Numerical modeling is increasingly used as a tool for environmental assessment and planning, including for Drinking Water Source Protection in Ontario as outlined in the Government of Ontario's 2006 Clean Water Act. However, modeling procedures are often inadequate in the organizational context and tight budgets. It remains a challenge to adapt these procedures such that they are transparent and efficient for watershed practitioners. This paper reviews and analyzes the application of the event-based approach, as defined in the technical rules to the ‘Clean Water Act’. Four limitations are then identified in a case study using the common procedure. Addressing these requires iterative model refinements, which likely result in cost overruns and undermine sound budget planning. An alternative method is then suggested, demonstrated and validated, which uses numerical modeling for creating a generic diagram. Such a generic diagram offers a transparent decision-making tool for planners, other non-technical employes and land owners.

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