Precipitation trends in Canada
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Jeff Fritzsche, Environment Accounts and Statistics Division
The data in this article consist of annual and seasonal time series of precipitation over a 62 year period (1948 to 2009) for eleven climatic regions as well as for Canada as a whole (Map 1). The time series are expressed in precipitation percentage departure from normal, which is the difference between the observed precipitation values and a precipitation 'normal' (the average of observed precipitation over a specified time period) divided by the normal and multiplied by 100. The period used to calculate the normal employed in this analysis is 1961 to 1990, as reported by Environment Canada in the Climate Trends and Variations Bulletin (CTVB) for Canada. 1 The departure from normal data used in this study were taken directly from the CTVB and consist of annual and seasonal precipitation percentage departures.
A continuing data collaboration
This article is the third of an ongoing series in EnviroStats showcasing data related to Canada's climate and the impacts of climate change. The focus of these articles is short statistical analyses of climate-related data, such as sea ice extent and snow cover. The second in the series was released in March, 2011 (www.statcan.gc.ca/pub/16-002-x/2011001/part-partie2-eng.htm) and examined temperature trends across Canada.
The articles are the product of an ongoing collaboration among Statistics Canada, Environment Canada and Natural Resources Canada.
The data featured in the articles will be made available through the Statistics Canada website, both in free CANSIM data tables and through new articles re-examining trends in the data every few years.
Precipitation is considered by the World Meteorological Organization-Global Climate Observing System as an Essential Climate Variable, 2 part of a group of variables related to the atmosphere. Precipitation is also one of several variables used to support the work of the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC). 3
Background and methodology
The precipitation percentage departures from normal data used in this study were taken directly from the Climate Trends and Variations Bulletin (CTVB) for Canada. 4 To compile a set of data that reflects both the national and regional variations, precipitation data from approximately 470 stations were used. These data are housed in the Adjusted and Homogenized Canadian Climate Data (AHCCD) 5 archives.
The precipitation data adjustment methods used by Environment Canada follow the steps described in Mekis and Vincent. 6 Precipitation data were evaluated for any known issues due to deficiencies or changes in observation procedures or instrumentation. Adjustments were made to rain and snow measurements separately. Corrections were made to each rain gauge type to account for wind undercatch, evaporation, and gauge specific wetting losses. Snowfall is the depth of newly fallen snow, measured using a snow ruler and converted to water equivalent with the aid of the density correction factors derived from Nipher 7 gauges. Traces of precipitation are also accounted for. Monthly total precipitation was calculated by adding the station's daily rain gauge and snow ruler observations over the month.
For each station, monthly precipitation percentage departures were computed by subtracting the monthly precipitation normal from the actual monthly total precipitation, then dividing the difference by the normal and multiplying by 100 to get the value in percent. The 1961 to 1990 normal 8 is used in the calculations. In order to obtain annual departures, monthly precipitation and monthly normals are added over the 12 months (January to December), while for seasonal departures they are added for each season defined as follows: winter (December of the previous year, January, February), spring (March, April, May), summer (June, July, August), and fall (September, October, November).
Using departures rather than actual observations makes it possible to relate all regional data to the same reference point. Since weather stations are not evenly distributed across the country, the precipitation departures are first interpolated to evenly spaced grid points using Gandin's Optimal Interpolation 9 method covering the entire country. Annual, seasonal and monthly precipitation departures are interpolated to individual grid points separately. The gridded point departure values are averaged within the geographic boundaries of each climatic region and for Canada as a whole (see Map 1).
The annual and seasonal data were tested for the presence of serial correlation and for anomalous observations (outliers). A Statistical Analysis Software (SAS) procedure, PROC ARIMA, was used to compute the overall trend. The PROC ARIMA process produces a linear trend and the associated significance level adjusted for any existing serial correlation and anomalous observations. 10 This study presents only original percent precipitation departure from normal and PROC ARIMA-produced linear trends. All of the linear trends shown are statistically significant 11 unless otherwise noted.
Analysis of the national annual precipitation time series (Chart 1) shows an increasing trend across Canada over the period 1948 to 2009 resulting in an increase of 17 percentage points. 12 Compared to the 1961 to 1990 normal, the national precipitation trend has gone from drier to wetter over the course of the study period. The annual precipitation departure trend was 8% above the normal in 2009.
Analysis of the seasonal trends (Chart 3) shows that nationally, precipitation increased overall in all four seasons compared to the normal, with the largest increases in spring (an increase of 24 percentage points for the study period), and the smallest increase in summer (increase of 13 percentage points for the study period).
Annual departure from normal
Most climatic regions (Charts 1 and 2) showed increasing precipitation over the study period compared to the normal, particularly in the northern climatic regions: Arctic Mountain and Fiords (+34 percentage points), Arctic Tundra (+36 percentage points) and Mackenzie District (+23 percentage points). Although some other climatic regions like the Northwestern Forest and South British Columbia Mountains received more precipitation compared to normal over the study period, the trend was not as pronounced. Some climatic regions did not show a positive or negative trend, such as the Pacific Coast and Prairies, indicating these areas did not experience significantly more or less precipitation compared to the normal period.
The three climatic regions that make up much of eastern Canada (Great Lakes and St. Lawrence, Northeastern Forest and Atlantic Canada) all had similar precipitation trends. The three regions show an upward trend compared to the normal over the study period.
Seasonal departure from normal
On a seasonal basis (Charts 4 and 5), two of the northern climatic regions (Arctic Mountains and Fiords and Arctic Tundra) had the largest percentage increases in precipitation from 1948 to 2009 compared to the normal. All four seasons in these areas received increasing precipitation, particularly spring (and winter in the Arctic Tundra). The Mackenzie District also received increasing precipitation in all four seasons and particularly in winter. It is important to note that the percentage departure in the north can represent a smaller absolute difference in precipitation due to lower normal precipitation amounts than the same percentage departure in the higher precipitation regions.
The regional precipitation time series trend for spring increased the most in the Arctic Mountain and Fiords (an increase of 52 percentage points), Arctic Tundra (+53 percentage points), Mackenzie District (+25 percentage points) and South British Columbia Mountains (+24 percentage points) climatic regions. Winter precipitation also increased the most in the north, rising 61 percentage points in the Arctic Tundra, 32 percentage points in the Arctic Mountains and Fiords and 31 percentage points in the Mackenzie District.
Over the study period, the Pacific Coast (-7 percentage points), and South British Columbia Mountains (-18 percentage points) climatic regions experienced decreased precipitation during winter, but data show increased precipitation during spring (+24 percentage points) and fall (+17 percentage points) in the South British Columbia Mountains and during spring (+19 percentage points) and summer (+7 percentage points) in the Pacific Coast as compared to the normal period. The results did not show a significant trend for fall precipitation in the Pacific Coast and summer precipitation in the South British Columbia Mountains.
As noted earlier, the Prairies climatic region did not show a significant trend at the annual level. Seasonally, only the spring season showed a statistically significant percentage increase in precipitation but the rate of increase was below the national average. The analysis also indicated that winters were receiving less precipitation over the study period, but the results were less statistically significant.