Ambika Paudel

As urban development continues in large cities across North America, existing tree and impervious cover is significantly altered. These land cover changes affect stream flow and water quality, and such modifications to landscape can lead to increased runoff, flooding causing infrastructure and property damage, as well as degradation of water quality. i-Tree Hydro, a model built by the US Forest Service to quantify the impact of city trees and impervious surface on urban stream characteristics, has been used in several Canadian studies. Based in the Greater Toronto Area (GTA), these investigations lack a detailed scenario-based analysis of the impact of plausible land cover changes on both quality and quantity of urban water. Using i-Tree Hydro, this study investigates the impacts of tree and impervious cover alteration on stream flow characteristics and water quality in the Highland Creek Watershed, the most urbanized watershed in the GTA. Results show an inverse relationship between tree cover and runoff volume. An average reduction in total runoff of 0.2% was found when tree canopy was increased by 1%. Findings suggest that pervious land cover and proportion of impervious surface that connect directly to the stream (e.g. storm sewers) are highly influential land cover types on surface runoff generation and pollution loading in urban watersheds. The Base Case model scenario developed in this study can be adapted as an initial planning tool to explore urban forest and development practices that minimize storm runoff and mitigate pollution loading in local streams.

Key words: i-Tree Hydro, Highland Creeks Watershed, Tree Canopy, Impervious Surface, Urban Forest Management