by Dr. Stefan Kienzle
One of the key objectives of the VACEA project is to provide new information on the impact of climate change on the availability of water resources in various study watersheds. In Canada, three watersheds are the focus of the impacts assessment (Figure 1): The Oldman River Basin (ORB), the Castle River Watershed (CRW) – a headwater tributary of the ORB – and the Swift Current Creek Watershed (SSCW). The impact of climate change on water resources is simulated using the ACRU agro-hydrological modelling system (ACRU), a distributed physically-based hydrological modelling system.
Oldman River Basin (ORB)
The ORB has an area of 26,700 km2 and is located in the southwest corner of Alberta. Its headwaters are in the forested Rocky Mountains, with mountain peaks reaching over 3000 m in elevation. The ORB also contains foothills and prairie grassland landscapes. The Oldman River flows for 362 km, where it confluences with other rivers to form the greater South Saskatchewan River Basin, and has a mean annual discharge of 105m³/s at its mouth, at an elevation of about 700 m. The large range in elevation and topography result in a substantial variation in climate. Mean annual precipitation ranges from over 2200 mm in the wettest parts of the Rocky Mountains to under 400 mm in the eastern part of the semi-arid prairies. The continental climate results in annual temperature variations ranging from over 30˚C in the summer to -30˚C in the winter. Most of the ORB lies in the rain shadow of the Rocky Mountains, and chinook winds, dry and warm foehn winds, are frequent in the winter. About 40% of precipitation falls as snow. Spring melt in the mountains provides much of the streamflow, which declines during the summer to a low in early fall. Extreme variability in flow through the growing season has resulted in the construction of reservoirs on most major rivers to capture spring run-off and release it throughout the growing season to meet the water demands of a growing population, agriculture, industry, and to augment low river flows in the late summer. The Plains comprise about 80% of the watershed and contain extensive irrigated agriculture. Extreme droughts have occurred in the 1930s, in 1988 to 1989, and in 2001. Flooding in 1995 was caused when heavy rains coincided with spring melt and caused extensive damage in the watershed.
The ORB is sparsely populated with only about 220,000 people, and has a predominantly rural character. The largest city is Lethbridge with a population of 85,000 people. Two First Nations' reserves lie within the watershed, occupying about 1850 km2.
On average 30% of the mean annual streamflow is produced on the US portion of the watershed, in Montana. Water demand, mainly from agriculture (87%), has outgrown water supply to such an extent that a moratorium on new water allocations was implemented in 2006. It is expected that climate change will result in a decline in average water supply and increase its variability, particularly the frequency and duration of droughts, resulting in the increased risk that future demands for water will not be met.
The Castle River Watershed (CRW)
The 825km2 CRW contributes on average about 15% of the streamflow of the ORB. The watershed consists of alpine, sub-alpine, montane, and foothills landscapes located on the eastern slopes of southern Rocky Mountains. The watershed has been identified as a pristine wilderness area and is free of any major industrial or agricultural activity. There is a ski resort located in the Westcastle River valley. The watershed has an elevation range from 1188 to 2677 m and is predominantly covered by broadleaf and coniferous forest (64%), herb and grassland (16%), shrub (11%), and non-vegetated land (9%). The CRW has a continental climate with cold winters and has a mean annual precipitation (1971-2000) of 925 mm. It is important to understand the impacts of climate change on this watershed, as it characterizes future water supply changes for the entire ORB, in particular water availability for irrigated agriculture.
The Swift Current Creek Watershed (SCCW)
The 4,300 km2 SCCW is located in southwest Saskatchewan, and is part of the South Saskatchewan River Basin. It has an elevation range of 550 to 1150 m, a mean annual precipitation of about 400 mm, and a range in air temperatures from 30 °C in the summer to -30 °C in the winter. Its major land use is agriculture, of which only a small portion is irrigated. The watershed is sparsely populated, with the largest town (Swift Current) having approximately 15,000 residents. Being a prairie watershed, with low precipitation and high evapotranspiration, the mean annual discharge is about 2m³/s. The dynamics of the impacts of climate change on this watershed are expected to be different from the CRW, and only the set-up of a physically based hydrological model will allow the simulation of the expected impacts on water resources.