Page 16 - Efficient Irrigation and Landscape Design Standards

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This section includes irrigation design criteria for reducing irrigation water use. The standards should be reviewed to ensure compatibility with the water purveyor’s existing water system (i.e. requiring a separate meter and backflow prevention device). Similar to the above provisions for landscape design, this section should be reviewed where a local jurisdiction has existing irrigation standards to identify where they may conflict with water efficient irrigation requirements.
Two methods exist for regulators to control irrigation water efficiency: Distribution Uniformity and Water Budget Allowance. The first method targets the design of the system directly through regulation of the Distribution Uniformity of the system.
5.1 DISTRIBUTION UNIFORMITY
Distribution Uniformity (DU) is a measure of how evenly water is applied across a landscape during irrigation. For example, if one inch of water is applied in one part of a lawn and only half an inch is applied in another part of the lawn, this is considered poor DU. DU is expressed as a percentage between 0 and 100%, although it is virtually impossible to attain 100% in practice. DUs of less than 50% are considered poor, DUs of 60 - 74% are good, and DUs greater than 75% are excellent. Poor DU means that either too much water
is applied, costing unnecessary waste, or too little water is applied, causing stress to plants.
If an irrigation system is 50% efficient (common for most systems) it will take twice as much water to keep a lawn looking green and healthy.
Examples of public facilities from Idaho to Washington that illustrate how poor DU and poor system design looks in
the field across the landscape. This is observable by the numerous dry spots and “green doughnuts”. Most irrigation managers and homeowners adjust watering times to cater to the dry spots. Regardless of how much water is applied or how much you increase in run times, these dry spots will never improve because the system isn’t spraying head-to-head.
C o e u r d ’A l e n e , I D L i b e r t y L a k e , W A S p o k a n e V a l l e y , W A
IRRIGATION DESIGN CRITERIA
DU%
PLANT WATER NEEDS
÷
DU DECIMAL
=
AMOUNT OF WATER NEEDED TO KEEP DRY AREAS GREEN
30%
1 INCH
÷
0.3
=
3.33 INCHES
50%
1 INCH
÷
0.5
=
2.00 INCHES
70%
1 INCH
÷
0.7
=
1.42 INCHES
EXCELLENT (ACHIEVABLE)
GOOD (EXPECTED)
POOR (COMMON)
75%
60%
50%
16 IDAHO WASHINGTON AQUIFER COLLABORATIVE