THEORY & PRINCIPALS
Water Harvesting Ditch on Contour
A swale is a water harvesting ditch on contour with a mounded, non-compacted berm on the downhill side. Swales, within the context of Permaculture, are meant to be tree growing and forest establishment systems, where runoff water (mostly from major rain events) is retained and infiltrated into the landscape for productive means.
Installing earthworks like swales into the landscape before planting means that the system will be self moderating over time. There will be no need to irrigate or fertilize as the system matures. Properly designed swales can rehydrate the landscape, replenish the aquifers and can recreate or initiate new springs.
Following the principals of permaculture, designers will ensure the system is established in a way that allows for the elements of that system to thrive without much input. If we did not harvest rain water, ensure access was appropriate and install earthworks, our design would be heavily reliant upon our labor & external inputs.
Stop it, Spread it, Soak it
The Anatomy of a Swale
The great absolute law of water is that it flows to seek its level. This translates into the landscape with water moving from its source (origin, cloud, pond) to a sink (entropy, valley, ocean). The sea denies no river!
As water moves down the slope (from ridges to valleys) it can be passively captured in a level ditch where its flow is impeded on contour. Swales are not used to flow water; earthworks & swales are used to reduce pressure on the land, passively harvesting energy and ultimately eliminate erosion.
Contour is a level elevation across a landscape. A level elevation means that each point marked across the landscape is the identical height from sea-level or a center point. Swales run perpendicular to the slope of the line along a contour line.
The mounded berm of the swale is not compacted and thus does not work to retain the water because it is above the line of retention. This is where we generally plant and sow seeds.
A sill is a overflow point on the berm which is compacted. It is lower than the berm and slightly above the line of retention, which is defined by the ditch. It is designed into the swale to accommodate for overflow and deter a washing out of the swale’s berm.
A level trench spreads the water across the level ditch as the water seeks to find its level. The water will soak into the landscape at a rate determined by the infiltration rate of the soil type (sand, clay, loam). A swale will only fill when a heavy rainfall exceeds the soil’s ability to soak it in.
As the water soaks into the landscape, a sponging effect begins to hydrate the entire landscape down slope from the harvesting point. Over the course of the following years, as the water infiltrates the landscape, it will hit an impermeable layer that that water cannot infiltrate and thus the entire landscape becomes hydrated. The roots of trees will bring the water up and distribute it through the entire system. This creates resiliency in the system. Even in years of drought the landscape will remain moist. Waiting for this sponge effect to occur, irrigation for new plantings may be required, especially in drier landscapes.
Water is the bringer of life. The longer water is held within the landscape, the more life within it. Swales are tree growing systems, where the roots of the trees are meant to accommodate the great volume of water captured by the trench. Trees stabilize and hold the berm in place. Otherwise, the excess water collected in the trench has a high energy potential and could wash away an unstablized berm.
Why use Swales?
Swales add a texture to the landscape and can be utilized to hold water in the landscape for a longer period of time. Water runs off a site if it is not impeded or harvested and can take topsoil & nutrient along with it. Yet, the problem is the solution! By harvesting this water, we can hold it on the site longer & utilize it for productive means before it ends up in the valley & the aquifer.
Placing a swale as high in the landscape as possible ensures all water that enters the site is passively harvested and can be cycled via gravity through the landscape many times before leaving. Cycling the water into cisterns, ponds, other swales or earthworks gives the highest yield with little energy input. The harvesting effect generally raises the water table and restores the aquifer's capacity; allowing us to responsibly use water from the aquaifer. We can take when we give.
With a swale, we capture the water right where it is needed, in the root zone of our tree plantings. This means we irrigate less, we build compost & humus right in the swale’s trench and we create microclimates for species preferring flooding or wet feet and, at the same time, for those preferring drier soils. By designing with the contour of the land, we are recognizing the flow of energy within a system and we can intentionally intersect the flow to passively & gently hold the energy in the landscape for a longer period of time.
Swales can be used on landscapes with very gradual slopes as well. Although the land may not have a slope visible to the eye, if you use a transit, you are bound to find that the land is not completely flat.
"But, my soil is sandy, my water infiltrates quickly and my land is flat!"
There are always considerations when choosing your design patterns for your particular landscape. Swales may or may not be the best fit your Florida landscape, but it may be a technique to incorporate. Don't build swales, just to have swales. Here are some factors to consider:
Yes, Florida with its sandy soils means any rainfall is in the soil within minutes. However, within improperly managed landscapes there is more erosion, taking with it all of our topsoil and organic matter. This all can be prevented & harvested within a swale. Where wind is blowing topsoil and organic matter, swales can capture debris & spread fertility.
