Rainwater is a limited resource so lets learn how to use it wisely in our landscapes. Rain gardens have many benefits, including:
- increasing stormwater capture and reuse in the landscape;
- reducing pollutants that enter the storm drains, rivers and ultimately end up in the ocean;
- reducing demand on our potable (treated to drinking water standards) water supply; and
- helps to recharge groundwater.
This approach is more sustainable as it captures rainwater into mulched basins with beneficial plant materials to support ecosystems, wildlife and our water supplies. Also called a “watershed approach” to landscaping, more information can be found on the benefits of this approach here: Watershed Approach to Landscaping
Rain gardens designed into our urban landscapes must be designed properly to successfully capture rainwater and manage runoff. Design considerations include the following: setback requirements, soil infiltration rate and drainage, slope, climate, amount and duration of rainfall events, location (inland or at the coast), size, existing plant material (trees, shrubs, perennials or annuals) and irrigation.
A major consideration is the soil’s porosity (amount of available pore space), permeability (how interconnected pore spaces are), and infiltration rate (how quickly the water moves through the soil) which are critical to the success of a rain garden and its ability to absorb rain water. These soil properties affect the amount of air, moisture, and nutrients that are available in the root zone of the plants and how much runoff is absorbed into the ground (instead of running off into the storm drains and out to the ocean). Living soils that have organic material are like a sponge—they hold more water, with living mulch (not rocks) and microorganisms that add nutrients and airspace to help hold more water.
To size your rain garden, here is the basic formula for calculating the amount of water you can collect:
- Determine size of catchment area, for example a roof area from one downspout, Area = Length x Width
- Multiply Area x Rainfall (in inches) x .62*(converts inches to gallons) = Gallons of Rain Water collected
- EXAMPLE for 1,000 square foot Area: Area of 1,000 (sq. ft.) x 1” (rainfall) x .62* = 620 gallons
To calculate the size of your rain garden catchment area:
- Gallons of Water ÷ 7.48** (converts gallons to cubic ft)= Cubic Feet of Rain Garden (square feet of area 12” deep)
- EXAMPLE (using 1″ of rain on 1,000 sq ft captures 620 Gallons):
620 Gallons ÷ 7.48 = 83 Cubic Ft. area (at 12” deep) OR
620 Gallons ÷ 7.48 = 166 Sq. Ft. area (at 6” deep)
- Another way to calculate is if you have a 10′ x 10′ area of 100 square feet , 83 cubic feet /100 square feet = .83 feet deep which is 10″ deep (.83 feet x 12″ = 9.96″ rounded up to) 10″ deep.
*.62 is a conversion factor that converts the inches of rainfall per acre to gallons per square foot/year. Rainfall of 1″ over one acre of grounds captures 27,143 gallons of water (27,143 gallons/43,560 sq ft/acre=.623 gallons/sq foot)
**7.48 is the conversion factor to convert gallons to cubic feet, i.e. there are 7.48 gallons in one cubic foot.
= 7.48 gallons
This information was adapted from G3-Green Gardens Group training classes. More info here: G3-Green Gardens Group Simple Rain Garden Recipe