December 22, 2020
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This week’s Book of the Week feature is Water for Any Farm, by Mark Shepard.
Chapter 6: The Fundamentals of Keyline Design
One of the main objectives of keyline pattern cultivation is to influence the shallow overland flow of runoff water so that it remains evenly spread across the landscape instead of following its natural path straight down the slope to the valley floor. Remember, of course, that before any human patterning, water will flow from the high spots to the low spots. Water striking the ridges will flow downhill to concentrate in the valleys. Ridges functionally receive less than the actual rainfall amount because the water flows away to the valleys, and valleys receive more than the actual rainfall amount since they receive some of the water that originally fell on the ridges but flowed down the slopes to lower elevations. This flow is not all in the form of channeled flow. In areas with light rains and on more gentle slopes, the majority of the water movement in the landscape may be by sheet flow.
Water falls evenly on the land and soaks into the top layers of soil. Gravity, constantly at work, suggests that this water move to lower elevations. The incessant pull of gravity is part of what brings water deeper into the soil. If the pore space between soil particles is large (sand and gravel), most of the rain will be pulled straight down toward the center of the earth, especially on flatter landscapes. No sheet flow occurs.
Rills and runoff streams won’t happen. With smaller pore spaces in the soil, water is not pulled toward the center of the earth fast enough to counter the effects of slope and the entire sheet of water, oftentimes just barely below the surface of the ground, is pulled downhill toward the center of the valley. Slowly, this body of water migrates away from the ridges toward the valleys.
This principal can be illustrated with a simple experiment. Take a kitchen sponge and saturate it with water. Wring it out a little so that it doesn’t drip. Next, place it on a tilted surface such as a cutting board or cookie-baking sheet with one side propped up. At first no water will trickle out of the sponge, but, eventually, gravity will begin to pull the water from the sponge. The water will begin to flow out from the lower side of the sponge and travel down the board.
The flow of water moving within the sponge itself is sheet flow. It is this shallow surface and subsurface flow that the keyline cultivation pattern is masterful at directing. When all farming activities follow the basic keyline patterning, all wheel tracks, cultivating furrows, and subsoiler lines follow the keyline cultivation pattern, all sheet flow is nudged toward the ridges. In flatter landscapes with sandier soil, in regions where the rainfall comes in smaller, gentler rains, the keyline pattern cultivation may be all that is needed to effectively keep all rainwater on the ridges long enough to allow it to soak in deeply and remain as a resource for increased crop yields.
In places where larger rain events are the norm, such as areas that receive their rain via thunderstorms of various sizes, simple keyline cultivation may not be enough to significantly influence sheet flow. In areas with heavier, clay soils, even in flat country, keyline patterning alone may also not be enough. The patterning will still have an effect, but rainwater will pile up on the soil surface faster than it can soak into the soil. The surplus water not soaked into the soil will eventually overwhelm the cultivation ridges and resume its surface flow overland directly to the valley floor. Some water still drifts toward the ridges, induced by the keyline cultivation patterning, but much will be lost as it overwhelms the system that is undersized for that particular rainfall type, soil type, and slope.
Some other circumstances that were not addressed by Yeomans at all, which are extremely important in the United States and Canada, are rainfall on frozen ground, rapid snowmelt, and rain on snow events. When these occur, subsoiler rip lines and tiny surface furrows are totally inadequate to capture all of the available water, if they’re even able to capture any. In many places (this has happened at New Forest Farm in Wisconsin several times, on multiple occasions some years!) this water may be the only water a site gets for the entire season. The loss of this precious resource is entirely preventable with the introduction of a simple yet powerful tool.
About the Author:
Mark Shepard heads Forest Agriculture Enterprises and runs New Forest Farm, an 106-acre commercial-scale perennial agricultural ecosystem that was converted from a row-crop grain farm. Trained in mechanical engineering and ecology, Mark has combined these two passions to develop equipment and processes for the cultivation, harvesting and processing of forest-derived agricultural products for human foods and biofuel production. Mark is a certified permaculture designer and teaches agroforestry and permaculture around the world.