Yes, swales may not impact your design greatly in minor rain events, but when major rain events occur, this is where the swales are the most productive. There will be runoff during rainy season rains. The runoff rate depends on how long the dry season has been, vegetation, mulch, other capture points, etc.
No, your land is not flat. It may be non-conducive given a series of other considerations and landscape factors, but there is always a tendency for the landscape to grade one way, it just may appear to the eye that it is flat.
What do we need more off in sandy soils? Organic matter. If nothing else, the ditch when filled with woody material becomes a carbon sink, providing a resource of slow release water retention and rich compost within a year or two.
Considerations when using swales
Many sugar sand sites here in Florida may seem to not be conducive soil structure for swales. In soils of sugar sand, water can infiltrate quickly. Without proper mulching or mulch ditches, the sandy soil causes any water harvested to be infiltrated too quickly. A lack of slope combined with this sandy soil makes the work of installing a swale design less efficient in the long run. With that being said, it depends !! Because of high infiltration rates, soil erosion from runoff is the highest. If designed properly, swales will still, during major rainfall events, capture runoff water. Swales are means of erosion control and can harvest runoff (water & fertility) from wind and wash outs and spread it passively along the contour.
Sites with a high water table may not be suitable for swales as this could encourage flooding. Fruit trees & vegetables don’t like wet feet. Since swales raise the water table over time, this may encourage too much water on the site for productive yields.
Slopes of 18-20 degrees or greater is not only unsafe for machinery, the volume of earth moved versus the volume of the swale’s holding capacity makes construction inefficient. The yield does not outweigh the maintenance or construction.
If earth moving is desired & appropriate but the landscape’s contour is not to the advantage of swale design, consider using hugelkultur beds off contour to retain moisture and build soil. Ripping into the soil through keyline design in a broad scale setting may be a better than creating permanent swale beds. Research P.A. Yeoman keyline design systems.
Observations when Designing for Water Harvesting Systems
- Identify any water elements already on the site like springs, creeks, ponds, wetlands, wells or cisterns
- Identify seasonal flows or pooling water. This can really only be done through empirical evidence by having experience with the site over the course of a year.
- Identify sun, wind, flood, access, frost sectors and how they interact with the site
- Identify drainage patterns, wasted water and runoff
- Determine water need for particular species, crop systems
- Determine annual rainfall and rainy season months
- Determine quality of water runoff and pollution vectors
- Determine water holding capacity & water catchment requirements
- Use common sense when siting near a building, property line, utilities, etc. Don’t harvest water where pooling may be a detriment to the system.
- Stay away from drain field when possible. Septic tanks are gross but currently a reality for many of us. Harvesting water off a leech field can cause pollutants to enter a food system
- Infiltration rate should be at least 1”/hour [Hint: we should all be fine in Florida unless you are harvesting in a clay mine]
- Swales may not be appropriate for areas with high water table or extremely steep slopes [See section: In opposition to swales]
- Consider access, structure site and windbreaks before situating swales. Its a lot more difficult to move a swale than to properly design one.
Zoning & Swales
- Zones are planned patterns of how we use & interact with our systems and natural systems. Zones can be reliant upon slope, grade, soil structure & vegetative cover. For example, having a Zone 1 kitchen garden on a steep slope is not conducive for our energy usage, but having it close to our home, where we can visit it each day is.
- Zoning is scaled by our visits per year according to the requirements of the element.
- Zone 2 is our food forest system. It will require less maintenance than Zone 1 and thus less our time devoted to this space. Our Zone 1 system is probably already established and with regular maintenance is providing for most of our needs.
- Zone 2 is where we link to more complex natural systems. We learn how dependent we are on Nature itself. We realize that we not just one person, but within a ecosystem where we all dependent on each other. This zone is where trees work in tandem to function as a forest. Louder animals take residence here and can be feed from grown & cut forage. Main crop gardens for our storage or main use are grown here. Coppiced trees for timber or fuel are also grown here. Windbreaks begin to be a crucial aspect to this zone.
- Swales can be used in any zone to benefit the system being established by provide nutrient cycling and water harvesting. It is the size of the swale that is relative to the zone's size, not necessarily the technique itself.
- Swale length: 100'
- Rise [Head from well casing to fence line]: 36'
- Run [Length from bottom (fence) to top of Hill (gate)]: 572'
- Grade [Rise / Run X 100]: 6.3 degrees
- Maximum rainfall event: 1.5"
- Grade above swale: varies depending on swale, will use overall grade
- Estimated run off percentage: 20-30% with grass and some bare ground
- Infiltration Rate for Sandy Loam with 6.3 degree grade: .7inches/hour
- Water Holding Capacity of Sandy Loam: 1.2”/ft
Determining Water Catchment to Accommodate Runoff
- Start with: Square feet of runoff x .1 = Water Volume in a cubic feet your swale needs to hold in a 1” rain event
- Runoff area between swales is 80’ long by 100’ wide (800sqft.)
