Home rainwater harvesting is a great way to improve the appearance, efficiency and sustainability of your garden while lowering your water bill.
Things You'll Need
Shovels
Bunyip (2 yard sticks, length of clear plastic tubing, duct tape)
Gravel
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Instructions
Site Assessment
1. Determine how much rain falls on your property during an average month. Use rainlog.org or a similar site to look up the average rainfall for your area. Measure the dimensions of impermeable surfaces, especially roofs, compared to the rest of your property. Multiply the area, in square feet, by the monthly rainfall, in feet, to determine cubic feet of rain.
2. Examine the topography of your property. Start from the top down. Where does water flow? What are the low points where it collects? Either observe during a rainfall event or use a bunyip to compare elevations.
3. Begin planning your water harvesting design. Each plan must be uniquely tailored to an individual property. Keep in mind a few goals. You want to prevent water from flowing off your property, except in big storms when even the most well designed water harvesting scheme cannot contain all the precipitation. To avoid erosion, you want water to flow slowly and spread out into wide, shallow basins containing plants. Keep it simple at first and remember that water flows downhill.
Water Harvesting Design
4. Watch where water flows off of your roof the next time it rains. Where does it flow after that? There are two ways to use water that falls on the roof of your home. The one you choose will depend on the yearly cycle of precipitation in your area and also involves your budget. The simpler method is to see where most of it falls to the ground, and direct its flow into wide, shallow basins with trees, flowers, or vegetables growing. This method is cheaper, but it may not be effective in areas where most or all of the rain falls during a brief rainy season. In these cases, you may wish to invest in a cistern to collect water from the roof for use in dryer times.
5. Slow down rainwater falling from the roof by laying rocks and gravel under the primary downspouts. This will prevent erosion and keep the force of the rainwater from destroying your work.
6. Dig trenches or build up berms to direct the flow of water. Berms are ridges of tightly packed dirt, preferably studded with rocks to slow the flow of water and prevent erosion from rain. Roofs collect water over a relatively large area and usually concentrate it in just a few places on the edge. Your goal is to slow this water down and direct it toward planting beds.
7. Make your trenches wide rather than narrow, and line them with small rocks or pebbles to reduce erosion. This will prevent your whole design from turning to muck at the first heavy rainfall.
8. Direct the flow of water into one or more wide, shallow basins. The wider the better, because you want the water to infiltrate into the ground, not form stagnant pools to drown plants and breed mosquitoes.
9. Use the earth that you remove while digging basins and trenches to make berms elsewhere on your property. Berms should be a little wider than they are tall - a bit less than a foot wide and four or five inches high, for example. Place them at the edges of basins and trenches or near roof downspouts to direct the flow of water.
10. Plan for overflow. Either visualize where excess water will flow when it fills a basin, or test it by filling it with a hose. Rather than letting this happen randomly, make one side of a berm lower than the rest, so you can control where overflow will go.
Plants
11. Plant your garden based on the water requirements of plants. Thirsty plants should be planted in the basins themselves, while more drought-tolerant plants can be planted on the edge of basins so their roots can access the water but they aren't swamped by it.
12. Dig basins around trees to collect water. Saplings, generally between one and five years of age, should be planted so that the lowest point is around the base of the trunk. This way, water will collect around the base of the tree where its roots can absorb it. Do not construct basins for older trees in this way. Mature trees do not absorb most of their water from the ground around their trunk. The zone of maximum absorption is actually a ring, just below the edge of the tree's canopy. If you design your landscape so that water collects at the base of a mature tree rather than in this wider ring, it will not absorb water as effectively, and it can cause rot and decay of the trunk, making it susceptible to parasites.
13. Plant lots of things. Flowers, bushes, vegetables and trees provide ground cover to reduce erosion, improve infiltration and use as much of the water as possible.
Maintenance and Observation
14. Take notes the next time it rains. Is your design working? Is water still being wasted by flowing off your land of collecting in areas without any plants growing? Is it flowing so fast that it's washing away soil or destroying your berms? Constantly re-assess and improve your water harvesting design.
15. Repair erosion damage, and make trenches wider or place rocks in areas where water is moving too quickly.
16. Time how long it takes for water to infiltrate into your basins. There should not be any standing water after 48 hours in the case of heavy rainfall, or preferably 24 hours in most cases. If there are still pools of water at this point, your basins are not wide enough or there needs to be a better overflow route.
