Role of Watershed Size in Quantifying Imperviousness

Imperviousness or Percent Imperviousness (PI) has been widely used in watershed management, especially in formulating storm water regulations and in simulating watershed hydrology in watershed models. Coefficient method is the widely used approach in quantifying watershed PI. In this approach, a unique imperviousness coefficient between 0 and 1 is assigned to each land use category. It is apparent that with the development of GIS technology, estimating impervious cover using the coefficient method is becoming a relatively easy task, especially for the users with limited technical capability. This paper demonstrates the importance of watershed size or scale in employing coefficient method to quantify watershed PI through an investigation of selected urban watersheds in Vermont. It also presents an important criterion in choosing the coefficient method to estimate PI as this method works well for large watersheds, but not for small watersheds. The results revealed that employing coefficient method to estimate PI for watersheds larger than 200 acres resulted in negligible error. In the mean time, employing coefficient method to watersheds with less than 50 acres in size resulted in substantial error. Although these conclusions are limited to the data presented in this study, the concept may be applicable to all data sets and geographical conditions.