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Rainwater catchment at Pedregal
- 1 Introduction
- 2 Utilizing Sloped Terrain
- 3 Utilizing Roofs
- 4 Tanks
- 5 References
Rain catchment systems can be constructed in many areas, both to increase the regeneration of the local aquifer with rainwater and to reduce the soil erosion from rainfall. Rain catchment during wet periods can also help full fill needs during dry periods. Rainfall can be used for drinking water, but here at Pedregal it is mainly used for irrigation and use in aquaculture. This is one project in a series of soil conservation, groundwater recharge, and farming demonstration projects at Pedregal a Permaculture Demonstration Center in San Andres Huayapam, Mexico.
Utilizing Sloped Terrain
System Components of Sloped Terrain Catchment
Storm water or creek water is guided to the place by gravity flow, but rather then following its natural path, artificial basins, channels and ditches are dug so that the rainwater is diverted from its natural path and into the tank. Tanks can be made of a shallow pan of rammed earth, an artificial pond, or a cistern of ferrocement.  The tanks at Pedregal are artificial ponds where they have added plant cover to synthesize a pond habitat.
Should divert water and be placed so that water can easily flow down into the tank.
Microfilter dams are small walls of loose rock wrapped together by fencing or strong mesh and placed in canals. No mortar is used and water is supposed to be able to flow freely through the small dam, while preventing soil, silt, and pebbles from going through the canal and into your tank. This can help reduce maintenance issues with your tank by reducing the need to drain your tank and shovel out sediment. Click on the link for more information on Micro filter dams.
Look at section 4. Tanks for more info.
System Components of Roof Catchments
The system design should include a sloping roof with a fixed gutter of PVC, galvanized steel laminas, or some other materials in the lower part, so you can collect the water that falls on the roof. This gutter must have a small slope and its lower end should lead into a hose or tube which will allow the water to flow into a storage tank. To find the slope of your gutter, first figure out the slope of your land, do not rely on the slope of your roof to tell you this. A small self cleaning filter can be installed between your gutters and your PVC to prevent problems of leaf litter or other things from clogging the system, however Pedregal does include this component. The system can also include a first flush in order to clean the roof of unwanted accumulated debris, leaves, and bird excrement, without having to store it in you tank. This first flush allows the first ten to fifteen minutes of rain to pass your system without going into your storage tank. The remaining water goes to the tank, which in this case is made of ferro-cement. If the end use is drinking water, then some type of extra filter should be added. At Pedregal they created a carbon filter, with alternating layers of sand and charcoal for the drinking water. The rest of the water is used for drip irrigation and aquaculture ponds.
For more info on Collection Calculations from roof size and rain fall.
Canals both of galvanized sheeting and of PVC
- While galvanized sheeting was shaped into canals in the front, this design took more work, but was more adaptable to the curving roof.
- PVC tubing was used to make the back canals, and did not take as much work, since they simply cut off the top of the canal, but these canals needed a straight edged roof to go along.
- Canals need to have a slight slope so that water does not pool up in canals after rain and so that rain can easily flow from canals to the PVC tubing.
- Though a screen was not used as an intermediary between the canals and the PVC tubing, this can cause clogging and more maintenance issues. If any organic leaf litter did end up making it to the tank the rotting material would also pose health issues.
- A screen is not necessary if your canals already are designed with a protective covering that prevents leaf litter from getting into the tubing. If one is already using PVC for the canals a simple alteration in the design could include rather then cutting the top half of the canal off, only cutting a small slit in the canal that only allows water into the canal.
- Tubes should run from the lower end of the canals to the tank.
- Since there are contaminants that can accumulate on your roof: from air pollution, your smoke chimney, or bird defecation, its good to have a self cleaning design for the system. The first flush makes it so that the first 10 gallons of water when it rains is flushed out of the system before it reaches the tank.
- Pedregal's first flush includes a separate PVC pipe about 15ft long that goes off the main tubing and is the first to fill with water when it rains. After the first flush is full, water passes to the tank. The first flush has a cap on the end with a small hole so that it slowly, slowly leaks out. This means that some small amount of water after it is full will also be flushed out.
Look at Section 4. Tanks for more info.
Carbon Filter for use in domestic drinking water
- Carbon filters utilize charcoal as a medium for filtering out impurities in water. They can filter out most deadly contaminants.
- Minerals in water can still flow through a carbon filter, so it is advised to have some other filtration elements beyond charcoal in order to ensure that sediments wont clog up the filter long before its normal lifespan. In Pedregals carbon filter they utilize a series of layers starting with: gravel on top, then sand, then charcoal, then sand, then pebbles, then more gravel. The filter in total is only a 10 gallon trash barrel and the charcoal layer is only about 2 inches tall within it.
