Threats to Water Availability in Canada: Droughts
Threats to Water Availability in Canada: Droughts
The following chapter of Threats to Water Availability in Canada, produced by the National Water Research Institute and the Meteorological Service of Canada, outlines the current drought status, trends and variability, and knowledge gaps and program needs.
Barrie Bonsal,1 Grace Koshida,2 E.G. (Ted) O’Brien3 and Elaine Wheaton4
1 Environment Canada, National Water Research Institute, Saskatoon, SK
2 Environment Canada, Adaptation and Impacts Research Group, Toronto, ON
3 Agriculture and Agri-Food Canada, Prairie Farm Rehabilitation Administration, Regina, SK
4 Saskatchewan Research Council, Saskatoon, SK
Since most human activities and ecosystem health are dependent on reliable, adequate water supplies, droughts present a serious national threat to Canada. Large-area droughts have major impacts on a wide range of water-sensitive sectors including agriculture, industry, municipalities, recreation, and aquatic ecosystems. They often stress water supplies by depleting soil moisture reserves, reducing streamflows, lowering lake and reservoir levels, and diminishing groundwater supplies. This in turn affects several economic activities: for example, decreased agricultural production, less hydroelectric power generation, and increased marine transportation costs. In addition, droughts have major environmental implications such as reduced water quality, wetland loss, soil erosion and degradation, and ecological habitat destruction.
Droughts are complex phenomena with no standard definition. Simply stated, drought is a prolonged period of abnormally dry weather that depletes water resources for human and environmental needs (AES Drought Study Group, 1986). However, each drought is different depending on factors such as area affected, duration, intensity, antecedent conditions, and a region’s capability to adapt to water shortages. Droughts also differ from other threats (e.g., floods) since they have long durations, and lack easily identified onsets and terminations. Furthermore, their recurrence in drought-prone areas is practically certain since drought is characteristic of dry environments (Maybank et al., 1995). Droughts occur on a variety of temporal and spatial scales with their impacts dependent on timing and sequencing of dry periods. For example, a shortage of water and soil moisture at a critical time for crop growth may initiate agricultural drought, but hydropower generation would not be affected if reservoirs have adequate supplies. Climate anomalies that last from a month to years are the root of most droughts; however, human impacts on resources and climate and changing demands for water are also major contributing factors (McKay et al., 1989).
Droughts in Canada
Although most regions of Canada have experienced drought, the Canadian Prairies (and to a lesser extent, interior British Columbia) are more susceptible mainly due to their high variability of precipitation in both time and space. During the past two centuries, at least 40 long duration droughts have occurred in western Canada. In southern regions of Alberta, Saskatchewan, and Manitoba, multi-year droughts were observed in the 1890s, 1930s, and 1980s (Phillips, 1990; Wheaton, 2000). Droughts in eastern Canada are usually shorter, smaller in area, less frequent, and less intense; nonetheless, some major droughts have occurred during the 20th century. In 1963/64 in southern Ontario, for instance, several wells ran dry, necessitating shipping of water from other areas. Great Lakes’ water levels also fell to extreme lows with major losses incurred by the shipping industry (Gabriel and Kreutzwiser, 1993; Brotton, 1995). Droughts in the Atlantic Provinces occur even less frequently, but reduced occurrence results in lower adaptive capacity, making the region more susceptible to drought impacts (Nova Scotia Department of Agriculture and Fisheries, 2001). Droughts are less of a concern for northern Canada mainly due to their lower population densities; nevertheless, increased frequencies of forest fires during drought years can have serious economic impacts.
The recent 2001/02 drought was unusual in terms of its vast spatial extent. Intense dry conditions encompassed most of southern Canada extending from British Columbia, through the Prairies, into the Great Lakes-St. Lawrence region and even the Atlantic Provinces. Over much of the Prairies, several consecutive seasons of below average precipitation have led to one of the most severe prairie droughts on record, devastating many water-related resources in 2001 and 2002. In 2001, the aggregate level of the Great Lakes plunged to its lowest point in more than 30 years, with lakes Superior and Huron displaying near record lows (Mitchell, 2002). Over Atlantic Canada, three consecutive years of drought conditions have forced Nova Scotia to seek advice from the Prairie Farm Rehabilitation Administration (PFRA) on procedures to augment on-site water supplies for agricultural communities.
Droughts are the result of disruptions to an expected precipitation pattern and can be intensified by anomalously high temperatures that increase evaporation. The major factor in the onset and perpetuation of drought involves circulation patterns in the upper atmosphere. Over Canada, the most extreme warm-season droughts are associated with a persistent upper-air ridge of large amplitude over the affected area. This flow pattern creates 'blocking conditions' that displace the jet stream, cyclonic tracks, and moist air masses and fronts (Chakravarti, 1976; Dey, 1982; AES Drought Study Group, 1986). Droughts can also be initiated and/or perpetuated during the cold season when a lack of precipitation results in lower than normal spring runoff and, thus, in reduced stream flow and reservoir and soil moisture replenishment. These precipitation deficiencies are also caused by anomalous upper-atmospheric circulation patterns and, in particular, a split in the jet stream over North America (e.g., Shabbar et al., 1997).
