At about this point in a drip design a question usually comes up similar to this one. "You have used the highest number for P.E.T.,the higher factors for plant type, and further bumped up the gallonage by the efficiency factors. We thought drip irrigation was supposed to save water. Where's the preciseness so often touted for drip systems?"
The answer to this question is through system control. You designed for mature plant material, but that's a few years down the road. You designed for mid-summer water use, but what about the other seasons?
The reduced application, for plant immaturity or cooler seasonal water requirements, is an operational or maintenance responsibility. Any reductions from peak water use and availability are accomplished through the controls of the delivery system. Irrigation time is the main means of controlling the amount of water delivered.
The controller (automatic irrigation timer) for the system requires some adjustments to tailor the delivery of water to the requirement. It may be switched off in winter, switched on with adjusted time for spring and adjusted once again for hot or dry weather.
Some of today's modern controllers not only time in hours for drip systems, but have water budgeting for reducing or adding time in percentage increments. From your knowledge of the project site, landscape plan and local climate you are now ready to requirements for your plant materials. Here's an example of the G/P/D formula applied.
Problem: What is the daily water requirement for a 15' diameter tree in a warm, dry climate?
Mature Tree = .80
Plant area 15 x 15 x .7854 = 176.71 sq. ft.
Climate factor worst case = .25
Drip efficiency for warm dry area = .85
.623 x 176.71 x .80 x .25 = reduced with your calculator to: 22.018
= 25.9 or 26 gallons per day peak use
Example Number Two:
What's the daily water requirement in a hot humid region for a plant area in a shrub bed where the plants are spaced three feet on center in a square pattern?
Shrub = 1.0
Plant area 3' x 3' = 9 square feet
Climate factor worst case = .30
Drip efficiency (humid) = .95
.623 x 9 x 1.0 x .30 = reduced to: 1.682 =1.77 gallons per plant area per day, worst case
In the drip system design process you would figure up the water requirements for all the various types of plants on the project and have that information ready for your next step, selecting the number of emitters per area.