4.2 Spring temperature
The clusters of springs are grouped into three spring branches (S1, S2,
and S3 in Figure 4). Thermal infrared images of the spring area were
taken on August 14, 2019 at 12:15 to better visualise mixing of the
three spring branches (Figure 5). Water surface temperatures were
between 1.1 and 6.2°C. Distinguishable features include a cold side
spring (Figure 5A) and a sharp thermal boundary between water
originating from S1 and S2/S3. It can also be seen that GS1 is situated
on the warmer stream side which must be considered during interpretation
of the temperature data from the gauging station.
To better resolve the temporal variability, temperature sensors were
placed in each of the three spring branches from June 11 to October 17,
2019 (Figure 6). Data gaps were the result of sensors being exposed to
air (August) or full datalogger memory (July). Initial temperatures in
the spring branches were equal at ~1.3°C but these
rapidly diverged at the onset of snowmelt (Figure 6). The maximum
recorded temperature in each spring branch was not reached
simultaneously: S2 reached its maximum on July 15 with temperatures
remaining high until late August. Temperatures at S1, on the other hand,
were rising during this time and reached a maximum on August 22. By
mid-September temperatures in S2 were back down to S3 levels. S1
temperatures had still not been reduced to S3 levels by October 17.
Diurnal variations are likely due to heating of spring water following
surface discharge. This is especially noticeable in S3 as it is the
longest spring branch above the temperature sensor.
The observed spatial and temporal variability in spring temperatures was
described above. Below, changes in spring temperature and discharge and
changes in lake temperature and water level are explored (Figure 7).
Note that stream temperature measured at GS1 was used, which was heavily
influenced by high temperature discharge (Figure 5A). Even though this
does not represent the spring system as a whole it captures the
relationship between the lake and springs satisfying the purpose of this
study. Additionally, water levels measured in P5 were used to include
days when water levels were below the lake stilling well. True lake
water levels were up to 5 cm higher than heads measured in the lake
piezometer due to the downward flux into the lakebed. The downward head
gradient beneath the lake accounts for the differences in readings
between the lake piezometer and stilling well. To capture both spring
snowmelt and the lake drying up, across two different seasons, the study
period extends from May 1 to October 1, 2018 and 2019.
During the 2018 season the water table under the lake rose to the
surface on May 18, after the onset of snowmelt. The maximum water level
was 1.8 m on June 24, coinciding with a maximum spring discharge of 0.31
m3 s-1 (Figure 7B). After this,
spring and lake temperatures increased rapidly until air temperatures
dropped on July 18 (Figure 7C). By August 15, the lake had dried up and
spring temperatures began to fall (Figure 7B and 7C). There are some key
distinctions between the 2018 and 2019 seasons. Snowmelt started two
weeks later in 2019 meaning that water levels reached the surface on May
31. Snowpack, measured at AWS1, prior to snowmelt was 60 cm deeper in
2018, however summer precipitation was greater in 2019. Also, the
maximum lake water level was 20 cm higher in 2018. Lastly, the lake
dried up later in 2019 and resurfaced following heavy precipitation on
September 2.
Prior to snowmelt in late May 2019, temperatures at GS1 remained steady
at ~1.3°C (Figure 7F). In 2019, after the surface water
appeared in the lake, they continued rising rapidly during the following
four days to 1.47 m (Figure 7E). On June 4 stream and lake temperatures
reached seasonal lows of 0.9°C and 0.7°C, respectively. Daily average
lake temperature proceeded to rise to 4.4°C by June 13 (Figure 7F).
During this time, changes in lake water level and spring discharge
corresponded to fluctuations in air temperature (Figure 7D). For
example, in the first week of June daily average air temperature
decreased by 10°C, lake water level decreased by 0.5 m and spring
discharge decreased by 0.13 m3 s‑1(Figure 7D, E, F). As air temperatures rose water levels and spring
discharge recovered.
Lake water level reached a maximum of 1.62 m on June 18, 2019 and stayed
high until mid-July when it began its downward trajectory (Figure 7E).
The decline in lake water levels coincided with a rapid increase in both
lake and stream temperatures (Figure 7F). By August 25, the lake had
dried up. However, the lake briefly appeared again following
precipitation on September 2, which was confirmed by a field visit on
September 4. This pattern was reflected in stream temperatures as a
decrease when lake water level was below the surface and an increase
when it resurfaced.