Water conservation is critical to sustainability everywhere. In the Tucson area, where we receive only 10 to 12 inches of rain per year on average, water conservation is even more crucial. Since 1940 the water table in Tucson has dropped up to 200 feet in some places. As the population of the Tucson region continues to grow, reducing our water use is imperative.
Most homeowners use 50% or more of the water they purchase from their local water utility to irrigate their landscape. Fortunately, there are several ways to reduce both water consumption and water bills while still caring for one’s plants. One important way to do this is by using a rainwater harvesting system to capture rainwater and direct it where water is most needed.
- A house with 1000 square feet of roof area can collect over 600 gallons of water from 1 inch of rain
Passive Rainwater Harvesting
Passive rainwater harvesting uses earthworks to direct surface water flow and the soil as the storage container. A passive system requires planning and observation of the natural water movement on the land but requires no gutters or storage containers. The primary concept is to slow down the flow of water, allow it to soak into the ground, and keep as much of it on the property as possible. Once established, native landscapes using harvested rainwater require irrigation only during times of drought.
The easiest way to start is to direct extended roof runoff through simple techniques to move and control surface water flow with berms, swales, catchments and basins.
Swales are shallow ditches, usually perpendicular to the direction of water flow, that collect storm water and allow it to sink slowly and directly into soil. Their placement permits the redirection of water from its natural direction of flow to another desired location.
Basins are excavated shallow areas, often lined with rock or mulch, that accept rainwater runoff and permit water to percolate into the soil. Basins connected by shallow, sloping (1/4 inch per 1 linear foot) swales allow water to flow from one basin to another.
Berms are low mounds of earth formed to help hold water in basins or swales. Often the soil used to create a berm was excavated by digging a basin or swale.
A simple French Drain is a trench or hole filled with gravel that allows surface water to soak into the ground, alleviating standing water and providing more irrigation to nearby vegetation. A more sophisticated French Drain can use perforated pipes to direct water elsewhere and allow the water to slowly soak into the surrounding soil.
- The Gardens has seven (7) French Drains throughout the historic west section of the property. These drains allow rainwater that falls on hardscape areas (non-permeable surfaces) to soak into the soil to nourish the surrounding plants.
Rain Chains are an elegant alternative to traditional gutter downspouts. Originally developed in Japan several hundred years ago, they offer an aesthetic, pleasing and functional method for directing the flow of water from a structure into the landscape.
- In Japan, they are referred to as “kusari doi”, which simply means chain gutter.
A curb cut is literally a cut in a street curb that allows storm water to flow from the street into a mulched basin, swale, or French Drain. Many neighborhoods in Tucson use curb cuts to move rainwater to community landscaping areas.
Active Rainwater Harvesting
Active rainwater harvesting systems add to a passive system a container or cistern to store collected rainwater runoff for later use.
A cistern is an above-ground or in-ground water storage device. There are many different types, all of which are part of a roof cathment system. One simple example is a trash barrel used to store rainwater for irrigating houseplants or pots on the patio. Larger cisterns can be made of plastic, metal or ferrocement. In-ground cisterns need pumps to move the water to a desired location.
Cisterns are generally expensive, and require maintenance and space. Sizing a cistern requires calculating the surface area of a catchment (generally roofs) and the amount of rainfall. 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. Cisterns require maintenance and care to protect against mosquitoes and debris collection.
The Gardens has 2 aboveground cisterns and one below grade cistern for storing rainwater. These 3 cisterns store over 15,000 gallons of water to irrigate the landscape without using potable (drinking quality) water from the municipal water supply.
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 information on rainwater harvesting visit the Tucson Botanical Gardens’ monthly Rainwater Harvesting Workshop the fourth Saturday of every month.Email this page to a friend