Snow Surveys in the UTRCA Watershed

With all the snow the Upper Thames River watershed has received this winter, residents may be wondering if major flooding is in the forecast. The 糖心直播 (UTRCA) has been manually surveying accumulated snow across the watershed since 1957, as part of the flood forecasting and warning services it provides to watershed municipalities to inform residents and businesses of impending dangers. To assist us in forecasting floods – and managing our flood control reservoirs – we operate:

• stream gauges,
• snow survey sites, and
• precipitation and temperature measuring stations.

Snow measurements help quantify the amount of water available to melt during the winter or early spring. The data is collected as part of the provincial Snow Survey Program, which is a collaboration between Conservation Authorities, Ontario Power Generation, and the Ministry of Natural Resources, and follows the Ministry’s Snow Surveying Manual (1985).

Snow Sampling Program

The UTRCA established eight snow survey locations in the 1950s. Over time, some sites were added and others were discontinued as property access changed. Currently, the UTRCA samples snow at 14 locations across the watershed, including seven locations that are at or very near to the original eight sites. We also use data provided by the Grand River and Maitland Valley CAs from nearby sites.

Considerations when selecting a site include access, vegetation, topography, and exposure to wind. The best sampling sites are relatively flat, well-drained, grassy areas that are protected from the wind and located on public property.

Measurements are made twice a month during the winter, as near to the 1st and 15th of each month as possible, as well as at other times if melting is imminent. UTRCA staff collect and record snow depth and snow-water equivalent data at each site:

• Snow depth is measured with a calibrated snow core tube that has a scale along the outside of the tube. Staff measure the snow depth at 10 locations across a site and calculate an average snow depth for that site.
• Snow-water equivalent is calculated to determine the amount of water in the snow pack. At each depth measurement location, staff collect a snow core within the tube then empty it into a common bucket to be weighed at the end of the 10 measurements. The snow tube is calibrated so that 1o grams of snow equals 10 millimetres of water. Dividing the total weight of the 10 snow cores by 10 produces the average snow weight in grams for that site. That number is then converted to mm and represents the snow-water equivalent.

Other observations collected at each site include date, time, weather conditions, temperature, snow drift information, coverage, and ground conditions.

Current Snow Conditions

The UTRCA snow survey taken on February 2 reported an average of about 75 mm of water equivalent in the watershed, with an average depth of about 30 mm. Compared to the long-term historical record for February 2, this snowpack water equivalent is nearly 200% of normal, and 60% of the maximum measured in the period of record (since 1957 for most sites).

The UTRCA’s water resources engineers determine the snow density by dividing the water content by the depth and multiplying by 100. This calculation tells us that the snowpack is approximately 25% water and 75% air.

Predicting the Snowmelt

As snow absorbs rainwater, or melts and is compacted by warmer temperatures, it gets denser. When snow density is more than 35-40% water, the runoff process begins.

Snowmelt models help UTRCA staff estimate when the snowmelt will begin and how quickly the runoff process will occur once it starts. Air temperature and rain are the main factors. Degree days of melting are determined by the average above 0掳C daily temperature. For example:

• a day where the temperature does not go above 0掳C yields no degree days of melting;
• a day with a 10掳C high and an overnight low of -1掳C yields five degree days; and
• a day with 10掳C high and a low of 5掳C yields (10 + 5)/2 = 7.5 degree days.

Snowmelt begins when there are 15 to 20 consecutive accumulated degree days. Rain on top of snow speeds this process up, adding more water to the snowpack and some additional warming. Most of the largest floods on record in the Upper Thames River watershed have been generated by a combination of rain on top of melting snow.

An ideal snowmelt happens when daily maximum temperatures stay below 10 掳C, overnight temperatures drop below freezing to slow the thaw process down, and there is little to no rain. This combination results in a long, slow melt period, usually with only minor flooding.

February Outlook