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Impacts on Water Supply
                                (A Canadian Perspective)
As flow patterns and water levels respond to the changing climate, our water supplies will be affected.  Diminishing surface-water and ground-water supplies, coupled with increasing demands for these resources, would challenge all aspects of water resource management.

 Quantity of Freshwater
It is difficult to predict future changes in the availability of freshwater. While there is confidence that warmer temperatures will affect variables such as evaporation and snow cover, uncertainties concerning the nature of regional changes in precipitation patterns, as well as the complexity of natural ecosystems, limit our ability to project hydrological changes at the watershed scale. However, it is reasonable to generalize that, for many regions of Canada, climate change will likely result in decreased summer flows, warmer summer water temperatures and higher winter flows. This is particularly true for the snowmelt-dominated systems that are found across most of the country.
Some of the most vulnerable regions of Canada with respect to the impact of climate change on water resources are those that are already under stress, with demand approaching or exceeding supply. This is most apparent in the driest regions of the southern Prairies, commonly referred to as the Palliser Triangle, where drought and severe annual soil moisture deficits are recurrent problems. Even Ontario, perceived to be an especially water-rich province, suffers from frequent freshwater shortages, and more than 17% of British Columbia’s surface-water resources are at or near their supply capacity for extractive uses.
For much of western Canada, snowmelt and glacier runoff from mountainous areas are primary sources of water supply for downstream regions. With warmer conditions, the seasonal and long-term storage capacity of alpine areas may decrease, due to thinner snowpacks, more rapid spring runoff, and decreased snow and ice coverage. This, in turn, would result in lower summer river flows and therefore greater water shortages during the period of peak demand. Recent trends observed on the eastern slopes of the Canadian Rocky Mountains suggest that the impacts of diminishing glacier cover on downstream flows are already being felt (see Box 1).  Across southern Canada, annual mean streamflow has decreased significantly over the last 30–50 years, with the greatest decrease observed during August and September.  Continued decreases are projected to occur as a result of climate change.

BOX 1:  Diminishing flows in Prairie rivers

Glacial meltwater is a key source of water for rivers in western and northern Canada.  Along the eastern slopes of the Canadian Rocky Mountains, glacier cover has decreased rapidly in recent years, and total cover is now approaching the lowest experienced in the past 10 000 years.  As the glacial cover has decreased, so have the downstream flow volumes.
This finding appears to contradict projections of the Intergovernmental Panel on Climate Change that warmer temperatures will cause glacial contributions to downstream flow regimes to increase in the short term.  However, historical stream flow data indicate that this increased flow phase has already passed, and that the basins have entered a potentially long-term trend of declining flows.  The continuation of this trend would exacerbate water shortages that are already apparent across many areas of Alberta and Saskatchewan owing to drought. 


                                         Peyto Glacier
The Great Lakes basin is another region where there are significant concerns over the impact of climate change on water resources. More than 40 million people live within the basin, most of whom depend on the lakes for their water supply.  Many studies have suggested that climate change will result in lower water levels for the Great Lakes, with consequences for municipal water supplies, navigation, hydroelectric power generation, recreation and natural ecosystems.
Although summer stream flows are generally expected to decline, many researchers project a corresponding increase in winter flows. This is because warmer winters would increase the frequency of mid-winter thaws and rain-on-snow events, a trend that is already evident on the upper Saint John River. This, in turn, would increase the risk of winter flooding in many regions as a result of high flows and severe ice jams. For example, on the Grand River of southern Ontario, researchers project that warmer temperatures and increased precipitation will extend the risk of severe flooding to the months of January and February. However, since snow accumulation will likely be reduced by frequent, small melt events throughout the winter, the magnitude of spring flooding will likely decline. Similar patterns are anticipated for snowmelt-dominated rivers across much of southern Canada.
Climate change affects not only the quantity of surface water but also that of groundwater. Every region of Canada is reliant, to some degree, on groundwater. For example, the entire population of Prince Edward Island relies on groundwater for potable water, while approximately 90% of the rural population in Ontario, Manitoba and Saskatchewan depend on groundwater resources. Despite groundwater’s importance, recharge rates for groundwater across the country are virtually unknown, groundwater dynamics are poorly understood, and research on the impacts of climate change remains limited.
The depth and nature of groundwater affects its sensitivity to climate change. In general, shallow unconfined aquifers will be impacted most significantly.  This is clearly demonstrated by historic variability, in which shallow wells in many parts of Canada run dry during drought periods. In many regions, unfortunately, these shallow aquifers also contain the highest quality groundwater and are a critical source of potable water and water for livestock.  Although deeper aquifers are less sensitive to the direct impacts of climate change, the failure of shallow aquifers could encourage their exploitation.
These deep aquifers can take decades to recover from pumping, due to slow recharge rates.
Local factors, such as the permeability of the material (e.g., soil, rock) above the aquifer, and the timing of precipitation, strongly affect the rate of groundwater recharge and therefore sensitivity to climate change.  An increase in winter precipitation is expected to benefit groundwater levels more than an increase in summer precipitation.  This is because snowmelt tends to recharge groundwater, whereas summer precipitation is primarily lost through evapotranspiration.
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