Headwaters Hydrology of Alberta’s Eastern Slopes: Principles, Processes, Prediction
Professor John Pomeroy
Canada Research Chair in Water Resources and Climate Change,
Centre for Hydrology,
University of Saskatchewan,
117 Science Place,
Saskatoon, Sask.
The hydrology of headwater basins in the Eastern Slopes is dominated by snowmelt
runoff and runoff responses to extreme rainfall events, with runoff efficiency being much
greater for snowmelt than for rainfall events. These hydrological processes are examples
of water and energy cycles that operate at the interface of the climate system, mountain
topography, terrestrial ecosystems (tundra, grassland, forests, wetlands) and soils.
Hydrological processes both mediate and are sensitive to changes in any of these systems.
Snowfall is intercepted in forest canopies where over half of it sublimates back into the
dry Alberta atmosphere. This is part of generally higher evaporation and
evapotranspiration losses from forests compared to open areas. In spring, forest cover
protects snowpacks from solar radiation and so slows down melt by weeks on south
facing slopes. Forest soils are more porous and drier than those of open sites and water in
forests tends to infiltrate soils to drain slowly to the stream rather than forming rapid
runoff as at open sites. Valley bottom wetlands have an important regulating effect on
streamflow from forested slopes, absorbing high flows and releasing groundwater during
low flows. Forest cover changes as a result of stand growth and succession, harvesting
and wildfires and now from implications of the mountain pine beetle. The overall effect
of a mature forest cover in the headwaters is to reduce annual streamflow and to delay
and moderate the peak flows. Conversely, forest removal increases annual and peak
streamflows and increases the flashiness of streamflow response to extreme rainfall and
melt events. But not only forest cover is changing in the headwaters. The climate and
hydrology of well-studied high altitude forested basins in the Kananaskis have changed
significantly since the early 1960s. Air temperatures in winter have increased 3 to 4 oC,
rainfall is sparser but more intense when it falls, less precipitation falls as snow and
spring peak streamflow has decreased by 30% causing annual streamflow to decrease by
29%. The challenge for forest hydrology is to predict the cumulative effects of changing
climate and forest cover on water supply and to indicate to forest managers how forests
can be managed to better preserve the natural water and energy cycles. Several examples
and the use of physically-based hydrological models for virtual forest management
simulations will be shown.