How to build a low-cost rainwater pond.

The main way to store rainwater in rural situations for multiple uses is in the form of artificial ponds. The idea is very simple: you harvest rain runoff by means of gutters, which then take that water into an impermeable tank dug into the soil.

Artificial ponds are extremely versatile and incredibly useful. They may store substantial volumes of water for relatively little cost. They may serve countless purposes, such as water supply for the house, irrigation, aquaculture, microclimate creation, landscaping, recreation, fostering wildlife, etc.

In theory, any pond can be dub by hand, with spades and hoes and wheelbarrows, etc. However, it is arduous and lengthy work. Unless it’s a rather small pond, or you have a good number of motivated people and no hurry, the most common is to use machinery such as diggers and bulldozers to dig your pond.

In rather gentle slopes, the digging of a pond produces a great amount of earth, which can be used in earthworks, construction, earth banks for a variety of purposes, or simply to make a small hill beside the pond, which can create an interesting look, or help creating a microclimate, working as a windbreak, giving shade or reflecting sunlight, etc. Of course those effects and benefits depend on an adequate positioning, which demands careful planning. Now, in places with a steeper incline, digging a pond won’t produce any surplus earth, because the dug out soil must be deposited at the lower edge of the pond and compacted, forming the dam that will give the pond its shape and capacity.

Rainwater ponds must be strictly sealed, as any water loss by infiltration may mean an empty pond in the middle of the dry season. The simplest and most effective way to waterproof a pond is with a plastic liner. They are 100% impermeable, but are photodegradable, which means that if exposed to sunlight they will deteriorate over time. However, if protected from the sun they can last virtually forever. Therefore, the liner must be covered with a permanent soil layer.

Before fitting the liner, the pond interior must be adequately prepared. The inner walls must be made in “steps”, with flat platforms about 20 cm wide, and raisers with an inclination of no more than 45° and about 45 cm high. That profile is important to make the soil cover stay on the liner, preventing it from being washed down to the pond bottom with the rain.

pond inside raisers platformsThe inside of the pond must be well compacted with a dirt tamper (compactor), or even with your feet. Any stones or bits of wood must be carefully removed, and the inner surface be made as smooth as possible to prevent any damage to the liner.

Also the water inlet must be prepared before fitting the liner. The inlet is a depression, a concavity on the inner side of the pond wall, with larger size and capacity than that of the runoff catching gutter. It must be sculpted keeping the step pattern, and its edges must be raised to help confine the water flux within the inlet, even with heavy rain.

Once the inside of the pond is prepared, it’s time to fit the liner. There are several types of thick, sturdy plastic liners that are made specifically for that purpose, which are often referred to as geomembrane. Unfortunately, those tend to be quite expensive. As a cheaper alternative, you can use a 200 μm thick high-density polyethylene (HDPE) silage wrap—they cost a fraction of the geomembrane price and can be perfectly effective; moreover, they are easier to get and to handle, and represent a reduced use of plastic relative to the geomembranes. However, they are much thinner, so they require extra care to avoid damage by tearing and sun exposure.

The liner must cover all the inside of the pond, going over the borders at least some 50 cm. Oftentimes it is not possible to cover the entire pond with a single piece of liner. You can use duct tape to join the pieces, and also for repairs. You must ensure the liner is well fitted (not too tight) to the bottom and sides of the pond.

The liner must be covered with a layer of loamy soil at least 10 cm thick. Add chopped grass, which helps keep the soil layer, preventing it from collapsing or being washed down. You may need to amend the soil, correcting its acidity or adding fertilizers before adding it onto the liner, at least for the last, superficial layer, to allow for plant growth in and out of the pond water, which will definitively stabilize the soil layer, keeping the liner protected. Aquatic plants will also be important to keep the water quality allowing for the production of fish, frogs, etc., which is vital to avoid mosquito proliferation.

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The water inlet is a critical point. It must be built with stones and mortar (or, alternatively, concrete tubes, ferrocement, etc.), to safely contain the water flow into the pond without overflowing, thus avoiding erosion inside the pond and exposure of the liner.

ponds water inlet.pngThe excess water must leave the pond through an overflow outlet. It is a critical part of every rainwater tank, and must be carefully designed, built and maintained.

When harvesting rainwater, we catch surface runoff from a large area. In strong rains, that may cause the concentration of large water volumes, exceeding the storage capacity of our systems, which means they will overflow. Now, we must never let the water choose its own course, as it will always choose the shortest possible way, that is on the steeper slopes where it picks up more speed, with immense potential to cause damage, especially erosion, besides damage to crops, facilities, etc.

To avoid such problems, we must make the excess water flow smoothly across the land. Some people may think of using tubes, but in most cases the best way to deal with that water is on the surface, through off-level troughs, which are just the same as the runoff catching gutters described above. The outlet point must be determined with the aid of a flexible tube water level to ensure that the outlet point is actually the lowest point around the pond border, at least some 50 cm lower than the dam level (or more, in the case of larger ponds). That is critical to prevent the overflow from running over the pond’s dam, which would cause damage and possibly even its breakage, with potentially serious consequences.