- Also a charcoal filter can not filter out sodium, nitrates, or heavy metals.
- If using the water for drinking you'll use the carbon filter and some piping to your end location.
- If using the water for other domestic uses a slow sand filter and some piping to your end location will be fine.
- If using the water for irrigation compare drip hoses to pressurized sprinklers for your ag land.
- Trees and perennial crops are better on drip hoses, since: the hose will not need to be moved to meet the plants change in location every year, the loss of water is greatly diminished, its cheaper, and the drip can be placed wherever the tree or plant is at.
- At Pedregal they also use drip irrigation for their annual plants, which requires that the drip hoses be moved to match the new planting each year for accurate watering.
- At Pedregal the nursery of baby trees and shrubs that they grow to help reforest the area is irrigated with gravity pressurized sprinklers. Their rain catchment system is at least 60ft above their nursery and this height provides enough pressure for their system, which rains out water from the ceiling of the nursery.
The tank's placement is one of the most important aspects of the system. You want the tank to be placed above the site of use, to diminish costs and to eliminate the energy demands of a pump. If your tank is below your area then there are a variety of water pumps.
The dimensions of your tank depends on the volume of water you need to use. If using this water for drinking take into account that the average adult needs to drink 64 oz of water a day. The following dimensions and capacity are based on cylindrical ferocement tanks.
Open or Closed Storage:
If using the water for domestic use it is suggested to have a closed container. Closed containers offer the benefit that garbage, organic matter or other pollution sources for fresh water do not accumulate inside. It also helps prevent algae or bacteria that rots quickly from growing inside your fresh water container.
If using the water for irrigation then an open container is very functional as well as cut down on the costs of storage and hold large amounts of water. If the container is open it is very desirable to have the water frequently moving to oxygenate the water and prevent mosquito problems. Rather then using pumps to circulate the water, animals like ducks and fish can provide the movement necessary to keep water moving as well as help control mosquito populations and through their waste they can add a high nitrogen fertilizer to your crops. Read more on aquaculture practices. At Pedregal they have one open storage container that is a fish hatchery, another one is a pond with water loving plants, and the last two are positioned so that fresh creek water is constantly moving in and old water out.
Ferrocement Closed Tanks
The following is for a tank with the capacity of 9,950 liters.
- 7 packages of cement
- 48 sheets of sand sifted
- 25 meters squared of chicken wire (or double if necessary
- 190kg of electro-welded mesh with a thickness of 3/8 (but the tank size will dictate the necessary size of the mesh. 
How to Build a Ferrocement Tank for Closed Storage:
Start with a level foundation. Choose the right place - preferably a non-sandy or rocky terrain. Dig a cylindrical hole, leaving the preparation to build a parapet on the top edge. If the the foundation is uneven one needs to refine it with a mixture of soil, lime and a little cement. To make the cylindrical walls the chicken mesh will be placed on top of the electro-welded wire and tied together with a thinner metal wire.
Once tying the electro-welded wire to the chicken wire, you want to lift the wire frame into place and set it on top of you foundation, wrapping it into a cylindrical shape with an open top and no mesh for the bottom. On top of the metal frame you apply one part cement by two sand. This mixture should have a thick consistency and pressure should be applied with the back of the trowel. The cement mix should completely fill the holes and cover the metal frame. The cylindrical shape gives it more resistant walls, so a layer 3 cm thick is sufficient to resist collapse from pressure exerted by a full tank. To avoid cracking the mixture must stay moist, so it is recommended protect it from the sun, while drying. 
After the walls, the tank is covered with a slab of cement, which has an opening for the water to enter and an opening that allows for continued maintenance and interior cleaning of the tank when needed.
If ground conditions are suitable and the stored water is being used for irrigation you can build a open tank with no lid and reduce the costs seen by closed tanks. The ideal is to use some of the natural hillside to facilitate construction of the wall.
You can dig out a slight edge of the hillside to form the open tank, but this is not necessary. Stones can be stacked on each other to make the frame of the walls and the frame for the foundation. Leave a opening for a spout at the bottom to retrieve water from the tank by gravity. Next line the inside of the rock frame with the cement mix of 1 part cement to 2 parts sand, remembering to leave the small bottom spout open. 
- Soil and Water Conservation Committee, State of Alabama. "Block and Gravel Inlet Protection". http://swcc.alabama.gov/pdf/Handbooks&Guides/2006_sections/12%20Sediment%20Control%20(practices)vol1.pdf
- Lopex, Laura. L., Esteva, Gustavo., Consejo, Juan J., Padilla, Eugenio., Robles, Marcela., and Alejandre, Virginia. Defensa Ecologica: Manual de Tecnicas. 2a. edition. Oaxaca, 1998.