located at the outlet springs, rose by 7 mm
during PE1 but no pronounced stream cooling was observed. During PE2
groundwater levels only rose by 1 mm (Figure 10D). The time-lapse camera
captured the accumulated snow melting after sunrise.
The temperatures simulated by the HFLUX model are shown in Figure 11A
for the original model and the modified model including the latent heat
of fusion (Eq. 2). During the snowfall event on August 17, the modified
model had a better match with observed temperature compared to the
original model. This resulted in a reduction in the overall RMSE between
measured and modelled stream temperatures along the study reach from
0.46 °C to 0.38 °C for August 17, and from 0.31°C to 0.29 °C for the
6-day period.
The energy fluxes computed by HFLUX is shown in Figure 11B, where
positive values correspond to an energy gain by stream and negative
values an energy loss. Net shortwave radiation was the major source of
energy with an average of 58.9 W m-2. Net longwave
radiation and the sensible heat flux were mostly positive with averages
of 28.7 and 1.7 W m-2, respectively. For most of the
period the latent heat flux was the major heat sink with an average of
-11.0 W m-2. This was followed by bed conduction, the
smallest flux, with an average of -1.6 W m-2. The
direct precipitation flux was mostly negligible as there was either no
precipitation or only liquid precipitation. However, the precipitation
flux became the dominant energy flux during the snowfall period,
reaching a minimum of -615.5 W m-2, the single largest
flux during the 6-day period. The average total energy flux during the
period was 67.6 W m-2 resulting in a net increase in
water temperature downstream.
Hyporheic exchange was not included in the heat transfer equation (Eq.
S2-1) in HFLUX. Nevertheless, the stream temperature model had a
reasonably low RMSE. Therefore, it could be inferred that either little
hyporheic exchange was occurring in the system or that hyporheic
exchange had a negligible effect on stream temperatures in the channel.
With this in mind, a tracer test was used to quantify transient storage.