The US Environmental Protection Agency has published a Stormwater Calculator that runs on a desktop computer and provides useful advice on management of water that runs off from buildings and paved areas. The calculator is intended for owners of industrial, institutional and commercial properties and for developers of all types of property. It can even work for residential property though it is unlikely to be quite so useful to a homeowner with a small urban lot.

Unfortunately the National Stormwater Calculator only accesses US data, though a quick review by GallonDaily suggests that the calculator might still be useful to Canadian property owners if site specific data on

• Soil Type
• Soil Drainage
• Topography
• Precipitation
• Evaporation
• Land Cover
• Low Impact Development (landscaping designed to capture and retain stormwater) Controls
• Runoff

are available. In many areas of the US these some of these data can be imported automatically from government databases.

The Calculator allows the user to make changes to various practices, such as

• Disconnection of roof drains from storm drains
• Rainwater harvesting (retention for use in such applications as toilet flushing and landscaping irrigation)
• Rain gardens
• Green roofs
• Street planters
• Infiltration basins
• Porous pavement

and to immediately see the impact that one or more changes would have on off-site stormwater flows.

With increasing concerns about the environmental and pollution effects of stormwater on streams, lakes and oceans, the stormwater calculator is an excellent approach to encouraging landscapers to find better ways to reduce off-site impacts of stormwater. Maybe Environment Canada could look at licensing the software for use here with appropriate data to be made available to Canadian users.

The National Stormwater Calculator, user guides, and more description of the system are available at no cost from http://www.epa.gov/nrmrl/wswrd/wq/models/swc/