Snowmelt season

Fine weather snowmelt conditions. Stage = 50cm, Tw = 8C.

From the hydrograph below we can see that fine weather snowmelt conditions begin over the last few days of March and into April, as indicated by the daily fluctuations or sine wave pattern. This is characteristic of streamflow hydrographs in snowmelt regions. As warm sunny days continue, the daily fluctuations rise higher and higher as groundwater levels rise. On top of this we have the influence of rainfall events, most notably on 30 March (18mm) and 7 April (36mm). However, these events do not reach the high stage of over 1m that was recorded on 14 February this year due to rain-on-snow.

Larch lysimeter site after snow has completely melted

Cedar lysimeter site after snow has completely melted

Time-lapse cameras showed that the snowpack had completely melted by 27 March at the larch site, while it remained four days longer until 31 March at the cedar site.

February rain-on-snow flood

Larch snow lysimeter site (depth = 63cm, density = 0.38, SWE = 24.0cm)

Today's snow survey showed that the snowpack is more or less the same as in the last survey in mid-February. Snow densities have increased from about 0.3 to nearly 0.4 at the larch site, indicating that the snowpack has ripened considerably and reached a melt condition. The snow water equivalent is up slightly to 24 cm at the larch site, whereas it dropped about 2 cm to 16 cm at the cedar site.

4 March: Stage = 51cm, Tw = 5C

Downloading the stage data and plotting the hydrograph as below, we can see a very prominent flood event on 14 February. This is a typical late-winter rain-on-snow event, but the size of this event has only been exceeded once in the last 16 years of stream gauging for the months of January and February. Daily snowfalls occurred during the first 10 days of February, followed by rising levels of sunshine and rapidly warming temperatures, with a Tmax of 11.8 degrees and over 9 hours of sunshine on 12 February. This was followed by warm, cloudy and windy weather when overnight temperatures remained above freezing. On 14 February heavy precipitation of around 10mm/h and 40mm/d fell in the upstream area and Tmax reached 12.3 degrees, producing perfect conditions for rain-on-snow melt to occur throughout the basin. No sunshine was recorded, but wind speeds reached 11.3 m/s.

Rain-on-snow flood event on 14 February (peak Q estimated to be 18.5 m3/s)

The three largest January/February rain-on-snow events over the past 16 years:

Date               Stage (m)    Q (m3/s)   Daily Precipitation (mm)
2004 Feb 23      0.994           17.0                  44
2009 Feb 14      1.278           32.0                  52
2016 Feb 14      1.042           18.5                  37

Snowmelt time-lapse

This time-lapse video shows the spring melt-off for the snowpack at the larch lysimeter site from 27 March to 6 April 2015. The 10 day period was captured at 5 minute intervals with a time-lapse camera. On 18 March the snowpack was measured at 25.3 cm SWE (depth 65 cm). The period starts with nighttime temperatures below freezing, but mean daily temperatures rise steadily from around 6 degrees C to around 12 degrees C, and there is frequent rainfall of up to 6.5 mm/h and 32 mm/day.

Low levels of snowpack

Snow survey in the middle of February should give us close to the maximum snow accumulation for the season, at least for the low elevations. Melt conditions become more common during the second half of February, and precipitation amounts decrease. However, current snowpack amounts remain low compared to average conditions.

The chart above shows that the snow water equivalent for the 60 m and 140 m sites is the same at about 22.5 cm. The snow density at the 60 m site is slightly higher at 0.30 for a snow depth of 74.5 cm, whereas the values for the 140 m site are 0.28 and 79.5 cm respectively. These snowpack amounts are some of the lowest recorded since 2002, with only 2007 having snowpack significantly less than the current year.

El Nino delays winter

Stage = 39cm, Ta = 2.0C, Tw = 3.3C

Heavy snows over the last couple of days signaled the start of mid-winter conditions and seasonal snowpack at all elevations in the basin. The formation of low elevation snowpack is about a month later than an average winter, mainly due to the strong El Nino effect and unseasonably warm temperatures at the start of this winter season.

Larch forest lysimeter site - checking and cleaning drainage

Snow survey at the larch forest lysimeter site showed there to be about 67cm depth of snow, with a snow density of about 0.25 giving a snow water equivalent of 16.5cm. Typically there would be more than a metre of snow depth by the end of January, with a density of more than 0.3 for an older snowpack. The snowpack was temporarily removed around the lysimeter tray drains to clean and confirm free drainage of the trays. For the larch site and tributary cedar site free drainage was confirmed, while at the lower cedar site I found 2 of the 3 trays to be blocked. Unfortunately a blocked drain means the runoff cannot be determined accurately up to that date, but after cleaning the lysimeter should drain freely for the rest of the winter season.

Free drainage condition confirmed in one of the trays

Snow depths at the upper (100m higher elevation) tributary cedar site were similar to the larch site, while the snow depths at the lower cedar site were the least at about 46cm due to snow interception (sublimation and melt-drip loss) in the mature canopy.

The stage hydrograph below shows a moderate peak around early New Year due to heavy rain. With continued cold and snowy conditions we expect the flow to decrease steadily towards a winter low-flow condition. However, ground-melt at the base of the snowpack and minimal evaporation loss will keep the flow level higher than the summer low-flow condition.

River scene time-lapse

This time-lapse video shows the period from 2-25 January 2016 during which several rainfall and snowfall events occurred. By the end of the video, the seasonal snowpack has finally accumulated, about three weeks later than average.

Barely any snow yet

Multiple small peaks due to rainfall - barely any snow yet this winter season