For water management, agroforestry and silvopasture, swales can be an important tool for the biodiverse farm.
In agriculture, swales are not usually the topic in farming circles but they can be a valuable tool in the earthworks tool chest. A ditch-berm combination, often confused with terracing, swales can be both part of a water management system, as well as a tree growing system.
This article will outline the construction of three on-contour swales on a sloping pasture at Hebron Acres in Clay County, Missouri. Though each application of swales is different from property to property, the effort at Hebron Acres can provide learnings for other property owners.
No escaping gravity
As gravity pulls precipitation downhill through the landscape, a smart strategy is to slow that movement and hold the water on the land as long as possible to make it available for soil and to hydrate growing things. Gravity cannot be totally escaped, but it can be used to the farmer’s advantage instead of a daunting foe.
Similar to P.A. Yeoman’s keyline subsoil plowing, swales arrest the downhill flow of water allowing it first to be captured in the ditch and then percolate into the adjacent berm and also down into the top soil deeper than if simply allowed to flow through the topsoil normally. These earthworks can be scaled to nearly any size and can be built by many means.
Right for every situation?
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Swale on contour holding water after a spring rain.[/caption]
One does not wake up one day and decide they need swales. As one form of earthworks, swales are a design element to be planned and integrated within the larger, holistic farm design. If a farm plan calls for water capture and management and increasing fertility and biodiversity, these landscape elements may be relevant.
As a farmer assesses the landscape and understands the layout and topography, they need to strongly grasp the way water moves through, on and under their property. If swales are determined to be a beneficial as part of an overall plan, the next step is to determine number, size and placement, as well as the interconnection with other farm design elements. Connecting swales to ponds and wells is one common example of integration. Another is to move water from one area to another, such as from the high valley out to the lower ridges of the rolling landscape.
Once placement of a swale is determined, a level-seeking device will be needed. This can be an A-frame level, a bunyip water level, or the sophisticated laser level. At Hebron Acres, a bunyip water level was used. These tools help determine a contour line which will be the linear guide for building a swale.
Pegging the contour line
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Survey pegs showing the contour line of the future swale.[/caption]
Once it was determined where the first swale was needed, we placed one stake of the bunyip level at the very far end of where the swale would begin. The level was stretched out to its length, which is approximately 20 feet. The second bunyip stake was then set down near where the next level spot would be. We take a reading on the calibrated stakes to see where the water levels are. We move the second stake up or down hill until we get the water in the bunyip water tube to read exactly the same on both stakes. Then, we pound in a survey peg or flag at both points on the ground.
Now we have two points that are on what we call contour. These two points are on the same level of elevation. We simply swing the first stake around and stretch the level out again from the second stake and find the next contour point and insert a survey peg. With this process and length of bunyip level, we can cover the ground pretty quickly at 20 feet at a time. Most A-frame levels are about five feet apart, though they can be operated by one person instead of two with a bunyip level.
We continue finding contour points for the entire distance of what we plan for the length of the swale. When done placing survey stakes, we can look back over the distance and see the pegs form an imaginary contour line. Staying on contour, or level, is important so your swales do not pool up or allow water to run out one of the ends of the swale ditch. Keep the bunyip level handy because it will be your measurement tool when you begin to excavate earth to a consistent depth for the length of the swale.
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Using a two-bottom plow behind a 1948 Ford 8N tractor to cut the swales and move the backfill down hill to form a berm.[/caption]
Now, the real labor comes. You will either dig the ditch by hand or by mechanized power. Your budget, schedule, and ability will factor in how you dig the ditch. But, your first decision is to determine how deep and wide you want your swale ditch to be and how tall you want the berm to be. At this point there are several schools of thought about swale dimensions. At least take into account what your average annual rainfall is, what an average rainfall event is, what your goals for the water are, how many swales in the same flow pattern there will be and how you will handle overflow.
