As our climate changes in the Cowichan Region summer rains are becoming less frequent and winter rains more prevalent. But lucky us, if we just do a little planning, we can collect rainwater for the dry times. Rainwater is a valuable and free resource that you can capture, store, and use for your garden and lawn irrigation in the summer months. What’s not to love about self-sufficiency?  

We want to challenge you to think about your water usage and assess your household need in the summer months. How can I collect and store rain to supplement my summer water needs? Don’t worry we’ll help you with this. 

How much water can I potentially collect?

Lets do a simple calculation. First you’ll need to calculate the catchment area you have access to, which is generally a roof area, whether from your home or business, or maybe just a shed. If you have a catchment area of 100 m², and in the Cowichan Region we receive approximately 1050 mm of precipitation annually, then in theory you could potentially collect at least 84,000L per year. Combined with efficient watering systems with no leaks, that’s enough water to irrigate a 10m² vegetable garden for 355 weeks! 

Here’s a calculator to make it really simple:


Table source: CMHC “Collecting and Using Rainwater at Home: A Guide for Homeowners” c. 2013

‘Adjustment for losses’ is part of the calculation because not all rainfall can be collected. Heavy rains may overflow the eavestroughs reducing rainwater capture, some rainfall will be lost through pre-filtration (gutter guards, downspout filters, etc.), evaporation and leaks, and some rainwater may be absorbed by the roofing material. A rough rule of thumb is that approximately 80 percent of the annual rainfall that falls on the catchment area can be collected if the tank is large enough and doesn’t overflow. 

 Rainwater Harvester Profiles

In the Cowichan Region there are many people collecting rainwater for their gardens. Learn what your neighbours are doing to reduce their reliance on our groundwater resources.

DeLisa Lewis - Greenfire Farm Irrigation Pond

Sandy MacPherson & Alan Philip - Underground cistern

Gordon MacKay - Rainwater cistern replaces perimeter drains

Don & Jeanne Ross - Small-scale rainwater catchment


Rainwater Harvesting Workshops - COMPLETED

To help you capture rain, the Cowichan Watershed Board is hosting two workshops on harvesting rainwater for your home and garden use. 

December 9, 10am-12noon: In this first workshop of the series, rainwater harvesting consultant and educator Ken Nentwig of RainwaterCanada will discuss how we can better manage our summer water needs as we adjust to our region's changing climate. He will review the basic components of an efficient rainwater harvesting system and how you size your system to meet your water requirements. 
This will be an interactive session with time for your questions and ideas. Now is the time to capture the rain! 

January 20, 10am–12 noon: 
The second workshop will focus on case studies and examples of a variety of rainwater harvesting systems. Ken will discuss the pros and cons of common recommendations, and applications for irrigation, flushing, fire suppression, and stormwater management.

Each workshop is $10 and pre-registration is required. Register at: 


Rainwater Resources

CMHA Guide to Collecting and Using Rainwater at Home

CVRD Water Balance Tool

The Partnership for Water Sustainability in BC

Canadian Association for Rainwater Management

American Rainwater Catchment Systems Association

Rain Gardens: Improve Stormwater Management in Your Yard

Reinventing Rainwater Management in Canada's Communities

Rainwater Harvesting for Drylands and Beyond



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