Part 2 of a three-part series on irrigation water requirements

by Bruce K. Ferguson School of Environmental Design,
University of Georgia

Irrigated landscape plantings must contribute to both the creation of livable communities and the efficient management of water resources. Designers need to know the amount of irrigation water a proposed planting requires in order to knowledgeably balance water use against the ultimate landscape effects.

The previous article in this series pointed out the different water demands of different types of plants. This article describes how to estimate the water requirement of an overall site.

If the estimated water requirement for a proposed plan appears unnecessarily high, a designer could reduce the demand by reducing the areas of plants with high demands and increasing the areas of materials with lower demands. Some techniques for advantageously arranging water-conserving plantings were discussed in the previous article.

When the design is finalized and its water requirements recalculated accordingly, the peak monthly water demand tells the designer what short-term capacity the irrigation hardware and its water supply must have; the total volume of water over an average irrigation season tells how much the site will draw upon water supplies year after year.

Much as estimates of water demand are needed for design, and much as designers try to make those estimates as reasonable as possible, the results are still only estimates based on general knowledge. Actual on-site water requirements are ultimately determinable only by experience with operating systems after installation is complete.

The Blaney-Criddle
Method for Evapotranspiration

The major component of a planting’s water demand is plant evapotranspiration. One way to estimate evapotranspiration is the Blaney-Criddle method (Blaney and Criddle, 1962; U.S. Soil Conservation Service, 1970.

The t and p factors take account of the climate where a planting is located. Values for the monthly proportion of daytime hours of the year (p) at given latitudes are listed in Table 1. Local monthly air temperature can be obtained from local climatic records such as those of Ruffner and Bair (1987). For establishing system capacity, the month with peak short-term evapotranspiration rate can be identified by calculating (tp / 100) for each month; the month with the highest product has the highest irrigation demand.

The annual Blaney-Criddle formula applies to total evapotranspiration over an irrigation season, which is assumed equal to the growing season, the time between the last frost in the spring and the first frost in the fall. Local frost dates can be found in climatic records.

To derive the annual climatic factor F, construct a table listing t and p for each month in the irrigation season. Find (tp /100) for each month, and sum the products.

Total Irrigation Requirement

The total quantity of water that must be applied to a site is influenced by irrigation efficiency and rainfall, as well as evapotranspiration Researchers have proposed many different ways to take these and other factors into account. The following formula takes account of the important factors (Ferguson, 1988):

Ir  =  Et /E?EUR??,,????'??+R
where,
Ir  =  total irrigation requirement, inches per unit of time;
Et  =  evapotranspiration, inches per unit of time;
E  =  efficiency of the irrigation system in delivering water to the plant, decimal fraction (see Table 2); and
R  =  rainfall, inches per unit of time.

Values of rainfall (R) can be obtained from local climatic records such as those of Ruffner and Bair (1987).

To establish a system’s required capacity, estimate the total irrigation requirement (Ir) for each planted area of a site during the month with the peak irrigation demand. Use Blaney-Criddle’s u (inches per month) for evapotranspiration (Et). It is prudent to assume dry weather conditions, by assuming that monthly rainfall (R) is zero and assuming that humidity is low when reading system efficiency from Table 2.

When estimating annual IR, use Blaney-Criddle’s U (inches per year) for evapotranspiration (Et). For rainfall (R); use the total rainfall during the irrigation season.

Water Volume

The number of gallons of water that are applied to an area over a period of time can be estimated to compare water requirements of alternative site designs, or to compare requirements of proposed developments with available water supplies. Water volume is given by (Ferguson, 1988):

Wv  =  0.623 Ir A
where,
Wv  =  volume of irrigation water, gallons per unit of time;
Ir  =  total irrigation water requirement, inches per unit of time;
A  =  area of irrigated plants, square feet; and
0.623  =  conversion factor, gallons per inch-square foot.

The area (A) of a lawn or a densely planted bed of shrubs or ground cover is equal to the area of the lawn or bed. For individually irrigated shrubs or trees, A is equal to the area of each plant’s crown (the area inside the drip line) multiplied by the number of similar plants.

The total volume required for a site containing different types of plantings is the sum of the volumes for all the planted areas.

The final article in this series will show an example of using these evaluations to develop alternative water-conserving planting 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 Number 29. Washington, D.C.: U.S. Agricultural Research Service.

Ferguson. Bruce K. 1988 (in press). Using Water Effectively. Chapter 3 of Irrigation, Landscape Architecture Handbook Volume 3, Washington: Landscape Architecture Foundation. Ruffner. James A, and Frank E. Bair. 1987. Weather Almanac. Detroit: Gale Research Company.

U.S. Soil Conservation Service. 1970. Irrigation Water Requirements. Technical Release 21. Washington, D.C.: U.S. Soil Conservation Service.

Bruce K. Ferguson is a registered landscape architect with 14 years experience in practice, teaching and research. He is a specialist in water resources in site development. He has written more than 100 articles, books and reports in this field including On-Site Stormwater Management. He is Associate Professor of Landscape Architecture at the University of Georgia, and is instructor for ASLA’s popular continuing education courses on stormwater management. In consulting, he has worked with design firms and Federal and State agencies on unique, difficult and innovative problems of water conservation and watershed management. The water conservation project reported in the final article in this series resulted from such a consulting project. He studied architecture and landscape architecture at Dartmouth College and the University of Pennsylvania.