Monthly variations in drinking water production, 2005 to 2007

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Cindy De Cuypere, Terence Nelligan, Mark Henry and François Soulard, Environment Accounts and Statistics Division

Potable water of sufficient quality and in adequate quantities is fundamental to human health and the economy. Of the total 42,058 million cubic metres (Mm³) of water withdrawn from the environment for household and economic activities in Canada in 2005, about 14% was treated ¹  by drinking water plants. ^2 , 3  Drinking water plants are defined as facilities that abstract raw water from the environment and produce potable water for consumption. They range from simple systems that provide minimal or no treatment to large facilities with complex treatment processes.

For the first time, Statistics Canada has collected national data on drinking water production volumes. This article will examine temporal and geographic variations in these data. It will also show how they can be analyzed together with environmental measures to highlight important supply and demand issues in a region.

The results indicate that nationally, the average daily volumes of water treated in the lowest production month of December and the highest production month of July varied by a range of 32% when compared to the annual daily averages for each of 2005, 2006 and 2007. The annual production remained stable over the three years. Depending on the drainage region, the variation from low to peak production months ranged from about one-half to over five times the Canadian average.

National production volumes ranged from 396 Mm³ in February to 576 Mm³ in July

Chart 1 plots monthly treated water production volumes for Canada and selected drainage regions, while Chart 2 plots the average daily treated water volumes per month for the same areas. ⁴  On average for 2005, 2006 and 2007, total monthly treated water volumes for Canada ranged from 396 Mm³ in February (a short month) to 576 Mm³ in July (Chart 1). Using average daily volumes (Chart 2), the lowest production occurred in December (13.7 Mm³ per day) and peak production occurred in July (18.6 Mm³ per day). The annual daily average was 15.4 Mm³ per day.

While volumes were relatively stable from November to April, the rise and fall between April and November can be attributed to a wide variety of seasonal uses, including lawn and garden watering, agricultural irrigation and livestock watering, tourism, car washing and outdoor swimming pools, among others. Total annual production was stable, with an average of 5,628 Mm³ treated per year. ⁵  In general, the variation in production from year to year was far less than the annual seasonal variation.

Using Statistics Canada's standard drainage regions ⁶  (Map 1), the Great Lakes and St. Lawrence regions produced the most treated water (Charts 1 and 2). Monthly volumes for the Great Lakes and St. Lawrence drainage regions were similar, representing 30% and 27% respectively of total Canadian production. The Great Lakes drainage region had roughly one third of the total population served by drinking water plants in 2007, while the St. Lawrence drainage region had one fifth, indicating greater water use per capita ⁷  in the latter (Table 2). However, not all drinking water is used by residents. On average in Canada in 2006, 43% of treated water use was commercial, institutional, industrial, or system losses. ⁸ 

Treated water production varied widely from low to peak months, depending on the region

To examine the seasonal patterns in water use on a standardized basis by drainage region, the differences between the average daily treated water volumes per month (as shown in Chart 2) and the average daily treated water volumes per year were plotted in Chart 3. Over the three years, national average daily production in December was 11% less than the annual daily average, while in July it was 21% greater.

This seasonal variation is important to drinking water plant operators, who strive to have both an adequate supply of source water and sufficient plant operating capacity to meet the demand during peak periods. Otherwise, they may have to seek other supplies, build new capacity, or manage the demand by implementing conservation measures or restrictions on use. In this analysis, production is used as a reasonable indicator of demand.

The Great Lakes drainage region had the largest production volumes of all the drainage regions (Charts 1 and 2) and seasonal variations very similar to Canada's (Chart 3). The Okanagan–Similkameen drainage region had low production volumes (Charts 1 and 2) but the greatest variation (Chart 3). Production ranged from 59% less than the annual daily average in winter to 118% more than the annual daily average in summer. This range represents an average seasonal variation in demand of 177% during the period from 2005 to 2007. The Newfoundland–Labrador drainage region had the least variation, ranging from 6% less than the annual average to 8% more, a 14% seasonal variation in demand. The combined Pacific Coastal and Yukon drainage regions had the third largest production volume (Charts 1 and 2) and are a good example of moderate variation.

Map 2 summarizes the variation as shown in Chart 3 for all the drainage regions. It shows that drainage regions in the interior south of British Columbia and at the southern limits of Alberta and Saskatchewan experienced the greatest seasonal variation in demand. Drainage regions in the Atlantic provinces experienced the least variation in demand.

The seasonal variation in any given area is related to the demand for seasonal water uses such as residential and agricultural irrigation. Precipitation received during high demand periods would offset some of that demand. Areas with a combination of intensive seasonal use and low precipitation are those where demand is most likely to exert significant pressure on supply during the summer season. Environment Canada reports that in 2004, 72 of 510 responding municipalities stated that they experienced water shortages. ⁹  Seasonal water shortages occur in areas of British Columbia, Alberta and Saskatchewan, southern Ontario, as well as in municipalities in Atlantic Canada. ¹⁰  Analysis of monthly frequency data within a region can highlight such supply and demand issues.

Water supply and demand in the Okanagan–Similkameen drainage region

This drainage region was selected for a case study because it experiences the greatest seasonal variation in drinking water demand (177%). The region includes both the Okanagan and Similkameen basins and is characterized by hot, dry summers and areas considered to be semi-arid and prone to drought. It has a number of growing urban centres, extensive tourism, farms, orchards, and vineyards that are irrigated and is subject to water shortages and restrictions. ^{11 , 12 , 13}  On average, over one third (36%) of the total annual treated water volume was used in the hotter and drier months of July and August. ¹⁴  In 2007, 84% of its drinking water was drawn from surface water sources and 16% was drawn from groundwater sources. ¹⁵ 

In the Okanagan basin portion of the drainage region, during the irrigation season residents use nearly 600% more treated water outdoors for landscaping than they do indoors. ¹⁶  In comparison, in the Regional Municipality of York in the Great Lakes drainage region, which has a greater population density and a wetter climate, residents use about 27% of their treated water outdoors during the irrigation season. ¹⁷  Within the Okanagan–Similkameen drainage region, the City of Penticton attributes a large portion of the high water demand during warm months to lawn watering, since demand decreases significantly on rainy days. The highest water users there are residents with irrigation systems. ¹⁸  Many of the communities in the region have implemented watering restrictions. Some treated water is also used for agriculture—for example, 5% of the water provided by the City of Kelowna. ¹⁹ 

Chart 4 illustrates standardized monthly variations in drinking water production and water yield ²⁰  in the Okanagan–Similkameen drainage region. Water yield represents the renewable freshwater that is produced in an area for a given period of time. It is a good measure for comparing against withdrawals to better understand what pressures may exist on our freshwater resources. Like Chart 3, Chart 4 plots the differences between the average daily values per month and the average daily values per year. While treated water demand peaks in July and August, the water yield peaks at 351% of its annual daily average in May, two to three months before.

In August, only 2.5% of the total annual water yield (about 96 Mm³) is produced, while drinking water plants withdraw for production 18% of their annual total (about 19 Mm³). ^21 , 22  Treated water demand thus accounts for one fifth of the water yield in August.

This disparity between peak water supply and peak demand is an acknowledged concern, especially for fruit growers and residential users in the Okanagan valley portion of the drainage region. ²³  As the Canadian population and economy grow, and climate change shifts the patterns of temperature, precipitation and extreme weather events, ²⁴  analyzing monthly data on a regional basis for all water use sectors would help to evaluate areas of potential stress.