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Drought

Drought is expected to become more frequent and severe in parts of Canada.

Drought is defined as a shortage of precipitation over an extended period, usually a season or more, resulting in insufficient water availability that adversely impacts vegetation, animals and people.

Areas of western Canada are already experiencing frequent and severe droughts. Scientists expect new areas across the country to be affected and drought to become even more frequent and severe. The consequences could have far-reaching impacts on Canada’s forests.

Read how drought and its indicators are defined

Why knowing about drought is important

Drought threatens Canada’s forests by limiting the available water that trees need to survive. When water is limited, trees become weakened. Weakened trees cannot grow at a normal rate, may not be able to regenerate, or could die. It is also difficult for trees to defend themselves against insects and diseases as they become stressed. Similarly, during wildland fires, weakened trees are at higher risk. For the Canadian forest industry, these issues directly affect the available wood supply.

Canadian Forest Service researchers have developed a measure of drought called the Climate Moisture Index (CMI). CMI is calculated as the difference between the annual amount of precipitation and the expected amount of water that evaporates each year and can be used to indicate the amount of moisture available in a given year. 

Tracking drought helps forest managers anticipate and manage for a changing climate. For example, the SeedWhere program can be used to predict where similar climates will be located under a range of future climate scenarios and timeframes. Forest managers can use this tool to select the planting stock (e.g., species and provenance) that is best adapted to predicted drought conditions.

What has changed

Several regions of Canada experienced substantial droughts between 1951 and 2010, but with significant variability between decades. However, during the first decade of the 21st century (2001–2010), exceptional droughts were observed across the country – for example, the 2001–2002 drought in the Prairies (Figures 1 and 2), caused abnormally high aspen mortality (see Tree mortality).

Similar trends have been reported in forests around the world. With droughts expected to become more frequent and severe in most of Canada’s forests, there are growing concerns about the impact of drought on forest distribution, tree health and regeneration success.

Figure 1 – Two maps, one showing the mean Climate Moisture Index drought in the aspen parkland between 1951 and 2000, and the other map showing the 2001–2002 drought in the same region.

Figure 1 – Mean Climate Moisture Index (CMI) for 1951–2000 and the 2001–2002 drought in the aspen parkland

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Graph displaying annual variability in the Climate Moisture Index in Canada’s aspen parkland between 1891 and 2010. Higher values denote wetter years, whereas lower values denote drier years.

Figure 2 – Long-term changes in the Climate Moisture Index (CMI) in the aspen parkland

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Set of five maps of Canada showing the mean annual Climate Moisture Index (CMI) for the reference period 1981 to 2010 compared to the projected mean CMI for the short term (2011–2040), medium term (2041–2070), and long term (2071–2100) using greenhouse gas scenario RCP 8.5 and again, for the long term, using climate scenario RCP 2.6.

Figure 3 – Reference period (1981–2010) and projected mean annual Climate Moisture Index (CMI) for the short- (2011–2040), medium- (2041–2070), and long-term (2071–2100) under the Representative Concentration Pathway (RCP)Footnote * 8.5 (continued emissions increases) and, for the long-term (2071–2100), under RCP 2.6 (rapid emissions reductions) for Canada

Larger image [174 Kb]

