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Imperviousness has been identified as a key indicator of general watershed conditions. Numerous studies have documented declines in the health of streams and rivers with increasing levels of imperviousness. One of the advantages of imperviousness as an indicator is that it can be measured relatively simply and at low cost using land use information. The use of imperviousness as a tool in watershed protection and land use planning, however, requires that the methodology for measuring imperviousness is sufficiently accurate and reliable.

A study was carried out to contribute to the development and promotion of an sufficiently accurate method for measuring imperviousness, to be used in a consistent manner for watersheds throughout the Georgia Basin. Available methodologies were reviewed, including ground surveys, stereo-photogrammetry, air photo interpretation, and satellite image analysis. A number of methodological issues were addressed, including: the importance of using a scale appropriate for the intended use; the distinction between land use and land cover; the use of (semi)automated image analysis in land use/cover mapping; the use of imperviousness factors; and the difference between total and effective imperviousness. A number of example data sets for several small watersheds were obtained from local agencies and community groups. These data and their associated methodologies were reviewed and quantitatively compared to provide estimates of the accuracy and reliability of imperviousness estimates, and the cost, technology and skills required to carry out these methodologies.

The following factors were found to influence the accuracy of imperviousness measurements (in order of decreasing importance): selection of imperviousness factors; accuracy and scale of land use mapping; consideration of land cover in addition to land use; and watershed delineation. Selecting appropriate imperviousness factors is by far the single most important factor in determining the accuracy of the final estimates, suggesting the use of photogrammetry and/or manual analysis of high resolution orthophotos is required for a high level of accuracy.

Photogrammetric methods at 1:5,000 scale or larger provide the most accurate estimates of imperviousness, at the highest cost and highest level of skills and technology. Manual analysis of 1-meter orthophotos can be used at scales from 1:5,000 to 1:20,000. At the 1:5,000 scale, the accuracy of the imperviousness results is very good, rivalling that of photogrammetric methods, at similar cost, but moderate level of skills and technology. The availability of high-resolution digital orthophotos will keep the cost of land use mapping at 1:5,000 in the Can 2 to 3 per hectare range, using either semi-automated or manual methods. At the 1:20,000 scale, manual analysis can provide fairly accurate results, at lower cost. Satellite image analysis provides the least accurate estimates of imperviousness, at the lowest cost and fairly high level of skills and technology. While it does not provide sufficient accuracy for imperviousness measurement at the sub-watershed scale for municipal planning purposes in the Georgia Basin, it does provide a moderate cost alternative for large areas where forested and agricultural land uses are also of interest.

Recommendations were formulated for carrying out imperviousness analysis by local governments and community groups and a manual for this purpose is under development.
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Document Type: Research Article

Publication date: 2000-01-01

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