Several studies have found relationships between sea surface temperatures (SSTs) over various regions of the globe and large-scale atmospheric patterns with associated temperature and precipitation anomalies over Canada. For example, significant relationships between El Niño – Southern Oscillation (ENSO) and winter/early spring temperature and precipitation patterns for several regions of the country have been identified (Shabbar and Khandekar, 1996; Shabbar et al., 1997). Associations between North Pacific SSTs and atmospheric ridging over the Prairies leading to more intense droughts during the growing season have also been shown (Bonsal et al., 1993; Bonsal and Lawford, 1999). However, these summer relationships are much less robust as compared to winter. Relationships between Canadian temperature and precipitation and other large-scale oscillations such as the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) are also evident during the winter season (e.g., Bonsal et al., 2001a). Droughts tend to persist in that warm, dry springs are followed by hot, dry summers. In addition, there appears to be a tendency for warm summers to follow other warm summers, and so on. Reasons for this are not clear but are likely related to feedback processes that enhance or prolong drought situations (e.g., soil moisture anomalies) (Maybank et al., 1995).
Monitoring, Modelling, and Prediction
Real-time reports of lake and reservoir levels, stream flows, snowpack accumulations, water-supply volume forecasts, dugout water levels (for the Prairies), and precipitation anomalies are currently used for drought monitoring in Canada. The status of these water supplies is critical to activities such as irrigation, water apportionment, storage, flood forecasting, hydroelectric power generation, navigation, fisheries, and wetland habitat. In the Canadian Prairies, provincial water resource agencies have been publishing monthly reports of stream, lake, reservoir, and groundwater levels since the late 1970s. Pasture conditions, on-farm surface water supplies, and seasonal precipitation accumulations are monitored by Agriculture and Agri-Food Canada (AAFC). AAFC maintains the Drought Watch web site that provides real-time information on prairie drought conditions, and promotes practices to reduce drought vulnerability. The Canada-wide drought of 2001 prompted the expansion of Drought Watch to monitor the risk and status of drought over the major agricultural regions of the country. A national map illustrating precipitation accumulations is now prepared in collaboration with the Meteorological Service of Canada (MSC).
Numerous indices that are measures of drought severity are also used for monitoring and modelling drought conditions. These range from simple approaches that only consider precipitation, to more complex indices incorporating a water balance approach using precipitation, potential evapotranspiration, antecedent soil moisture, and runoff (e.g., the Palmer Drought Severity Index [PDSI]; Palmer, 1965). Various soil moisture indices have also been used to monitor and model soil moisture changes from daily precipitation and actual evapotranspiration. A problem with these more complex indices is that evapotranspiration is difficult to compute since it relies on meteorological measurements that are generally not readily available (net radiation, vapour pressure deficit, wind speed). The high spatial variability of summer convective rainfall and the difficulties in modelling snowmelt and blowing snow also hinder regional-scale moisture modelling (Maybank et al., 1995). There are currently several meteorological and surface water indices under investigation and/or consideration for use over all of Canada. Plans are underway to incorporate these indices to monitor near real-time drought conditions across the entire country, similar to the Drought Monitor project in the United States (Svoboda et al., 2002). Satellite and radar measurements can potentially provide solutions to the spatial-scale problems associated with drought monitoring and modelling. MSC currently uses Special Sensor Microwave Imager (SSMI) to produce snow water equivalent maps for the Prairie Provinces, available to water resource agencies.
Drought prediction involves anticipating climatic anomalies that produce unusually dry conditions for an extended period of time; however, at present, there is no completely satisfactory method that can routinely predict Canadian climate over the month to season time frame required for drought analysis. Environment Canada currently produces seasonal forecasts for temperature and precipitation for lead times of 3, 6, 9, and 12 months using both statistical and numerical weather modelling techniques. The forecasts are updated quarterly at the national scale but this is often too infrequent for regional and local drought analyses.
Adaptation involves adjusting to climate change, variability, and extremes to avoid or alleviate negative impacts and benefit from opportunities (Watson et al., 2001). Drought adaptations include short- to long-term actions, programs, and policies implemented both during and in advance of drought to help reduce risks to human life, property, and productive capacity (Wilhite, 2000). Canadians have a great deal of experience in adapting to droughts; however, their adaptation strategies vary by sector and location. Areas with a greater risk of droughts are often better prepared to deal with dry conditions. Drought adaptation decisions are made at a variety of levels ranging from individuals, to groups and institutions, to local and national governments. There are various adaptation processes or strategies including sharing and/or bearing the loss, modifying drought effects, research, education, behavioural changes, and avoidance (Burton et al., 1993). Adaptive drought measures include soil and water conservation, improved irrigation, and construction of infrastructure, including wells, pipelines, dugouts and reservoirs, and exploration of groundwater supplies. The usefulness of each set of strategies varies with location, sector, and the nature and timing of the drought. Better management responses may be made with improved drought and drought impacts monitoring and advanced prediction. Adjustments that occur after drought are generally less effective than planned anticipatory adaptation.
Drought adaptation research and planning strategies are in their early stages although risk management plans for drought-prone regions of the country have been established (e.g., the Agriculture Drought Risk Management Plan for Alberta). Many adaptive strategies have been devised and tested for their effectiveness in reducing drought impacts (Maybank et al., 1995). However, intense, large-area droughts that persist for several years still result in severe hardship, even to those regions used to coping with droughts. An improved capability to estimate the numerous impacts associated with drought is required for enhanced adaptation. In addition, future national, provincial, and municipal level coordinated and proactive drought planning is needed, since vulnerability to future droughts could be exacerbated by increasing development, as well as by increased summer drying and risk of drought projected to occur over most mid-latitude continental interiors as a result of climate change (Watson et al., 2001).
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