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.