Rainwater Harvesting

In desert cities like Tucson, outdoor watering accounts for as much as 50% of household water use. Much of water for the landscape goes to maintaining lawns and non-native plants with high irrigation needs. Tucson, so far, doesn’t have any enforced residential water restrictions but with rainwater harvesting everyone can reduce the water required for landscape irrigation.

Rainwater harvesting systems are not new with documented archeological evidence in China dating to 6000 years ago and here in the US evidence dates back about 4000 years. Modern culture just forgot how to use the rainwater to the best benefit. So the goal of rain water harvesting is to create a yard that harvests 100% of the water from annual rainfall. Simply slowing water down allows the water to penetrate into the ground helping to recharge the aquifer, reduce soil erosion and irrigate our landscape. To achieve the goal both active and passive rainwater harvesting techniques are required in urban areas.

Passive rainwater harvesting is the place to start a rainwater harvesting system. A passive system uses earthworks to control surface water flow and uses the soil as the storage container.  The passive system requires planning and observation of the natural water movement on the land but requires no gutters or storage containers. Active systems integrate a storage container into the system to catch rainwater runoff for later use on the property. Once established, a native landscape using harvested rainwater requires irrigation only during times of drought.

The easiest way to start is to direct the roof runoff through simple techniques to move and control the surface water flow with berms, swales, catchments and basins. A swale is a depression or ditch, dug to promote the movement of water. Berms work in conjunction with swales by utilizing the dirt removed from a swale to create a mound to contain the water flow along the edge of the swale, at edges of basins and catchments to contain the water. Basins are shallow planting depressions used in the landscape to collect water and create areas of water penetration around groups of plants. Catchments are berms on the down hill slope of a planting to catch surface water and let it infiltrate into the soil at a plant’s root zone.

Active rainwater harvesting system adds a storage vessel or cistern to provide on-demand use. Cisterns consist of many different types. A simple trash used to store rainwater for use on houseplants or pots on the patio is a cistern. Larger cisterns of plastic, metal or Ferrocement are all possible. Cisterns are generally expensive, require maintenance and space. Sizing a cistern requires calculating the surface area of catchment, generally roofs, and rainfall. The rule to follow is 1000 square feet of roof with one inch of rain yields approximately 600 gallons of water. As you can see, trying to save all the rainfall in cisterns becomes expensive and a space issue. A cistern used to supplement a passive system generally works well for urban landscapes not able to hold 100% of a rainfall event on the land surface.

The final step of the rainwater harvesting system is the use of mulch. Mulch use reduces evaporation, stabilizes slopes, holds bare soil, suppresses weeds and organic mulch amends the soil. Rock works as mulch, but it can be expensive and can create hot areas in the landscape. Organic mulch requires continuous renewal as the organic matter decomposes, but improves the soil. Any material dropped by trees or shrubs that is swept up and removed can be used as mulch. Besides leaves, compost, straw and even shredded paper works as mulch.

If you are still not convinced a rainwater harvest project is worth the work, think about the advantages.

• Sustainable and environmentally responsible
• Conserves ground water
• Reduces erosion and flooding
• Reduces salts and better for plants than municipal water high in salts
• Uses simple technologies and easy to maintain
• Saves money on rising water costs
• Recharges our aquifer
• Protects from runoff pollution
• Assures continued water source for years to come

The resurgence in the ancient techniques of rainwater harvesting will continue as more people discover the ease of use and many benefits. For hands on information on rainwater harvesting visit the Tucson Botanical Gardens’ monthly Rainwater Harvesting Workshop the fourth Saturday of every month.

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