The overflow gutters are dug with a 1–2% incline, zigzagging across the slope, gently taking the water down to the next pond in the system. Lastly, they should take the excess water to a “dry pond”, that is a pond that is not sealed, where the water will penetrate, replenishing the water table.

Catchment gutters

As previously mentioned, rainwater harvesting is done by means of off-level channels that collect the rain runoff and take it to the reservoir (pond). The gutters are easy to build: starting from the pond inlet point, you mark a line on the ground using a level and stakes. You can use a flexible tube water level or a simple bubble level attached to a trestle-like frame—a horizontal bar held by one leg at each end. Give the line a fall of 1–2% toward the pond. The line marked by the stakes is where you’ll dig your gutter. You can have 2 gutters, forming the right and left arms of your water catchment, that will bring all runoff from the section of the hill above, taking it to your pond. The gutters can be dug with a tractor or by hand, with mattocks and hoes.

Sometimes you’ll have a gulley, cut on the hillside over time by heavy rainwater. That obviously represents an opportunity to harvest a lot of water. However, it is generally a bad idea to try to store water right there or build the pond at the gulley, as it is normally the steepest part of the hillside. Rather, you should choose a spot with milder topography for the pond, and build just a diversion in the gulley, with a reinforced wall built with stones, rammed earth tyres, earth bags, concrete, etc. The diversion connects with the catchment gutter, taking the water to the pond.

Dig a cavity, maybe a 1.5 m wide, 1 m deep hole, at the end of the catchment gutter, just before the water inlet to the pond, to act as a sedimentation tank. This hole must not be sealed. Solid particles such as sand, pebbles, branches and other impurities carried by the water will settle in this tank, preventing them from entering and accumulating in the pond. The sedimentation tank must be cleared regularly.

sedimentation tank.pngIt is hard to predict the ideal size of the catchment gutters, so it is better to start small, as an excess of water can cause you trouble, especially in strong rains. The pond should not fill up at the first rain, but rather slowly, over many rainy days, maybe taking a few months to fill to capacity. So, it is safer to start with a modest catchment and observe the water flow during rains, check how the pond fills, and make the gutters bigger if necessary.

Once the pond is finished, plant grass seeds or a mix of herbaceous species that are native of your area in the soil layer that covers the plastic liner—their roots are the most effective way to fix the soil in place, preventing its washing down by the rain. Introduce also aquatic plants, such as cattails and water lilies, rush, etc. inside the pond once it is full. Frogs will come and colonize it, and you can also introduce fish, shrimps, etc. (we’ll talk a bit more about aquaculture in chapter 9, “Rural Permaculture”).

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Now, a very important safety tip: you must put up a very effective fence around your pond. This is critical to the safety of people, as there is always a drowning risk, and also to protect the pond itself—you don’t want a large and heavy animal such as a cow or a horse treading your pond, damaging the plastic liner with their hooves! Even if you don’t have such animals, it is always possible that someone will forget to close a gate, or your neighbour’s livestock may manage to cross a fence, etc.—that’s the sort of thing that happens all the time.

Lastly, plant trees around your pond, to shade the water, keeping it fresh and preventing excessive evaporation, besides making it look great. In cold climates, plant deciduous trees on the sunny side, allowing the sun to warm the water in the winter.

pond close to the house pleasant effect.png
Series of ponds

You can and should have more than one pond in the property. In fact, by the redundancy principle, it is always better to have two or more smaller ponds, rather than only one big pond—this way, if you ever have any problems with a pond, your water supply is still guaranteed.

Start by building a smaller pond and move on to building bigger ones later. This gives you some important advantages: you’ll have water sooner, as a smaller pond is quicker to build and fill. With that water, everything will get easier: building, planting, and even making new ponds. Moreover, it is better to start small to get experience—you’ll learn from mistakes and improve your skills, so everything will flow better when you get to the bigger ones. And the costs will be split in “installments”, which can be a great advantage. Lastly, you’ll have the benefits of a pond scattered all over the plot: favourable microclimate creation, the possibility of independent irrigation systems, and multiples opportunities for varied uses, such as aquaculture systems. You’ll also be creating multiple points for wildlife including migratory birds to benefit from water, shelter and food, etc.

The several ponds may have different sizes and shapes, according with the topography and other properties of the terrain, such as catchment potential, available space, etc.
The water catchment for those multiple ponds must be made in series, where the excess water from a higher pond flows passively to the next pond down in the system (the overflow drain of a higher pond is the catchment gutter of a lower one).

pond series.png

All this and much more in the book The Environmental and Civilization Crisis and the Permaculture Alternative.
You can download the e-book for free HERE!

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