- 800 square feet x .1 = 80 cubic feet ditch to support the runoff area
- Each swale ditch is 100’ by 1’ deep and 4’ wide = 400 cubic feet.
Determining Water Holding Capacity
- There are 7.5 gallons of water per cubic foot
- Our 400 cubic foot swale ditch will capture 750 gallons of water when full. Much of this will infiltrate quickly, saturate the soil & mulch and only then begin to fill the ditch.
- In a 1” rainfall event, this would accumulate 600 gallons of water over an 80’x100’ area.
- Since redundancy in our systems is important to increase efficiency and resiliency, a well designed sill or spillway will ensure that an overflow event will not be a detriment to any one particular swale but benefit all lower water harvesting systems.
Finding & Marking the Contour Line
There are several tools available for finding contour in a landscape. A-frame, bunyips (water level) & builder’s level are the most commonly used tools. Utility flags are positioned to mark the same contour point as the first designated point. The starting point is determined based upon the design and site considerations.
Again, contour is a level elevation across a landscape. A level elevation means that each point marked across the landscape is the identical height from sea-level or a center point. Swales run perpendicular to the slope of the line along a contour line.
Ditch & Berm Construction:
To build a swale, the soil is excavated from the swale’s ditch and placed on the downhill side of the ditch. This can be done by hand with shovels, an excavator, a front end loader, dozer with proper blades or a rototiller and box blade (as in our case). Some methods are more efficient, but the best option is usually the one readily available. Smaller swales dug by hand are incredibly efficient and should be dismissed as a viable option for any designer.
For our process, we do an initial loosening of the soil with a rototiller where the tractor will be digging the trench. This creates a workable soil for the box blade to easily manage. All loosening procedures should be done once & never again if possible, as the microbial life and root zones are affected negatively by continuous soil working.
The soil of the berm is not compacted, as would deter oxygen from entering the soil. The berm can also have a gradual slope downhill, it is not necessarily a large mound or a mirror image of the ditch.
In sandier sites with quick water infiltration rate, swale trenches should be shallower and wider than in soils that have longer water infiltration rates. For our specific design, the trench will be deeper and wider to accommodate the tractor’s implements and supplemented with wood chip mulch.
As the trench is dug the topsoil is removed and set aside on the downhill side of the berm where it can be introduced to the berm once its finished being built. It may be conducive for the driver of the machinery for us to resurvey and stake the outside perimeter of the contour line.
Adjusting the swale Trench
Once the trench as been dug to the desired depth, we resurvey to get a level bottom of the trench. Shovels and light scraping with the tractor in a few areas usually does the trick. Considering our sandy soils, silt entering the trench, filling the trench with organic matter, it seems that close tends to be close enough.
Water should not flow in the trench, which is why we aim to get within .5" difference from one side to the other. Its best to get both ends level with each other, allowing some leeway in the center being a bit higher or lower.
After excavating the swale trench, it is good practice to investigate the soil layers & take note of the topsoil’s depth, and make other observations.
- A sill is a level & compacted spillway that limits the water holding capacity of the swale’s trench as a discharge point.
- The berm does not hold the water or act as a retaining wall, the trench does the retention as the sill limits the holding capacity.
- The trench can only hold as much water as the sill allows. If there is a heavy rainfall event, the water will fill up in the trench to a height where it meets the sill and overflows out of the swale’s trench.
- The sill is the only compacted area on the berm area. It is a few inches lower than the swale’s berm and at least 3 feet wide. This can be in the middle of the berm or at the end of the berm.
- If a swale or sill does fail, the water will seep into a berm like a glass overfilling with water (slowly and evenly). This can lead to erosion of the berm or simply cave it in. The difficultly of rebuilding the berm depends on the scale and soil structure.
- Its important to realize that groundwater travels from the highest point (ridge lines) to the lowest point (valley) in the landscape. Thus, valleys are always wetter with more life, but less energy. Higher in the landscape there is typically less life and more energy.
- Velocity of the water increases as slope increases.
- Various metrics for the slope, the number the swales desired and the distance from the bottom to the top of hill can all be considered when spacing and positioning swales in the landscape.
- Swales can be placed closer together towards the top of the hill to encourage higher infiltration at the top of the hill where it is drier.
