Water resources should be used in ways which ensure that their suitability for economic and ecological uses, such as for crop production and as fish habitat, is maintained.
Such uses may be curtailed in cases where water is polluted, thus imposing economic costs on farmers, other economic agents, and the public. Although agriculture contributes to declining water quality in some areas, it depends on the availability of reliable supplies of good-quality water.
The link between farming and the quality of surface and groundwater has long been recognized, and is now a key environmental and health issue.
Agricultural inputs such as fertilizer, livestock manure and pesticides may cause water contamination when improperly stored, applied or disposed of. High concentrations of organic matter, phosphorus and nitrogen in surface water can lead to its eutrophication and deoxygenation, which in turn destroy aquatic habitat and produce taste, odor and aesthetic problems. Intensive agriculture in areas of high soil permeability and high water tables may cause groundwater contamination from the percolation of chemicals and nutrients through the soil profile. There is also concern that where certain bacterial or nitrate concentrations exceed drinking water guidelines in surface or groundwater, there may be negative health effects.
While agriculture has the capacity to adversely affect water quality, it can also enhance it through management practices that reduce erosion and reduce flows of agricultural contaminants into water. As agriculture requires access to reliable supplies of high quality water for irrigation, livestock watering, and processing, it is in the sector’s interest to reduce impacts on water quality.
The sustainable use of water requires that withdrawal rates do not exceed recharge or compromise other water uses. On a national level, agriculture uses approximately 9% of water withdrawn in Canada. Although agriculture returns less than 30% of the water it uses to its source, a much higher percentage is indirectly returned to the environment. This water is used mainly to grow crops, water livestock, clean farm buildings and equipment, and meet domestic needs.
Agri-environmental indicators related to water quality include the Risk of Water Contamination by Nitrogen, and Risk of Water Contamination by Phosphorus. Other indicators, such as Residual Nitrogen, Risk of Water Erosion, and Management of Farm Nutrient and Pesticide Inputs, are indirectly linked to water quality.