Desert Botanical Garden: THE DESERT HOUSE: Rainwater Harvesting

Rainwater harvesting, or collecting rainwater for future water needs, has been used for many centuries as a way to take advantage of seasonal precipitation that otherwise would be lost to runoff or evaporation. People, like the modern Hopi and O’Odham farmers, as well as ranchers in the Southwest United States, construct dams, terraces, ditches, cisterns and ponds to collect and store rainfall for use on their farms and ranches. Rainwater harvesting is a major source of drinking water for many people around the world.

The Desert House rainwater system can be divided into two components. First, rainfall that lands on the landscape is diverted to plants via contoured slopes and berms. Plants needing relatively more water are placed to collect more runoff. Basins have been built around particular plants to collect water and allow it to percolate slowly through the soil. This method insures that plants receive the water they need and very little water is wasted.

The second component of rainwater harvesting at Desert House uses the roof to collect and divert rainwater through downspouts, into a sand filter and store it in a 4,700-gallon cistern located in the basement. When necessary, the stored rainwater is pumped to the surface for landscape irrigation. Stored rainwater also can be used for evaporative air conditioners or fish ponds, or to water sensitive plants.

How much water can be harvested and stored using the Desert House roof? Average annual precipitation for the Phoenix area is around 7.5 inches; with this annual rate each square foot of the Desert House can accumulate four gallons per year. The Desert House roof, measuring 2,208.93 square feet can amass 9,277 gallons per year, half of which, about 4,700 gallons, can be stored. If the rainwater tank is full, the downspouts can be adjusted to divert water directly onto the landscape.

The Desert House landscape needs approximately 104,860 gallons of water per year. Rainwater can account for 9,277 gallons or 9%. Graywater can account for 11,370 gallons or 11% of total irrigation demand. Combined rainwater and graywater harvest can save 20,647 gallons of water per year that would have normally come from the city water supply.