Establishing a Framework for the Spatial Identification of Effective Impervious Areas in Gauged Basins: Review and Case Study

Abstract

Effective impervious area (EIA) is defined as the subset of the total impervious area (TIA) often hydrologically connected to stream networks via stormwater infrastructure. Its importance in runoff modeling and watershed health has been well established in the literature, making it a governing characteristic of urban watersheds. However, there is a critical need to move beyond quantification of the EIA at the watershed outlet toward explicitly identifying locations of connectivity. This paper reviews existing methods to quantify and identify the EIA and proposes a new model framework that builds on these methodologies to offer an automated and objective way to spatially identify the most probable impervious areas comprising the EIA. Impervious runoff is modeled on a volumetric basis with connectivity measured by accounting for attenuation across different surface types that can be calibrated to match observed direct runoff trends. A simplified model representation is presented for three watersheds in Knoxville, Tennessee. The results illustrate the influence of pervious attenuation in urban watersheds, showing that a threefold difference in the EIA is possible in a watershed on the basis of this variable. However, patterns of the EIA and watershed sensitivity to pervious attenuation are variable among watersheds, highlighting how watershed-specific characteristics influence runoff and should be accounted for in management plans. The model results capture the spatial variation in impervious connectivity between watersheds and offer insight into the potential for disconnection using targeted infiltrative enhancements such as green infrastructure. Further research is needed to inform the pervious attenuation variable and allow it to change on the basis of site-scale characteristics such as land cover, soil type and compaction, rooftop connections to stormwater networks, and antecedent moisture conditions. Such explicit quantification of pervious attenuation will allow the application of this method to ungauged watersheds. This work is the first step toward spatially identifying impervious connectivity in watersheds using this spatial modeling framework, providing the potential for more scientifically informed watershed management strategies through targeted green-infrastructure installation.