Water quality is affected by a combination of natural and anthropogenic factors, the relative influences of which change over the range of temporal and spatial scales investigated. Natural factors affecting water quality include precipitation intensity and amount, river discharge, geology and soil type, topography (slope length and gradient) and vegetation cover. Meybeck et al. (1989) provide a detailed review. Most of these factors can be, and have been, affected by humans; for example, changes in river discharge due to abstraction, urbanization or impounding; discharges from industry, agriculture or sewerage, etc. Many of these anthropogenic influences are part of the larger process of catchment land use or land cover change that can affect water quality in both rivers and lakes, as well as downstream estuarine and coastal waters. Investigation of the relationship between land cover and water quality is particularly useful when considering diffuse source pollution. Diffuse sources of suspended sediments, pathogens, nutrients and pesticides in agricultural land uses, and of suspended sediments, pathogens, nutrients, oxygen demanders, heavy metals, oil and road salt in urban areas, are often difficult to measure. One aim of understanding land use-water quality relationships would be to be able to use land use to estimate and understand water quality in rivers suffering from diffuse pollution. Another would be to use satellite-derived land cover data to predict water quality in unmonitored catchments. Finally, understanding the relationship between land use and water quality would improve the science and policy of land-use management. For example, it would enable a better understanding of future impacts of changing land use driven by, for example, climate change (Abler et al., 2002).
For a relationship between land use and water quality to be observed, a number of criteria have to be met: