Harvest & Store Rainwater with Contour Swales

Polyculture of corn, sunflower, buckwheat, and alfalfa growing on contour swale mound.

Polyculture of corn, sunflower, buckwheat, and alfalfa growing on contour swale mound.

You already know rainwater is good for your garden and may have noticed how green your plants look after a good rainfall. That’s because rainwater contains nitrogen which “washes out” of the atmosphere, providing your plants with a nutrient boost, giving them their green colour. So it makes sense to catch and store this free resource to irrigate and feed your garden, and one of the best ways to do that is a low-tech do-it-yourself method known as a contour swale.

Traditional Water Management

Traditional water management methods seek to remove rainwater from the property as quickly as possible, typically in an effort to reduce erosion or flooding. Ironically, these methods can increase these problems by removing water with impermeable surfaces that increase the rate of water flow and potential for erosion both on site and downstream. Increased rate of water runoff also overburdens waterways, disrupting their ecosystems by creating conditions unsuitable for the life that lives there.

Because of conventional water management, most of our landscapes are perpetually in drought, increasing the risk of fire and greatly reducing their resilience and capacity for regeneration and productivity.

Regenerative Water Management

Since water is such a valuable resource for creating fertility and for drought proofing, it makes sense to retain it for as long as possible, and the best method is to use water infiltration to store it in the soil. Regenerative water management principles seek to:

Slow the rate of water across the landscape, allowing it to take the longest possible route before leaving the site, maximising the value it provides for the longest time.

Spread the water over the largest area possible to benefit all parts of the landscape.

Sink the water into the soil to recharge aquifers and create underground water storage accessible during times of drought and preventing the loss of valuable nutrients due to runoff.

What is a Contour Swale?

Contour swale under construction. Trench on left and berm on downslope side (right).

Contour swale under construction. Trench on left and berm on downslope side (right).

A contour swale is a trench or ditch with a level bottom that follows the natural shape, or contours, of your property. One way to think of how a contour swale works is to consider a bathtub. When filling a bathtub, first the water level spreads out across its’ width, and then fills up evenly. This is because the bottom of the bathtub is level. 

Now consider your property. On a topographical map, contour lines represent the shape or lay of the land. If you walk along a contour, the elevation remains constant, or level. When you locate a ditch along a contour or across a slope on your property, water that flows or falls into it spreads out along the bottom of the ditch before it begins to fill up. This gives the water more time to sink into your soil, instead of just running off your property. A contour swale uses the same principle as a bathtub to catch water, spread it out across your landscape, and sink it into the soil where it will recharge the groundwater and be available as a long-term water source for your plants, thereby making it more tolerant of drought. You can even direct your roof downspouts into the swale to increase its’ catchment area!

The other part of the contour swale is an embankment, or soil mound. When creating the trench, remove the soil and pile it in a mound down slope of it. This results in a raised garden bed that is ideal for planting trees, shrubs, vegetables and herbs, or even ornamental plants. Plants preferring a well-drained soil are planted on top of the mound, with water-loving plants on the edges. The roots can grow down into the soil to access the water caught and stored by the swale. You can also fill the swale with woodchips so it doubles as a garden path.

As you can see, contour swales are a simple but very effective technique for harvesting and storing rainwater where it’s most useful, in your soil!

Contour swale trench filled with wood chip to function as a path, while helping to retain water and adding organic matter over time.

Contour swale trench filled with wood chip to function as a path, while helping to retain water and adding organic matter over time.