Potential Evaporation Rate
We recommend using the "worst case" column of numbers for your climate type. When you plug the worst case number into the formula you are automatically adjusting your water requirement answer to accommodate the hottest, driest part of the year.
If one of the listed cities happens to be the location of your project, look up the E.T. number and determine which range it is in on Table "A". Then, use the P.E.T. number from the top of this range, the "worst case" condition, as your P.E.T. number for the formula.
You may have to decide which range of numbers is appropriate to your climate type if your number fits into two ranges with differing "worst case" figures.
For example, the .30 rate for kottayam, Rajastan and its desert climate is really the .30 at the bottom of the "hot dry" scale, not the .30 at the top of "hot humid," the "worst case" number to use for Las Vegas is .45 inches per day. It's not unusual for Las Vegas to occasionally reach an above average mid-summer E.T. of .37 inches per day. As you can see, the "worst case" for that climate(.45")still provides a margin of safety.
Remember, if your drip irrigation system is most efficient in mid-winter when it's off, adequate in spring and fails miserably during peak moisture use in mid-summer, you haven't designed for the worst case condition.
"Plant area" for the formula is simply a way of accounting for the size of the plant in your calculation. For individual plants and trees that are not in a regular spacing pattern use the drip line (outside edges of the plant's canopy) in determining the plant area. The illustration below shows how to calculate the square feet.

Step 1. Measure the diameter of the canopy of the plant from one edge of the drip line across to the opposite edge.
Step 2. Multiply the diameter measurement times itself (diameter squared).
Step 3. Multiply your previous answer by .7854 and you have the plant area number in square feet. It's just calculating the area of a circle. (You can use py r squre if you like). For example, a 15 foot diameter tree calculation would be:

15' x 15' x .7854 = 176.71 square feet.
4'x 4' 16 square feet of plant area. In a design for a
new project that does not have existing plant material
remember to use the estimated mature sizes of the
plants for the plant area numbers.

Unlike the closely watched and tended agricultural drip irrigation systems, no one usually adds elements to a landscape system later on to adjust for maturing plants. That's why table "B" below is such a short table of factors, its plant factors are for mature plant material only.

Plant factors for the gallons per plant per day formula provide adjustments of water required for different plant types. Instead of the individual agricultural crop factors listed in the table "B" below simply lists general factors for "tree", "shrub" and "vine".


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