- Swale spacing caluclator: https://www.permaculturereflections.com/swale-calculator/
Access & Structure Outline
- Road for truck/trailer or tractor - access roads are placed on contour as well. This ensures the water that falls onto the road will be caught by the downhill swales. This will also serve as a point for harvesting within the design. These functions stack the purpose of the road as opposed to simply using it as an access consideration.
- Swale trench - Wood chips will be added intimately and periodically as labor & availability allows. The wood chips serve as a slow release water sponge and composting element. Any debris that drifts into the trench will add to the in situ compost. We can eventually shovel the compost/humus in the trench onto the berm. From there, we can refill the trench or plant flood tolerating plants like taro, rice, etc.
- Swale bed - There are various microclimates on a swale bed. The lip between the berm & the trench is the moistest area on the berm. The top of the berm is best for species that require more moisture and the back of the berm for hardier species who need less water.
- 2’-4’ walkway - A good walkway help in efficient harvesting, mulching with a wheelbarrow, chop & dropping, a space to observe without compacting the berm
- Strip tilling & cash crop rows - between the major swales, we are going to be using our tiller for establishing cash crop and mulch crop systems. Over time, these areas will be phased out to add more tree systems, however, initially, we will can build soil fertility with cover crops and interplant with cash crops like melon & papaya to obtain a yield within the first year.
- Never compact swale bank by walking or driving over them; this suffocates soil microbial life & compacts the soil, making it more difficult for roots to infiltrate.
- Planting & seeding should be done promptly if irrigation is possible or rainfall probable. If neither are possible or likely, trees should be planted after the rainy season has made the swale nice & wet.
- Stake or mark production trees to avoid losing their position. Once the system has matured and we are ready for chop & drop, it will be can be so dense that we can lose sight of our trees and end up cutting it out by accident.
- Growing shade through canopy & ground cover establishment is critical to prevent the sun & wind from drying the soil. These will serve to nurse our targeted productive plants (fruit trees, cash crops)
- Windbreaks should be considered to alleviate wind stress on future plantings & to mitigate frost from settling within the system
- Taprooted and deep root plants like Moringa, Comfrey, Mexican Sunflower, Strawberries, Rhubarb will work to stabilize the berm, as well as ensure the water in the ditch is migrated to the berm
- Fruit trees have roots that spread out & feed along the surface of the topsoil. This is where taproot plants will ensure stability in the system.
- Root crops & net/web root plants like lemon grass do well in the loose bank of the swale and will stabilize the bank.
- Interplanting a system with a polyculture of species increases the diversity and thus increases the potential yield of a system. Though not all species get a long, most species adhere to the principal that life acts in cooperation to achieve abundance. Various plant heights, root zone structures, nutrient & water requirements all reinforce a reliance upon each other. While one plant harvests sulphur from the soil, another requires it and a direct communication will cycle the nutrient throughout the system.
- Planting a swale berm with one species invites problems inherent in monoculture design; limited yield, pest & disease problems, less cooperation between plants leading to more reliance on human input.
Canopy Trees: Pecan, Avocado, Carob, Elephant Ear (L) Post oak (good acorns), Clumping Bamboo, Neem (frost-sensitive), Mexican Pinon (Possible?), Tipuana Tipu (L)
Sub Canopy Trees: Fig, pomegranate, Olive mulberry, Persimmon, Peach, Nectarine, Lemon, Lime, Wild Orange, Leuceana (L), Moringa, Kenaf, Papaya, Banana, Barbados Cherry, Jelly Palm, Acacia, Guava, Coffee, Ice Cream Bean (L)
Shrub/bush Layer: Pigeon Pea (L), buffalo berry, Goji Berry, Goumi Berry, Cranberry Hibiscus, Florida Cranberry, winged sumac, Cassava, Wild Indigo (L), Elderbermry, Saw palmetto, Mexican Sunflower
Herb layer: Asparagus, Basil, Lemon Grass, Cironella Lemon Balm, Rosemary, Comfrey, Artichoke, Dill, fennel, parsley, yarrow, cilantro, rue, lavender, nasturtium, millet, aloe vera, sunflower, Stinging Nettle,
Ground cover: Sweet Potato, Melon/Pumpkin, Citron, Peanuts, Cowpea, Austrian Winter Pea, Mint, Peppermint, Clover, Vetch, Perennial Peanut, Gooseberry, Cereal Grains, earth chestnuts, Lupine, Chickweed, Purslane, Dandelions, Chives
Root Layer: wild Leeks, Wild Onions, wild garlic, yams, Sweet Potato
Climbing/Vine Layer: Velvet Bean (L), Grapes, Malabar Spinach, jasmine, kiwi, beans
Fertilizing & Jumpstarting
- Cover cropping is the use of plants to quickly stabilize a system, build fertility, add carbon to the soil and organic matter on the surface. When beginning new earthworks, it is a race against the weeds, by introducing the species we want, we gain momentum towards achieving our goals.
