The work reported in this paper was supported by the Cobb County-Marietta Water Authority.

Alternative planting schemes can be designed with conservation of irrigation water in mind. Each alternative’s water requirement can be quantitatively evaluated. When the trade-offs of water requirement and landscape effects are known, design choices can be made.

Previous articles in this series showed how different kinds of plants require different amounts of irrigation water, and how the Blaney-Criddle method can be used to estimate a site’s total water requirement. An application of these principles to a site in the northern suburbs of Atlanta, Georgia illustrates how alternative designs can be produced, incorporating water conservation to different degrees, and how their evaluation can guide knowledgeable design choices.

The site is a new administration building for the Cobb County-Marietta Water Authority. The natural environment of the site is xeric, on a ridgetop with the Atlanta skyline on the horizon. During droughts in 1986 and 1988, the Authority had imposed restrictions on outdoor water use, which had consumed up to 50% of the water supplied by the Authority on some summer days. When the Authority began to build its new administration building, it wanted to make sure that adequate maintenance of the site would not require an excessive amount of irrigation water.

The planting plan shown in Figure 1 had been proposed. The plan employs plant materials which had all been time-tested in the Atlanta area, such as photinia, ilex, juniper, nandina, abelia, liriope, day lily, red maple, and Bradford pear. The plan includes detailed use of colorful shrubs, perennials and annuals near the building entrance, a few crape myrtles spotted in turf at the driveway entrance, a rhythmical sequence of large and small evergreen shrubs along the curving side of an existing white water tank, a bed of native Southeastern wildflowers at the back, and shade trees growing out of a flowing matrix of Bermuda turf. The design successfully interprets scale and space, appropriately focuses on the most visible and active parts of the site, and uses locally available plant materials knowledgeably. However, water conservation, the quantitative water requirement of the proposed planting was first evaluated so that any design alternatives could be compared with it. According to the Blaney-Criddle formula, the water requirement of a planting depends on both the type of planting and the climate where the planting is located.

Table 1 shows the resulting water requirement estimate, based on the long-term average data for the region. A table set up in a similar way could be used to evaluate water requirements in your own projects. The maximum water requirement in inches per month indicates the necessary capacity of the irrigation system for each planted area. The annual water requirement in inches per year indicates the planting’s long-term intensity of water use. In this design, the highest intensities are in turf and where shrubs or trees are layered over grass or lower shrubs. The lowest intensities are in native wildflowers and where trees and large shrubs are planted over mulch. (The wildflowers would require some irrigation during establishment and occasional dry summers; the zero irrigation listed in the table is only an approximation based on long-term average data.) The overall water requirement for the plan as a whole is nearly a quarter of a million gallons of water in an average year.

Two alternative designs were produced to try to reduce the site’s water requirement while being sympathetic with the aesthetic intentions of the original plan. This was done by modifying the original plan in the areas adjacent to the drive and parking lot and at the sides of the building, which are the least visible and active parts of the site. In these areas subtle modifications could be made that would be little noticed by a visitor to the building.

The next plan (Figure 2) modified the secondary areas of the site more aggressively. In addition to the modified ground cover, the species of trees and shrubs at the sides and rear of the site were exchanged for xeric species that would require irrigation only during establishment. Books about the region’s ecology (Godfrey, 1980; Wharton, 1977) listed native species that occur naturally on xeric sites, including five upland oaks, three pines, sourwood, mockernut hickory, highbush blueberry, huckleberry, sparkleberry and mountain laurel. From this list, species could be selected that are similar in size and shape to the species they replace, preserving the scale and rhythm of the original design. The risk of such an exchange is that the replacement species would alter color and texture in the secondary areas, be somewhat difficult to obtain, and not be as time-tested as the original species.

According to regional rainfall data, the second design required only half of the water required by the original design. This reduction is accomplished without alteration in the number, size or arrangement of trees and shrubs, and no alteration at all of the critical areas near the building and drive entries. In addition, eliminating the need for irrigation from the center and rear of the site reduces the required length of lateral pipes, and hence irrigation construction cost.

The planting that has now been installed at the site combines water-saving ideas from both of the alternative plans. The plants exactly follow the second alternative discussed, with modified ground covers but the original shrubs and trees retained for their aesthetic effectiveness and known survival rates and growth habits. The irrigation system covers only the areas suggested by Figure 2, so that a 46% water-saving is possible in an average year. The absence of long-term irrigation for the shrubs and trees in the secondary areas is compensated by water-holding soil amendments and the availability of temporary irrigation during establishment.

This case study shows that substantial water savings are possible. The planting design sets the water requirement. Alternative plans can be generated and evaluated. The need for water conservation can be balanced against the need for aesthetics, shade, screening, and other demands upon the environment which the planting design is intended to meet.

References:

Godfrey, Michael A., A Sierra Club Naturalist’s Guide to the Piedmont. San Francisco: Sierra Club. 1980

Wharton, Charles H., The Natural Environments of Georgia. Atlanta, GA: Department of Natural Resources. 1977.

Ruffner, James A., and Frank K. Bair. The Weather Almanac. Detroit, Ml: Gale Research Company. 1987.