Specific land use and land cover types are associated with human activities and physical characteristics that affect water quality. For example, land use defined as "cropland" could be associated with the application of pesticides and fertilizers, which might result in the movement of these materials into surface water or groundwater. Land use defined as "industrial" could be associated with the use or disposal of hazardous chemicals, which also may also pose a risk to streams or aquifers. Since most water quality problems have a land use origin, the understanding of land use practices will assist in addressing water quality concerns.
What is Water Quality?
It is often difficult to define water quality because it depends not only on the concentration of pollutants in the water but also on its intended use. Various parameters are used to measure water quality to provide an indication as to its useability for an identified use.
Various land uses can potentially introduce many pollutants into aquatic ecosystem. The U.S. has spent over $300 billion since 1970 on controlling pollution, only to find that many rivers and other water bodies are still heavily polluted by materials not emanating from these sources. Rather, they come from nonpoint sources (NPS), from activities such as agriculture, urban storm runoff, forest practices, lawn fertilization, construction activities and combined sewer overflows. Current data indicates that water quality management efforts should be mainly focused on controlling nonpoint sources of pollution, which may be best achieved through land use controls. (Rogers 1994).
The
effects of land use and land cover on the hydrologic cycle, on soil and
on water quality may either be direct or indirect. Direct effects might
involve a land use that increases soil erosion and subsequently degrades
water quality. Indirect effects may be the result of land use activities
or land cover characteristics on climate and the altered climate’s subsequent
impact upon water.
All precipitation that falls on land interacts with soil. Water either runs off the soil surface or infiltrates the soil where it is either taken up by plants, returned to the air via evaporation or transpiration, or percolated to a depth not reached by plant roots and enters the groundwater. The amount of water absorbed by the soil and the rate of infiltration is dependent on the soil's porosity and permeability. The amount of water retained or released is dependent on land use and land cover; therefore, the effects of land use on soil must be considered.
The distribution of land cover can result in less available water for groundwater recharge, while the base flow of perennial rivers could be affected by a reduction of groundwater discharge to the river. The effects of land cover on hydrology and water quality occur at the hydrologic unit, not only in the form of pollution, but also in change in downstream flows. Four major direct impacts of land cover upon the hydrologic cycle identified by Rogers 1994 are: flood and flood potential; drought; changes in river and groundwater regimes; and water quality.
Just as specific land use types associated with certain human activities affect water quality, land cover types associated with ecological processes also influence water quality. In combination, the effects on water quality are numerous and of critical importance in watershed management.
(adapted from Jim McEvoy, Wisconsin Department of Natural Resources 1984)
While land use and land cover activities may pollute surface waters and/or groundwater, land use management practices can minimize or reduce potential negative effects. The following land use types differ in their impact on water quality. Their effects on surface water and groundwater will be addressed. Additionally, best management practices which alleviate or reduce the negative effects will be presented.
Agricultural
Forest
Urban
Transportation
