Water Harvesting Techniques

This page is a compilation of various water-harvesting methods, mostly earthworks, presented in no particular order.  More will be added as time permits.

Swales

A swale is a level ditch, dug on contour, with the fill piled into a loose berm on the downhill side.

When rain falls and flows over the soil surface the water is stopped in the swale ditch. It then sits long enough to sink into the soil, creating a plume of moist soil downslope.

Over time, ground water builds up, raising water tables, preventing downslope wells from drying up, keeping surface streams and springs flowing, and ensuring adequate groundwater for crops.

Swales are not just water harvesting systems, they are also tree growing systems. The loose berm on the downhill side must be planted or it will erode away. Typically, fast growing ground cover plants are planted immediately. Nitrogen-fixing support trees and fruit trees are also planted on the berm. The swale ensures that they have adequate water. The shade they cast over the swale reduces evaporation, and the leaves and wood that fall from the trees increases the organic content of the soil, which increases the soil’s ability to store water.

Several swales can be installed on a farm, creating food-, fuel-, fodder-, and fertilizer- producing shelter belts that capture rainwater and effectively store it in the soil, where crops need it.

In this way, a few people using only hand tools can effectively capture a very large volume of water every time it rains. Swales are effective at very small scales. On a large scale, a system of swales can help to utterly transform a landscape.

Fruition Pits or Basins

A fruition pit might be the simplest water harvesting system. It is a wide, shallow pit, that is filled with garbage. Ideally, most of the garbage would be organic material, and none of it would contain toxic chemicals or heavy metals. Useful trees and shrubs are planted around the perimeter of the pit. The edges of the pit are gently sloped and are lower than the surrounding area, so that any runoff water coming near the pit will flow into it. The pit becomes a dump for all non-toxic, biodegradable garbage, like food scraps, weeds, broken ceramics, etc. The material filling the pit acts as a giant sponge, absorbing the water that flows into it during rainstorms and slowly releasing it into the soil throughout the dry seasons.

Water flowing in beyond what can be absorbed by the organic sponge is retained and sinks into the soil. The trees surrounding the pit shade the inside of the pit and protect it from wind. Their roots access the constant supply of moisture at the bottom of the pit. The trees are fertilized by the constant addition of organic waste into the pit. These systems can be used to grow food and other products with little or no irrigation and turn waste into a valuable resource.

Berm and Basin

A simple addition to a basin or fruition pit, where appropriate, can dramatically increase its capacity for harvesting rainwater. A berm can be built such that sheet flow is interrupted and moved into the basin, increasing the catchment area to everything upslope of the entire berm.

Check Dams

A check dam is a porous dam, installed across a drainage way, which slows but does not stop the flow of water.  They are often installed in erosion gullies and stream channels.  The simplest check dam is a few rocks placed across a drainage.  It may use many rocks to cover the horizontal distance, but the dam will only be one rock high.  As rapidly flowing, erosive water hits the dam, it slows down enough for some of the silt suspended in the water to fall out of suspension and be deposited just upslope of the check dam.

As the silt field builds up, the flow of water is attenuated – the same total quantity flows through the drainage, but it takes longer and moves slower.  Also, the bottom of the drainage or stream bed is built up and gets higher.  Over time, streams that only flow during the rainy seasons can begin to flow year-round.  The silt field above the check dam is typically very fertile and moist, allowing plants to get established that will prevent further erosion.  Often, a new check dam must be built every year or every couple years, as the dam becomes buried in silt.

A Gabion is a much larger version of a check dam, appropriate for large gullies with heavy seasonal flows.  A gabion is constructed out of large round rocks held together with chicken wire, roughly in the shape of a wall.  Gabions must be constructed and installed with great care, because if they are not they can fail and be washed downstream, potentially causing serious harm.  Often, a smaller, simpler check dam will work as effectively as a gabion.  It will just take a lot longer.  But we tend to prefer slow, simple solutions that can be easily managed by humans.

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