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What Is A Swale?

Conserve water and minimize your irrigation needs by making the most of your land’s natural contours to create swales.

by Amy StrossMarch 27, 2015
PHOTO: Amy Stross

An important part of sustainable farming includes learning how to maximize use of the land and its natural resources without upsetting its natural ecosystems. Water is perhaps our most valuable natural resource as farmers, yet it is often squandered in agricultural systems. According to When the Rivers Run Dry: Water—The Defining Crisis of the Twenty-First Century (Beacon Press, 2007), conventional agriculture consumes and contaminates the bulk of our freshwater resources. Farmers with an interest in sustainability recognize water’s importance and are often the first to seek out ways to conserve water.

“The first step for any ecological farmer is to optimize the land’s relationship with water,” says Mark Shepard, author of Restoration Agriculture (Acres USA, 2013).

In order to recharge groundwater and ailing aquifers, we must put more rain in the soil than we take out. One proven technique for encouraging infiltration and absorption is the swale.

A Swale Defined

A swale is a combination of a trench dug on a contour, with a berm on the downhill side. This technique, also known as a bioswale, infiltration swale and berm ‘n’ basin, is used to intercept water and surface runoff so it can be absorbed into the soil. Swales not only catch water, but also soil, seeds and organic matter so they stay on-site rather than washing away. They work best for harvesting water on land that’s gently to moderately sloped.

Helping Conserve Water

“[Swales are one way to] practice the art of ‘waterspread,’ emphasizing the gentle harvesting, spreading, and infiltrating of water throughout a watershed rather than the rapid shedding or draining of water out of it,” says Brad Lancaster in Rainwater Harvesting for Drylands and Beyond, Vol. 1 (Rainsource Press, 2006).

In turn, the land requires less irrigation. As an added bonus, there’s less runoff.

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“Think of the implications of this,” Shepard says. “If every farm property captured and held all of its surface water, there would be no flash floods. Water that would have flooded downstream towns and villages will slowly filter down through the soil and bedrock and emerge downslope as springs. Stream and river levels would remain more stable throughout the season, and seasonal streams might become perennial.”

Creating a Swale

What Is a Swale? - Photo by Amy Stross (HobbyFarms.com)

Starting at the highpoint of a watershed, look for the “keylines” in your property, those contour lines in the landscape where the grade changes from a steeper slope to a more gradual slope. This is where sediments naturally deposit. Constructing a swale along a keyline contour will impact waterflow on the maximum number of acres with the least amount of earth shaping.

Use a leveling device to help identify the contour lines, then mark them with flags or stakes. Dig the swale trench along the contour line, 1 to 3 feet deep and 1 to 4 feet wide. Pile earth dug from the trench on the downhill side to make a raised mound or berm roughly the same size as the trench.

By creating a series of swales and berms to capture rainfall, you can help spread water evenly across a landscape so it doesn’t collect in waterways and valleys. You’ll want to place swales closer together in steep, compacted or clay soils and farther apart in flat, dry or sandy soils. If you’re digging multiple swales, keep in mind the spacing you’ll need to allow tractors and other machinery to still traverse your property. Gretchen Vaughn, owner and operator of Greensleeves Farm, a 12-acre permaculture market farm in Alexandria, Ky., spaced her swales 30 feet apart based on her equipment size.

Swales are usually planted with trees and shrubs to make the swale more stable and multifunctional. “Their deep roots will hold the berm in place, and the leaves will add humus to the soil,” says Toby Hemenway in Gaia’s Garden, (Chelsea Green Publishing Company, 2001).

In wetter climates, perennial vegetation is planted on berms for better drainage, with the trench either mulched or seeded with grass, cover crops or ground cover. In dry lands, it’s the opposite: Trees and shrubs are planted in the trenches where water concentrates.

Vaughn planted more than 400 fruit and nut trees along the berms of her swales. “On terrain like this, contour plantings with swales are the only things that make sense,” she says. “The alleys will be fine for rotational grazing, and some fruit trees have started producing already. It’s a great system!”

For more details on creating swales, pick up Brad Lancaster’s Rainwater Harvesting in Drylands and Beyond, Volume 2: Water Harvesting Earthworks (Rainsource Press, 2008). You’ll find details on marking contour lines, calculations to help determine the appropriate depth and height of your swales, how to manage the overflow, information based on soil types and rainfall estimates, variations of swales for unique situations, and more.

Use Swales for Water Catchment and Irrigation

The primary goal of a swale is to increase the soil’s storage capacity. The soil moisture in and around a swale remains long after the spring rains have gone. Because of this, garden beds below a swale will be gravity-fed and remain lush for many weeks longer than the surrounding area. The moist microclimate will require much less irrigation. Humus will build and absorb even more moisture, storing water in the ground deeper and longer than water spreading across the soil surface. Trees and shrubs planted in the swale system will be naturally irrigated.

Tips for Integrating Swales into Your Farm

On steep, overgrazed or disturbed land, water will be fast-moving during heavy rain. In these cases, a series of swales may be needed, placed at close intervals going down the slope. On gentle slopes covered in thick grass and other vegetation, the watershed can absorb more rainfall, so fewer, more widely spaced swales may be sufficient, causing less disturbance to intact land.

Brad Lancaster suggests starting at the high point in your watershed and adding one swale at a time. See how the first one performs. If the swale overflows, make it bigger or build swales closer together. If little or no water backs up behind your swale, then build the next one smaller and space it farther away. The process becomes more intuitive with time.

If an area has trees and shrubs already established, dig around them when digging the trench. It’s helpful to retain the existing vegetation for the absorption capacity of their roots.

As we seek to optimize our relationship with our land, how we manage water will be crucial, and swales can be a useful addition to a water-management strategy.

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