Another naturally occurring event is precipitation, a necessary source of water for all life on the residential site. While essential, precipitation is not predictable in occurrence and varies widely in seasonal and regional amounts. Average rainfall is about 59" in Miami, 42" in Boston, 36" in Seattle, and 7-1/2" in Phoenix. May through October
are the wettest months in Miami, whereas April through June are the driest months in Phoenix. Natural precipitation is often supplemented in the residential landscape with various forms of watering to help plants and lawns survive dry spells. This provisional water can be substantial in terms of both volume and cost. It is estimated that the average family of four in the United States uses 400 gallons of water per day, 30 percent of which is devoted to outdoor uses. Additionally, water is projected to become an increasingly scarce and precious resource in the future. Thus, the sustainable residential site must integrate various techniques for conserving water and using it wisely.
A landscape designed for minimal use of water is called a xeriscape and is most commonly used in arid regions, though it has applications in all areas that experience periodic droughts. A xeriscape can encompass the entire site or be a designated low-use water zone within the site. A well-conceived master plan, proper selection of plant materials (see “Select Plants for Regional Precipitation” in this chapter), efficient irrigation, and good maintenance practices can all contribute to a xeriscape. The master plan should incorporate a host of concepts so that water conservation is integral to the entire design.
The optimal xeriscape relies only on naturally occurring precipitation. If irrigation is used to supplement this, then it should be designed on the basis of different requirement zones as a means of conserving water (Figure 3—37). For example, a
low-water zone would have minimal or no irrigation with drought-tolerant plants. A low-water zone should be as large as possible and located the greatest distance from the residence because it would not need pipes and other connections to support it. A moderate or mesic water zone would require some irrigation during dry spells but little to no extra water the remainder of the time. A moist water zone, often including the lawn, would require the most irrigation and should be relatively close to the residence to reduce the length of pipe or hose to service it. This zone should be kept as small as possible to conserve water.
Ideal in planting beds and containers, drip irrigation or soaker hoses are the best methods of irrigation because water enters directly into the ground. New technologies in irrigation such as subsurface capillary systems are emerging and promise to provide significantly more efficient distribution of water to the landscape as well. These systems use a small tube that is connected to a matlike material below the ground’s surface as a means of moistening the soil. Sprinkler – or spray-type irrigation should be minimized because some water is lost through evaporation and wind dispersal. Lastly, an irrigation system should have a rain-shutoff device and undergo regular maintenance to repair leaks, adjust volume, and correct aim of the sprinkler heads.
Other maintenance practices that conserve water include putting mulch on planting beds where there is exposed soil. Mulch reduces the amount of water that evaporates from the soil, lowers soil temperatures, reduces weeds that are unsightly and take moisture from landscape plants, and decomposes to form organic material. Lawns (also see “Select Plants for Regional Precipitation”) should be regularly aerated to create air space within the soil and encourage the infiltration of surface water. The grass height in lawns should be maintained at 2 to 3 inches so the grass shades the soil and helps to retain soil moisture.
Water harvesting is a different technique for conserving water by catching precipitation on a hard surface such as a roof or pavement and then storing it as a source for watering plants. A simple way to collect rain from roof areas is with a rain barrel placed at the bottom of a downspout (Figure 3—38). Similarly, runoff from a roof or paved surface can be retained in a cistern, a large tank typically constructed underground. A cistern was a common method for storing water on farms before electric pumps became commonplace in wells.
A less conventional means for conserving water on a residential site is to use recycled water or “gray water” as a source for watering the landscape. Gray water originates
from various household washing practices such as dishwashing, laundry, and showers and is typically drained into a septic system or municipal sewer. Yet this water is only mildly “polluted” if no toxins or chemicals are used in washing. It should be noted that gray water is not sewage that originates from toilets. With proper plumbing changes, the water from sinks, showers, dishwashers, and laundry machines can be drained to a holding tank where it can be a source for watering plants. It is often suggested that gray water be combined with tap water before applying it to the landscape. This is a particularly useful concept in arid climate regions where any water is beneficial, though gray water should not be used for root or potted plants.