Most gardeners have learned that large percentages of residential water usage occur outdoors, mostly as a result of irrigating lawn grass and other plants, and have adopted water-saving practices: replacing thirsty lawn grass with naturally drought resistant perennial plants, especially California native plants, using efficient drip irrigation, and mulch. These strategies involve relatively low expense, depending on implementation.
Some gardeners have gone to the next stage of water conservation, which includes water catchment and grey water recycling. These strategies require equipment, and its installation, both of which could lead to some initial expenses. A 5,000-gallon water tank, for example, would be a substantial investment, but one pays off through long-term water savings or even fire protection.
Today’s column introduces percolation ponds as another stage of water conservation.
When we are fortunate enough to have rain, much of the water from roofs and paved areas runs to storm drains, which in the Monterey Bay area lead eventually to the sea. An efficient storm drain system avoids flooding, but often delivers pollutants into the ocean. A better approach is to direct the runoff to the soil, which filters the pollutants and leads the water into the aquifer.
This approach involves the development of a percolation pond, which is simply a low area that collects and holds runoff so that it percolates into the ground.
The principal objectives for percolation ponds are to filter runoff to minimize pollution, recharge local groundwater, and conserve water.
To include a percolation pond in your garden, find a naturally occurring low area (or create one) that is at least ten feet away from your home and any existing septic field. This separation is needed to avoid having water migrate towards your foundation, or to interfere with any utilities close to the house.
The percolation pond should have good drainage, so that it holds water for no more than forty-eight hours. A retention pond, by contrast, holds water for longer periods, and could be designed as a water garden or bog garden.
Determine the surface area of the percolation pond to reflect the surface area of the capture area and the soil type. For example, multiple the surface area of your roof by 20% for sandy soil, 33& for loamy soil, and 45-60% for clayey soil. If you are creating a percolation pond, the bottom layer ideally should consist of about 60% sand, 20% compost and 20% topsoil. This composition would provide effective filtering of the runoff.
Then, adjust downspouts or a sump pump outlet to direct the water into the percolation pond. Depending on the situation, a bioswale could be used to direct the runoff to the percolation pond. A bioswale is a drainage course with gently sloped sides (less than six percent) and filled with vegetation, compost and/or riprap.
In areas that receive regular rainfall, the upper layer of the percolation pond can be planted with deep-rooted perennials, which can flourish under occasional deep soaking, followed by relatively dry periods. These features are called “rain gardens.”
In California, where we need to protect and restore our aquifers, and have current drought conditions, the upper layer might emphasize decorative stones, which can slow the flow of water that might otherwise overflow, and promote percolation. California native plants, once established, would do well in a percolation pond, and also provide both an attractive appearance and environmental benefits.
Several recent books offer design ideas for rain gardens. Here are examples:
Rain Gardens: Sustainable Landscaping for a Beautiful Yard and a Healthy World (2012), by Lynn M/ Steiner and Robert W. Domm
Rain Gardens: Managing Water Sustainably in the Garden and Designed Landscape (2007), by Andy Clayden and Nigel Dunnett
Garden Guide: 20 Proven Lessons on How to Build Your Own Rain Garden (2015), by Jodty Ford