[MUSIC] Where does water come from in the mountains? [SOUND] There are three main sources that we will discuss in more detail. Rain, snow, and glacier ice. Let's start with rain. Because of the higher elevation of mountains, rain tends to be limited to certain times of the year, mostly during the summer. But also the shoulder seasons of spring and fall. It's too cold at other times of the year, when precipitation falls as snow. As we've discussed previously, rain tends to fall at higher elevations because mountains will force any moisture-laden air masses to rise. As these air masses rise, they cool, which causes water vapor to condense and eventually form precipitation. Two different types of systems can cause rainfall in the mountains. There can be frontal rainfall, where low pressure systems move in from elsewhere and are forced to travel over the mountains. Or the rain may be from locally formed convective storms. These storms tend to form in afternoons and evenings from strong daytime heating on the landscape, which causes evaporation, and eventually condensation as the water vapor rises and cools. Rain can be prolonged with frontal rainfall. It may last for days or even weeks, as is often witnessed on the west coast of Canada. Where prevailing winds continually drive low pressure systems and their associated storms into the Coast Mountains. These rains are not necessarily intense. They often may only consist of a light mist. On the other hand, convective rainfall occurs on a much shorter timeframe, lasting mere minutes to hours. Convective storms often include another form of solid precipitation called hail as well as strong winds. While frontal precipitation occurs at any time of the year, convective precipitation is limited to periods when it is hot enough to cause strong evaporation during the day, such as during late spring, summer, and early fall. Convective storms are common in continental areas, and are often seen in places like the Canadian Rockies throughout the summer. The rain that results from these storms can take several routes once it falls on the ground. Runoff can be direct, moving downslope on the surface from the minute it hits the ground. Or runoff can be delayed, absorbed into the ground and slowly percolating through the landscape. Direct runoff can have an almost immediate impact on stream and river runoff, causing them to swell and run faster. Delayed runoff may also contribute to river runoff, but over a period of days to weeks. The intensity of the rainfall often controls how well the rain is absorbed into the ground. During really intense rainfall events, the top portion of the ground will become saturated first. Preventing further percolation of water into the soil, causing the surface runoff. This is not uncommon during convective storms. Steady rain that is not as intense will allow the water to percolate deeper. However, a steady long lasting rain can also eventually saturate the soil, causing increased overland to flow. During winter, late fall, and early spring, the predominant precipitation in many mountains, including even those in equatorial regions, is snow. The snow provides for the short-term storage of water, delaying any notable runoff until the warmer temperatures of spring arrive. Once the snow begins to melt, how that water travels over the landscape will be partially controlled by the snow's temperature profile. Fresh snowfall, which can contain upwards of 90% trapped air, is a very good insulator. The upper 30 to 40 centimeters will be colder, particularly if exposed to very cold air. But that cold air will not travel very efficiently through the snow pack to the ground. In addition, it takes time for the accumulated warmth that's stored in the ground over the summer and fall to fully dissipate. This means that the snow resting directly on the ground will often tend to be warmer than surface snow. There are many things, though, that can affect the exact temperature profile, such as whether the ground was exposed to prolonged freezing temperatures before the snow fell. The duration of the winter is also a factor. The density of the snow can affect the exact temperature profile, since the insulating ability of snow decreases as its density increases. Whether or not the ground is frozen will determine how the melt will flow. If the ground under the snow is frozen, surface runoff will dominate, leading to water quickly finding its way into streams. In conditions like this, any late winter rain on snow events can easily lead to flooding. If the ground is not frozen, the melt will tend to percolate into the soil and delay runoff. So the temperature profile of the snow pack is an important consideration when thinking about runoff into streams and rivers. Snow melt is determined by the time of the season and temperature. Obviously, the warmer the temperature, the more melt and runoff. This is complicated, however, because snow generally has a high albedo, which is a measure of the reflectivity of its surface. For example, up to 90% of the sun's incoming energy on fresh snow gets reflected. It's bounced back into the atmosphere. But seasonal changes are important too. In spring, much of the initial energy from the sun contributes to warming the snow pack, with only minimal melt occurring. So there tends to be a lag between warming of the atmosphere and melting of the snow and related runoff. Once the snow begins to melt, though, a positive feedback develops. Positive feedback is a scientific concept that describes a cycle where one event causes another event to occur, with the second one reinforcing the first, causing it to become greater, and so on and so on. For example, imagine a mosquito bite. You get bitten by a mosquito. The bite is itchy, so you scratch it, which makes it even itchier, and you scratch some more, making it even worse. That is a positive feedback. As the snow warms and there's some melt, water will start to build up in the snow pack. And water has a lower albedo than the snow itself. This lowering of the albedo means that less energy is reflected, encouraging more melt. As the snow begins to melt, the albedo typically lowers for another reason as well. Airborne particulate, including soil, dust, smoke, and organic material which falls with the snow, is exposed and increasingly concentrated as snow melts. This particulate matter makes the snow appear sooty. Darker snow is less reflective, and so more heat from the sun is absorbed, further warming the snow pack. And the beginning of the melt season, when the snow is bright and fresh, it will take a while for the melt to begin. But as it gets older, it will melt faster and faster due to the reduction in albedo. Glaciers are the third source of water in mountains and play a significant role in the hydrologic cycle. Glacial ice is an important component because it represents the long-term storage of water. It's essentially a natural reservoir. Sometimes hundreds or even thousands of years old. Glacier ice is also highly seasonal. You can probably imagine how in a dry, hot summer and fall, the glacial component to the runoff in the mountains can be very significant. And as the surface of the glaciers melt, they also experience a reduction in albedo that can greatly accelerate melting late into the summer. In addition to providing life-sustaining water, glacial melt also has other benefits. The water coming from the glacier is cold, not much above freezing, and will keep the stream and river temperatures quite low. This is very important for several fish species such as trout. However, the large amount of rock flour or very fine sediment that's formed by the grinding action of glaciers as they move over the rocky bed surface, which is incorporated into the runoff, may also influence which species live in glacier-fed streams and rivers. So melting glaciers can have many effects on rivers that have their headwaters in mountains. Here's University of Northern British Columbia glaciologist Brian Menounos. >> Well, glaciers in many mountains of the planet are excellent resources for water. And what makes them really unique is that they release this water in late summer, when seasonal snow packs are at a minimum. But it's not only the quantity of water that they release to these head water creeks, but it's also cool water. So in some cases they provide important buffering, thermal buffering, if you will, for many of these small headwater rivers. So the fate of glaciers in Western Canada is not a good news story. I'm wearing black today, much like a funeral director, telling you this. It is projected largely through human activity that these glaciers are not expected to survive. Those small alpine glaciers in the Canadian Rocky Mountains, our own research has suggested that close to 90% of the area and volume will be lost by 2100. People may ask themselves, why change things now if their fate is already sealed? We can't do anything about the alpine glaciers in the Rockies, the small ones. We can change the fate of the larger ice fields and glaciers in the Coast Mountains. By and large, we've already committed to the fate of the alpine glaciers in the Rocky Mountains.
