Water availability indicator: data sources and methods, chapter 4
Environment Canada estimated water demand for 2009 for each sub-drainage area (SDA) as the sum of municipal, industrial and agricultural water withdrawals from all flowing water.
Water supply for 2009 is calculated using streamflow data collected by the Water Survey of Canada's hydrometric stations. The water flow (cubic metres per second [m3/s]) data is extracted for the hydrometric station located at the basin outlet. Flow values from the outlet station are considered to be approximately equal to the water supply for the entire basin. Any water consumed in the basin was added to the water supply to estimate all water theoretically available for use. Adding water consumed gives the total water supply for the basin. For cases when the most-downstream station’s flow did not account for the flow in the entire basin, proportions were used to estimate water supply for the basin.
Environment Canada calculated water availability for 2009 by dividing water demand by water supply for each sub-drainage area (SDA). All SDAs are assigned one of the Organisation for Economic Co-operation and Development's (OECD) four water availability threat classificationsFootnote  based on the water availability ratio.Footnote  While not always applicable to Canadian circumstances, the OECD defines these classifications as follows:
- Low (less than 10% of available water is withdrawn): low water stress.
- Moderate (between 10% and 20% of available water is withdrawn): water availability becomes a constraint on development and investment is needed to increase water supply and reduce demand.
- Medium (between 20% and 40% of available water is withdrawn): both water supply and water demand need to be managed and conflicts among competing uses will need to be resolved. Aquatic ecosystems may require special attention to ensure they have adequate water flows.
- High (more than 40% of available water is withdrawn): severe water stress. At this level of consumption there is an increasing dependence on desalination and groundwater is being used faster than it is replenished. Water use patterns and withdrawals may not be sustainable and water scarcity can become a limiting factor to economic growth.
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