Swales and Keylines
Swales and Keylines for Water Management
We can make better use of limited water resources by first understanding the way rainwater runs across a slope, and then reshaping the land to control the water flow: both where it goes and the speed of flow. Yes, speed does matter. When water flows fast over a hard soil it does not soak in to any great degree and erosion may be high, but when it flows slower more water will soak (infiltrate) into the soil and the potential for erosion will be reduced.
These considerations are the foundation for sustainable farming practices which go under different names in different places.
This is an old concept practised in many parts of the world and promoted strongly by permaculture practitioners. It involves the creation of long, level hollows, furrows or other excavations, or barriers created across a slope (such as a long pile of rocks or rubble). They are used to intercept overland water flow and then hold it for long enough to let it slowly infiltrate into the soil.
It is important when creating these swales that their bases are treated in such a manner (e.g. ripping, cultivating, adding soil ameliorants) that helps improve the infiltration rate of water into the soil beneath. Swales can be stabilised by planting them out with pasture species, and/or planting their slopes with trees and shrubs. Infiltration rates will generally increase with time due to the effects of tree roots on the soil in the swale, and through humus accumulation. Swales are also effective in trapping eroded sediment.
This is a concept developed in Australia by the Yeoman family, incorporating the natural contours of the land in order to plan the positioning of dams, tree-lines, and irrigation layout.
Designing a keyline system for any property requires good initial planning. A contour map is essential in order to best understand the rise and fall, and flow of the land. The leading proponents of keyline design recommend the use of orthophoto maps (aerial photomaps which are available from keyline consultants) because of their increased contour detail. Planning should, if possible, be undertaken before the land is acquired as this will enable the viability to be assessed before substantial investment has been made. All land can be improved through the use of keyline design although obviously some parcels of land will be better suited for productivity purposes.
Three fundamental concepts that must be understood are keypoints, keylines and keyline cultivation patterns.
- Keypoint refers to a position located along the centreline at the base of the steepest part of a primary valley.
- Keyline is a line that runs through the keypoint and extends to where the contours of the valley start to become the sides of the ridge.
- Keyline cultivation patterns. The basic rules of thumb are that cultivation on ridges (above the keylines) should be parallel to and above the contour lines, whereas cultivation below the keylines should also be parallel, but below the contour lines. This means that water runoff above the keylines is directed towards the more gradual slopes for slower dispersal into the soil, and below the keylines it will be directed towards the greater slope for quicker dispersal, and hence will not result in swampy and therefore possible saline conditions.
The use of keyline cultivation is extremely beneficial for effective flood irrigation practices. It allows for inexpensive flood irrigation of undulating land as well as fast flood irrigation of flat areas. This is a crucial point because generally with flood irrigation, because generally the water tends to lie for greater periods in flats while the water is being coaxed to cover the entire area. This means that important aerobic microbial organisms are deprived of oxygen in these lower areas, also contributing to the poor quality of the land in low areas. Keyline design is initially concerned with the topography and climate of the land parcel, but also incorporates the use of treelines and plans the positioning of dams, irrigation channels, fences, farm roads and buildings.
Planning should take place in the following order as proposed by Yeoman's in articles on keyline design:
Water includes calculation of key points and keylines in order to aid placement of dams and irrigation channels. Placement and size is decided by the contours and the surrounding suitability of flood irrigation land. Roads should not cut across contours as this interrupts water runoff flow. Generally, road placement should be along the top of ridge lines and elevated contours.
The importance of treelines on a property cannot be understated. It is an area of farm management that has become increasingly scrutinised in order to achieve the best results without causing the loss of production or headaches during cropping or related farming methods. Trees are crucial to sloping land in stabilising the soil, providing shelter from the elements and controlling erosion. It is these same principles that make them an integral part of keyline design. Buildings are placed on considerations of comfort, aesthetic attributes and practicality. Fencing can be applied to correlate with the planned subdivision of the property based on its production requirements. Fencing should also be used to protect trees and dam access from damage by stock.
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