Limiting imperviousness to maintain ecological quality: Are threshold-based policies a good idea?

摘要

The effects of urbanization on flows and water quality within a watershed havebeen documented for some time. Further, many models have been constructed that aresensitive to land use and land use change as predictors to make estimates of the impactsof urbanization. Impervious cover is probably the most basic quantity that can be used toquantify the degree and extent of urbanization across a landscape. Despite the apparentsimplicity of recognizing imperviousness from the ground, measuring this quantity withaccuracy from commonly available GIS-based products is not trivial. The situation isfurther complicated if one takes the perspective of the hydrologist. What imperviousnessvalue should be used in a given analysis method? Is all imperviousness created equally?Here we investigate the several methods for characterizing imperviousness fromseveral perspectives: remote sensor measurement, inference from land use/land cover,and "direct measurement". Our findings indicate that although different methods formeasuring imperviousness are highly correlated, they can differ dramatically in theirmagnitude and there may exist local or systematic biases in the reported values associatedwith any one method. Therefore, if imperviousness values of 10% or greater are aconcern, they are sensitive to the method used.As a final step, we illustrate how different measures of imperviousness canintroduce error into a common hydrologic model – a regression model for peak flowestimation that uses imperviousness as a predictor. Peak discharge estimates generatedfrom one measure of imperviousness averaged only 68.7% of those from a differentmeasure. The implication is that it is imperative that methods for calculating andreporting imperviousness be standardized.