
This report provides supplementary guidance to perform a subwatershed stress and local area risk
assessment for lake-based surface water supply systems. In the Water Budget Guidance (MOE & MNR), the surface water assessment approach was primarily developed for water withdrawals from riverine systems, and may not properly assess the level of water quantity risk that may, or may not, be present for lake-based municipal water supply system.
The following guidance summarizes the methodologies used for assessing both subwatershed hydrologic stress and local area risk for five municipal systems in Ontario. Each of the five systems draws water from a lake source and has independently progressed through the Water Quantity Risk Assessment framework. The interpretation of the Technical Rules to define stress and water quantity risk for each of these systems varied. To assist SPCs in defining subwatershed hydrologic stress and local area risk for lake surface water intakes in the future, this guidance provides general requirements, additional technical considerations and alternative approaches for how to complete the technical assessments for lake-based surface water intakes.