For the occasional backyard fruit tree here are a few extra factors:
Cherries } - .85 for mature trees
Almonds } - .75 for mature trees
Citrus - .60 for mature trees
As you can see from the table and fruit tree factor examples, a landscape or ornamental tree will be very close to the .80 general factor. The 1.0 factor for shrubs means that unless we know the plant factor for the particular shrub, we are not factoring back or reducing the expected water requirements from the 100% level.
A warning note concerning existing plant material is appropriate here. If you have a plant that is thriving on your project site without any irrigation, even in mid-summer, you could possibly damage or kill it through irrigation. I have seen it happen on landscape irrigation projects particularly in arid climates. A large, native tree for instance, may be surrounded by a new lawn area and sprinkler system. The water to maintain the turf over waters the tree and so ends the life of a one-hundred year old climatic veteran.
The final variable in the formula for gallons per plant per day is the divisor used for the efficiency of drip irrigation systems. No irrigation system is 100% efficient. A 100% efficient system would be one where every drop of water delivered is used by the plant.
Irrigation efficiency, then, is defined as the percentage of irrigation water available for consumptive use by the plant material.The efficiency numbers in the chart below are in decimals representing those percentages for various climate types. These factors are fairly accurate as long as the wetted pattern on the soil surface from one emitter doesn't exceed two feet in diameter.
If, for example, you had a high-flow rate emitter, on tight clay soil, producing a nine-foot diameter puddle on the surface, you couldn't expect a very high efficiency rating.