There seems to be a lot of paranoia as we approach this years snowmelt season fueled by alarmist statements to the media by people who don’t fully understand what caused last June’s extreme flooding. The snow recording sites in the headwaters of the Sheep, Elbow and Highwood Rivers show that there is a little more snow high up than there was last year. Is this much of a concern? No, not really, not yet.
The contribution of snowmelt to the base flow in our rivers depends on the rate at which the snowpack melts, irrespective of the amount of snow remaining high up in the headwaters. Maximum melting occurs on hot, sunny, windy days.
The effect of a deeper snowpack is that it shifts the start of the main melt period from early May to early June, when more intense solar radiation increases the rate of melting, which in turn increases the base runoff.
If we have a deeper snowpack than normal that results in the highest melt rate being in mid June, and we have a period during normally rainy June of hot, sunny, breezy days, then we are going to get higher than normal water levels in the rivers. In most years it is not a big concern. Getting heavy rain during a high melt period is what causes flooding. In 2013 the weather was fairly cool leading up to the extreme rain and melting was progressing at a normal rate. Hold on to you waders if all the many contributory factors are ever aligned. For a more detailed explanation continue reading…
Let’s take a look at the Snow Water Equivalent (SWE) hydrograph for Little Elbow Summit at the headwaters of the Elbow River basin. SWE is a measurement of the amount of water contained in the snow pack. It is the depth of water that would theoretically result if the whole snow pack instantaneously melted. Click on the hydrograph (left) to bring up a larger version that you can use to follow my observations. Latest Little Elbow Summit snow data.
The blue line is the SWE for 2013. The two grey dashed lines are quartiles (50% of historical data falls within these lines). The pale grey rectangle highlights June 19 to June 21 — the days of extreme rainfall. Note that on the 20th the rain turned to snow, adding about 42 mm of SWE to the snowpack.
The snowmelt season
- The main snowmelt season is from the beginning of May to early July. As solar radiation increases the average rate of snowmelt, indicated by the steepness of the line of the hydrograph, increases from about 6 mm SWE per day in early May to 35 mm per day at the end of June.
- The quartile lines show that the average rate of snowmelt is remarkably consistent year to year. Other snow recording sites at Three Isle Lake, Mt. Odlum and Lost Creek show similar characteristics.
- A deeper snowpack at the start of the melt season shifts the period of most rapid melting into late June when the melt rate is higher. With a shallower snowpack the snow has melted by the middle of June.
- By mid June most years there is no winter snow at lower elevations or in open areas above treeline. The only old snow remaining is in lightly treed areas and some wind-drifted snow at the edge of meadows. However, there may be new snow.
- The base run-off in June is dependent on solar radiation, air temperature and wind speed at higher elevations. Maximum melting occurs on hot, sunny days with a brisk warm wind.
- For significant melting to occur the air temperatures needs to remain above 0°C at night and the snowpack needs to be isothermal (all layers are at 0°C) and wet throughout its depth.
How does heavy rain affect the snowpack?
- The weather a few days prior to heavy rain is important in establishing the base flow from snowmelt. A few sunny, warm, breezy days will prime the snowpack for maximum melting.
- Cold rain on 0°C snow results in only a small increase in the melt rate. If the rain turns to snow, as it did at Little Elbow Summit, then the contribution to the base flow will actually be reduced.
What happened in June 2013?
- Contrary to statements in the media the June 2013 flood was not a warm rain-on-snow event, nor did evaporation from the snow surface condense to create more intense rain. It was an extreme rainfall event, with cold rain on 0°C snow adding little additional run-off to the normal radiation-induced melting. Because the headwaters only had snow in protected meadows at higher elevations by June 19, any addition to the total run-off by rain-on-snow was a small percentage of the total run-off.
- As can be seen from the Precipitation/Temperature graph below, temperatures (red line) at the start were between 6 °C and 8.5 °C, falling rapidly from 8 pm on the 19th to O°C at 10 pm on the 20th. Look at the hydrograph to see what happened.
- The first day of the storm, when temperatures were relatively warm, and I am guessing the cloud cover not too thick, the snowpack was melting. Note the steepness of the line — I estimate about 65 mm SWS per day. The temperature dropped and the rain turned to snow adding about 42 mm SWE of snow. Once it warmed up a bit, late on the 21st, about the time the rain ended, melting resumed. On the 25th, warmer temperatures increased the melt rate and the snowpack melted steadily at about 35 mm SWS a day until it was all gone in early July.
- It is interesting to note how closely the 2013 melt rate parallels the melt rate indicated by the upper quartile (dashed grey line).