At Hebron Acres, we used a two-bottom plow pulled by a 1948 Ford 8N tractor. That approach turned out to be just as effective as an excavator (back hoe). We hitched up our plow with the left moldboard in perfect line with the contour line represented by the survey pegs. From there, we left the pegs in place and simply pulled the plow following the contour line exactly cutting a clean cleft on the uphill side and moving the soil downhill. We made several passes with the plow pushing more and more soil downhill into a berm on the downhill side of the forming ditch. To clean up the loose soil in the ditch, we switched out the plow for a drag blade which we swung to a trailing angle so it would scrape out the loose soil where the plow was digging and push it out on top of the berm.
Mulch to protect
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Mulching the swale to protect it from erosion before being planted with cover.[/caption]
Though we did have to do some hand work with a spade and rake to totally clean out the ditch and pile the earth up uniformly on the berm, the swales were formed very well with this technique. Since we were building the first swale in the fall and did not have our trees and understory plants yet, we covered the berm in a thick layer of hay mulch to keep the newly piled soil from erosion. We have rains in the winter in northwest Missouri, so it was important to protect the berm and not overexpose the soil to direct sunlight.
This first swale measured about 160 feet in length with a ditch two feet wide and about 18 inches deep. The berm itself was about two feet high, so there is about a 42 inch difference between the bottom of the swale and the top of the berm. This could cause pretty good pressure on the swale should it fill with water quickly in a heavy spring thunderstorm. To alleviate this pressure, both ends of the swale are designed to allow for overflow into the pasture.
Understory and overstory
Planning at Hebron Acres calls for use of polycultures and the swales are an example planted with a diversity of fruit and nut trees. The trees were planted on the uphill side of the ditch, on the berm and on the downhill side of the berm depending on their soil hydration preferences.
The first planting featured five groupings of hazelnut, pecan, false indigo, elderberry and persimmon. Since these were just seedlings, we are waiting until later to plant understory plants such as comfrey and appropriate guild plants.
Another consideration is avoiding frost pockets. Air moves the same way water does but is also affected by its temperature. Cold air wants to flow downhill. Keeping this in mind means you should not plant your swales without an outlet to let cold air continue to flow downhill and off the property. If a swale is planted so densely without any gap in its vegetation, there is a good chance of creating a frost pocket that will never seem to warm up and could damage plants or cause them to go dormant too early. The gap in the swales at Hebron Acres is planned to let the cold air out through each successive swale through the natural downhill flow zone.
Another good technique employed in the swales at Hebron Acres is the placement of organic matter in the swale ditch. In this case there was a layer of brush and old hay placed in the ditch to allow it to decompose adding organic matter and allowing the nutrients to leach out and be available in the hydration plume downhill from the swale.
An alternative to placing dead plant material in the ditch is to choose some mixture of plants to grow within the ditch. Some of these can be planted as a mulch and some can be planted for other beneficial reasons. For example, daikon can be planted in the ditch to provide deep natural tillage. The daikon can be harvested or left to rot and decompose. Either way, the long tapered root of daikon provides a channel for air and water to penetrate deep into the soil, as much as 18-24 inches.
Silvopasture and intercropping
Originally, the distance between swales at Hebron Acres were planned to allow for an even number of passes with equipment for mowing. As plans evolved, the space between the swales will be either grazing paddocks or for forage growth.
Between the upper two swales a plot of sericea lespedeza was planted among the pasture mix. This is intended as forage for goats as sericea lespedeza contains a natural de-wormer.
Between other swales electric netting will allow goats and laying hens to free range on the forage while keeping them off the young growing fruit and nut trees.
Monitor and assess
Finally, once swales are built and the growing systems are established it is a good practice to monitor and assess the structure each time when working around the trees and understory. At Hebron Acres, the ditch of the swale serves as a walkway to harvest from the adjacent trees and vines. It also serves as a good place to closely examine the berm for any signs of weakness or structural failure.
At Hebron Acres, the berms are kept covered in grass and forbs as a sort of early succession growth phase and were chopped and dropped to provide a mulch to decompose on the berm and in the ditch. The soil of the berm and directly downhill should begin to show signs of improvement even after one year if the location of the swale was where the land was being abused before.
One significant improvement for Hebron Acres was that even after a heavy rain, the bog that usually appeared at the bottom of the slope did not show up anymore. This was a sign that the swales were doing their job holding water up on the landscape instead of letting it flow downhill and eventually off the farm where it was lost.]]>