Forecasting spring floods sounds fairly straightforward. Snow falls in winter, turns into water in the spring, and fills up the streams and rivers, sometimes to the point of overflowing.

You'd think that lots of snow means lots of water and a good chance of floods. However, flood forecasting is not quite that simple, especially in the north.

"Flood forecasting is a pretty long and involved process," says DIAND hydrologist Ric Janowicz.

The backbone of the forecasting program is basic data collection, says Janowicz. DIAND's Water Resources division uses about 45 snow survey sites scattered around the territory. Most measure snow depth and -- more importantly, he says -- the water content of the snow. There are also several snow pillow sites, where the snow settles on a bladder of water mixed with glycol and a continuous record is kept of the weight of the accumulated snow.

To complement snow pack information, the Yukon's flood forecasters, in cooperation with Environment Canada, collect streamflow information from about 60 hydrometric stations and climatic information, such as temperature, precipitation, and humidity, from a string of meteorological stations. All this information is collected through the winter and plugged into computer models.

Modern flood forecasters rely heavily on computer models, Janowicz explains, but most models were developed for populated areas in the south and aren't directly applicable to northern conditions. They don't account adequately for cold-climate factors like permafrost and seasonally frozen soil, and sublimation, where the snow evaporates directly into water vapour without passing through the liquid water stage.

Northern flood forecasters have had to adapt the southern models to northern conditions or develop entirely new models, says Janowicz.

The Wolf Creek Research Basin near Whitehorse was established several years ago to calibrate hydrologic models. Studies at Wolf Creek have allowed researchers to measure the accuracy of the southern models when applied to the Yukon. They found some startling differences in Yukon conditions.

For example, in southern Canada, sublimation is generally ignored as a factor, says Janowicz. However, the Wolf Creek studies show that up to 60 per cent of the Yukon's winter snowfall can disappear through sublimation, never reaching the streams channel.

The southern models also miss the mark in assessing the relationship between snow-pack and elevation. The models assume that the snow-pack increases with elevation, which is broadly true in the south.

In the Yukon, vegetation also plays a significant role, says Janowicz. Above the treeline on Yukon mountains lies the subalpine taiga, an area of low brushy plants that trap snow blowing off the bare alpine tundra higher up. As a result, the depth of the snow pack varies according to vegetation zone.

"With the fall and early winter data, and the appropriate models in hand, we begin the forecasting season in February", Janowicz says.

In the spring, Water Resources publishes a series of Snow Survey and Water Supply Forecast bulletins. The March 1 bulletin provides an early indication of the amount of water likely to flow into Yukon streams. The April 1 bulletin covers the time when the snow pack hits its maximum. The May 1 bulletin provides an indication of quickly the snow is melting.

Then the forecasters hit the road. They shift operations to Dawson and use aerial surveillance and frequent on-site water-level monitoring to watch for ice jams that produce flooding in that community. Once Dawson is safe, they move on to Old Crow, which has a history of ice-jam floods a week or two later than Dawson. Then it's back to the office in Whitehorse and daily runs of the computer models to forecast snowmelt floods in places like Ross River and Mayo.

Even after the peak of the spring run-off is past, the forecasters continue to run their models and monitor the data, watching for signs of heavy glacial melt that can cause flooding late in the summer. Later they watch for freeze-up ice jams that can lead to winter floods. And they start collecting data again for the next flood season.

For more information about flood forecasting in the Yukon, contact the Water Resources Division of Indian and Northern Affairs Canada, Whitehorse, Yukon.