The fundamental choice of how much irrigation water a site will require is made during the site layout and planting design. Once the number and types of plants are decided upon, the amount of water required for adequate maintenance of the plants is fixed. Conservation of irrigation water starts by considering the water requirements of plants in the planting design. Selecting and arranging plantings for low water demand can among a designer’s most effective tools for water conservation. Which types of plants need less water than others? How can those plants be arranged on a site, and how do they alter landscape effects?

A common misconception about irrigation water demand is that it is the same as “potential evapotranspiration” (PEt). Technically, PEt is defined as the maximum amount of water that can be evaporated or transpired into the atmosphere based on such factors as air temperature, wind speed and solar radiation. PEt is a climatic maximum and does not take account of the lower rates of evapotranspiration that may occur in specific types of plantings.

Data on Plants

The best available data on how water demand varies with different types of landscape plants is listed in Tables 1 and 2. Those tables show the factors for plant types used in the Blaney-Criddle method of estimating evapotranspiration. The Blaney-Criddle method, like any formula based on general site conditions, provides only an estimate of evapotranspiration rates. However, unlike PEt, it takes into account the different water usage rates of different types of plantings (Blaney and Criddle, 1962).

The data in Tables 1 and 2 are of two kinds, a maximum monthly factor k and an average annual factor K. The annual K is always less than the peak monthly k, since the annual figure is a long-term average. The average annual factor is more important for long-term water usage; the maximum monthly factor is important for short-term peak rates of use.

The water demand for each type of planting is directly proportional to its plant-type factor. If you remove from a planting plan a plant with a factor of 1.0, and replace it with a plant having a factor of 0.5, you have reduced the water demand by half.

For many individual species of landscape plants, researchers have not yet derived quantitative plant factors. Until more detailed results become available, the general factors in Tables 1 and 2 can be used as the best available estimates.

More data is available for turf grasses than for other types of landscape plants. Table 2 lists complete monthly k factors for Bermuda grass as a function of month during the growing season and average temperature during the month. The Toro Company published monthly Blaney-Criddle Bermuda grass evapotranspiration estimates for 360 locations in the United States and Canada (Toro Company, 1966). Turf grass factors are available for use in specific locales, such as those for Bermuda grass in the Mesa-Tempe area of Arizona (Erie et al, 1982) and for warm-and cool-season grasses in California (Harivandi, 1986).

Tables 1 and 2 give a general idea of the types of plantings that can be most demanding of water, and most conservative. Turf grasses, particularly cool season species, are big water users, possibly because their fibrous root systems are adapted to picking up soil moisture as quickly as it becomes available. Trees and large shrubs, when planted over nonliving ground cover such as mulch, often have only moderate demands, possibly because their large root systems can draw on deep soil moisture. Layered plantings, where shrubs or trees are planted over grass or lower shrubs, have high water demands because they expose lots of leaf area to the atmosphere for transpiration. Plants that are native or adapted to a site’s natural conditions tend to have low demands for additional water.

List of locally adapted water-conservative varieties are available, such as those published by the California Department of Water Resources, the Denver Water Department, and the Southern Arizona Water Resources Association.

Arrangement of Water Conserving Plantings

The benefits of high water using plants are of most value in the parts of a site where levels of use and visibility are highest, such as pedestrian plazas in shopping centers, entry features at office centers, or the greens on golf courses. In areas with lower levels of use and visibility, lower irrigation rates can be allocated without compromising site functions or human experiences, such as in privacy screens around residences, service areas in shopping centers, and the roughs on golf courses. Although the irrigation rate is allowed to be high in the principal areas, water requirement for the site as a whole can be small if the principal zone is confined to a small area. Select plants to fit the allowed moisture conditions in each area.

Segregate types of plantings by moisture condition. Later, the zones (“sections”, “circuits”) of irrigation hardware will be based on, among other things, these zones of uniform water requirements. Then watering can be scheduled for the unique moisture requirements of each zone. Low water users will not be overwatered, and high water users need not be underwatered.

Proper design of the environmental conditions of a site may further cut down water demand. Shade (from arbors, tall trees, tall buildings or steep slopes) and wind screening (from fences, walls, buildings or planted windbreaks) might reduce local evapotranspiration by shielding plants and lowering temperatures. Mulch?EUR??,,????'??+rock, gravel, wood chips or fabric?EUR??,,????'??+acts like a “windscreen” over the root zone, reducing the amount of water that escapes by evaporation, even if the mulch is pervious enough to let precipitation and irrigation waters into the root zone. Laying out a site with such factors in mind can probably help to reduce water demand, but it is best to leave their effects out of quantitative water requirement projections until more is known about them.

Landscape Effects of Water-Conserving Plantings

A site lay-out that conserves water need not sacrifice aesthetics, shading, screening, or other desired landscape effects.

Thayer (1982) evaluated acceptance of water-conserving materials by having people fill out questionnaires as they walked through small landscaped enclosures during California’s dry season. In each enclosure the species of trees and shrubs and overall layout were the same, but the ground cover materials and irrigation rates were different. The results showed that people do not inherently prefer heavily irrigated turf to more water conserving ground covers such as coyote brush, decomposed granite, or bark chips, except for the special purposes of sitting, lying on the ground, or play in the backyards of residences. The only tested groundcover the people disliked for all purposes was inadequately irrigated turf.

Nelson (1985) surveyed residents of condominiums in the San Francisco Bay area that used only 40 percent as much irrigation water as other nearby communities. The residents reported that they were living in lush landscapes, did not know they were living in water-conserving communities, and were pleased with the greenness and variety of their surroundings. In these communities turf areas were consolidated into a few level specimen lawns located in the centers of dwelling clusters, which gave the impression of spacious green areas even though total turf areas were 17 percent smaller than in other communities. Turf was generally not placed near building foundations, along narrow median strips, or within drip lines of trees. Outside the turf areas, mostly water-conserving types of plants were used; high-water-using plants were confined to drainage areas where water accumulated naturally. Discrete splashes of color were added with small quantities of special plants.

The two remaining articles in this series will describe how the Blaney-Criddle method can be used to estimate total water requirement of a proposed planting, and show an example of using such an estimate to develop alternative water-conserving designs for a site in Atlanta, Georgia.

References

Blaney, Harry F., and Wayne D. Criddle. 1962. Determining Consumptive Use and Irrigation Water Requirements. Technical Bulletin No.1275. Washington, D.C.: U. S. Agricultural Research Service.

Erie, L.J., O.F. French, D.A. Bucks and K. Harris, 1982. Consumptive Use of Water by Major Crops in the Southwestern United States. Conservation Research Report Number29. Washington: U.S. Agricultural Research Service.

Ferguson, Bruce K.1988 (in press). Using water effectively. Chapter 3 of Irrigation, Landscape Architecture Handbook, Vol.3, Washington: L.A. Foundation.

Harivandi, Ali. 1986. Turfgrass water conservation?EUR??,,????'??+research and development. Pages 55-60 of Conference on Water Conserving Landscapes (Xeriscape 1986). Oakland, Calif.: East Bay Municipal Utility District.

Nelson, John O. 1985. Money and water saving landscape projects. Pp. 289-305 in Xeriscape ‘85, Proceedings. Santa Ana, Calif.: Mncpl Water Dist.- Orange County. Thayer, Robert L., Jr. 1982. Public response to water-conserving landscapes. HortSciencevol. 17(4): 562-565.