- Cover crops can be scythed, mowed, used as fodder, harvested, or left to live, die & reseed. Cover crops allow for us to initiate a system and undersow within their ground cover to assist in germination and fertility of the successional crops.
- Guilds can be planted with companion species to create a cohesive network of support.
- Fast growing trees are selective species of plants used as pioneers to quickly stabilize the system from the understory tree or canopy tree layer. These species can be likened to the hard working immigrant workers who migrate into a culture and do the work the natives have refused or choose not to do. These trees are used for their leaf litter, nitrogen fixation and/or chop & drop mulch
- Pioneering is a niche in which the work is the most labor intensive. This niche is usually degraded, desolate or highly disturbed sites where higher successional species can not grow. Pioneer trees are hardy species that require little attention, few nutrients and less water. These are often native species, leguminous trees and non-alleopatheic trees.
- Leguminous trees are the perfect pioneers in their ability of harnessing nitrogen (the energy of growth) from the atmosphere into their root zones. They make way for higher successional species like fruit trees by preparing the soil, providing shelter & shade, breaking up compaction and providing fertility & mulch.
- Compost tea treatment provide homeopathic solutions for soil and landscape health
- Bringing in fertility in the form of external inputs (such as manure, hay, straw, organic or inorganic fertilizers, rock dusts) should be done minimally and not done if possible
- We should hold our water in high esteem, we should respect that our use of water directly effects our entire community. Yet, we are establishing a perennial, long lived system and in the beginning, these young trees and new system needs water. Depending on our location, this may need to come from irrigation. We want to love our young trees without spoiling them & making them weak, shallow rooted duds. Give them plenty of water & ween them off.
- Within our design, will be using gravity fed drip tape (1gpm per 100 feet) for the irrigation of the entire swale berm. This will help us to establish cover crop and ensure our trees are taken care of while we wait for the rainy season to come.
- The water will be coming from water storage cisterns located on the North side of the house. These cisterns can be filled via gravity from the main house roof, the cisterns under the garage and the cisterns located the top of the hill.
- We expect to water 2-3 times per week for some time and reducing the irrigation after that as we continue to monitor and observe the system.
- Mulching with organic matter is of the upmost importance in all natural systems.
- Organic matter in the form of hay, straw, wood chips, food scraps, leaf mould & cover crop (living mulch) can be added to the berm & the trench.
- Mulching the trench is common practice to build compost in place and soak up the harvested water, buffering a 'wet feet' issue for our fruit trees.
- Mulch also harvests water by reducing evaporation of the soil & by retaining water like a sponge.
- Compost is not mulch. Quality compost should be covered by mulch to avoid drying out the microbes & loss of nutrients through runoff or evaporation.
Furnish the Swale
- Bury 50 gal drum or cisterns in the trench near the bank to irrigate trees during drought
- Add mushroom inoculated logs or wood chips
- Add rice paddy for flooding
Applying the concept of swales to a smaller garden
- Using the contour of land is to harmonize with nature. Nature avoids straight lines within the landscape.
- Building garden beds in staggered pattern on the contour of the landscape allows for a passive harvest of rain water that would have otherwise been missed.
- As water flows up to the garden beds, it will slow down and wicked into the beds.
- Place wood chips in your paths to soak up the water and slowly release it over time.
Other Methods of Harvesting Water
- Mulch Pit - essentially a deep hole where the soil that is dug out of the pit is used as a bed or berm for planting around the pit. We recommend filling pits or dry wells with mulch material, creating a carbon sink. This will decompose and provide nutrients for the plants around the pit. This is especially useful in sandy soils where nutrients tend to leech away during rainfall events and will increase the water holding capacity in the system.
- Earth Berm - Rainfall moving down slope will navigate around berms, which can be directed into ditches, pits, drains or dams. Earth Berms also act to harness water in of fog condensation. Fog rolling through a system will release moisture onto trees and berms. In our landscape this can account for 50% of our total rainfall per year. Other benefits include acting as a noise barrier, wind break, privacy screen, direct foot traffic, increase surface area of the landscape. Consider the directions the berm faces to create microclimates of wetter and drier areas & warm and cooler areas. Combine an earth berm with hugelkultur for increase nutrient cycling and water retention. Earth Berms should be planted densely with a polyculture.
- Ponds/Dams - Lined or unlined large pits designed to hold a large volume of water.