 
Graph data
Table listing the Climate Moisture Index (CMI) values for Canada’s aspen parkland for the years 1891 to 2018. Higher values denote wetter years, whereas lower values denote drier years.
Year Climate Moisture Index (CMI)
1891 -8.2
1892 -12.8
1893 -12.7
1894 -18.1
1895 -18.6
1896 -2.6
1897 -13.7
1898 -14.8
1899 0.7
1900 16.8
1901 22.4
1902 7.3
1903 -2.9
1904 7.9
1905 -15.0
1906 -3.9
1907 -1.6
1908 3.7
1909 -2.7
1910 -29.8
1911 -4.6
1912 -3.7
1913 -3.8
1914 -10.2
1915 -6.8
1916 -0.6
1917 -11.9
1918 -19.7
1919 -30.0
1920 5.5
1921 -0.1
1922 -16.9
1923 -5.8
1924 -18.3
1925 -8.5
1926 -11.2
1927 13.4
1928 -11.1
1929 -21.9
1930 -16.2
1931 -11.0
1932 -0.5
1933 -9.5
1934 -0.8
1935 -4.4
1936 -13.4
1937 -15.1
1938 -18.8
1939 -5.9
1940 -7.7
1941 -24.0
1942 4.5
1943 -2.3
1944 -0.3
1945 -15.1
1946 -12.9
1947 -14.5
1948 2.3
1949 -19.4
1950 -12.5
1951 4.9
1952 -0.9
1953 1.5
1954 4.7
1955 10.3
1956 3.7
1957 -18.1
1958 -6.2
1959 -13.9
1960 6.6
1961 -11.2
1962 -10.9
1963 -9.1
1964 -23.1
1965 13.7
1966 -7.2
1967 -7.5
1968 -12.2
1969 -3.6
1970 9.2
1971 -5.6
1972 -7.8
1973 3.0
1974 14.9
1975 -0.2
1976 -7.6
1977 -9.7
1978 1.6
1979 4.4
1980 -15.8
1981 -5.0
1982 -11.9
1983 1.2
1984 -5.0
1985 1.4
1986 5.9
1987 -7.2
1988 -10.5
1989 -6.9
1990 -1.8
1991 -6.9
1992 -12.2
1993 -3.3
1994 0.5
1995 -13.3
1996 0.8
1997 1.8
1998 -11.7
1999 2.6
2000 -0.5
2001 -15.1
2002 -35.8
2003 -5.8
2004 -1.3
2005 4.5
2006 -2.5
2007 1.3
2008 -8.9
2009 -18.1
2010 -2.7
2011 8.6
2012 -8.1
2013 5.3
2014 1.1
2015 -15.0
2016 -5.9
2017 -1.3
2018 -14.5

The outlook

Increases in drought could have far-reaching impacts on Canada’s forests, both directly, through impacts on tree growth and survival, and indirectly, through drought-related increases in the frequency of disturbances such as fire and insect outbreaks.

Drought is expected to become more frequent in several areas that are already relatively dry, such as the southern interior of British Columbia and the Prairie provinces (Figure 3).

Some areas that have not previously experienced frequent drought are also expected to become drier in the future. The current prairie conditions are expected to spread northwards into areas of the southern boreal forest. Such a shift would lead to significant changes in forest ecosystems.

Moist regions, such as the Pacific and Atlantic coastal areas, are expected to be less affected, with limited changes in annual climate moisture index (CMI) values over the next 100 years. However, these moist areas could become more prone to the impacts of seasonal droughts even if the annual CMI indices remain positive.

How drought and its indicators are defined

Drought differs from aridity, which is a defining climate feature in regions characterized by low precipitation, such as deserts.

Three indicators are used to understand drought:

  • The Climate Moisture Index (CMI) - calculated as the difference between annual precipitation and potential evapotranspiration; the potential loss of water vapour from a landscape covered by vegetation (see references). Positive CMI values indicate wet or moist conditions and show that precipitation is sufficient to sustain a closed-canopy forest. Negative CMI values indicate dry conditions that, at best, can support discontinuous parkland-type forests. The CMI is best suited to evaluating moisture conditions in dry regions, such as the Prairies.
  • The Soil Moisture Index (SMI) - calculated using daily or monthly weather records of precipitation and temperature, along with elevation and soil water-holding capacity (see references). Lower values indicate drier conditions. The SMI is designed to provide a direct measure of soil moisture changes within the rooting zone of trees. It is better suited than the CMI for representing seasonal droughts in regions with high winter precipitation, such as the Pacific coast and eastern Canada. However, as this indicator was developed quite recently, it is not as well-tested as the CMI, which was developed in 1997.
  • The Palmer Drought Severity Index (PDSI) - calculated using precipitation, temperature, moisture-holding capacity of the soil, and local infiltration (see equations). Positive and negative values represent conditions that are wetter and drier, respectively, than the long-term (≥30-year) historical mean for a given location. This indicator is most commonly used in agriculture, but can be applied in forestry.
Sources and references for drought and its indicators

Canadian Forest Service key contacts

Ted Hogg, Research Scientist, Vegetation Climate Interactions, Northern Forestry Centre
David Price, Research Scientist, Integrative Climate Change Impacts Modelling, Northern Forestry Centre

Adaptation tools and resources

Forest Change Toolkit – a list of tools and resources for climate change adaptation

Find out more
Related Canadian Forest Service research
 

 

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