# Drip Irrigation for Trees

by Mike Kuhns, Extension Forestry Specialist

## Design Considerations

• Water must be clean –  filtration often is needed.
• Pressure regulation may be needed. Most drip systems operate at 10 to 30 PSI.
• Consider elevation changes. For every foot of elevation change, pressure changes by 0.433 PSI (increases as elevation drops, or decreases as elevation rises). Keep this in mind when designing drip systems for sloped areas.
• The system must be able to wet at least 50% of the root area of each plant (60% or more is better). Root growth will be confined to moist soil. If the drip system wets too small an area, plants will become root-bound, like a large houseplant growing in a small pot.
• A plant's root area for these purposes is considered to be within the drip-line (under the crown), even though woody plant root systems typically extend well beyond the crown edge.
• The drip system must be designed to meet the needs of a mature plant in the hottest time of the year.
• Emitters must be spaced to avoid toxic salt build-up around plants.
• Emitters should be above ground. This allows visual inspection to see that the system is working. It also prevents root intrusion into emitters.
• Special low pressure backflow preventers should be used when connecting to culinary water systems.

## Calculating Water Use by Mature Plants

Water used or needed per plant is expressed as maximum daily potential evapotranspiration, and is calculated as follows:

Gallons per Plant = 0.623 x Plant Area x Plant Factor x Potential ET (evapotranspiration) per Day / Drip System Efficiency

## 0.623 Conversion Factor

This conversion factor is simply the gallons of water applied when you irrigate one square foot of ground with one inch of water.

## Plant Area

Plant Area = Diameter of drip-line or crown diameter squared x 0.7854 (or radius squared times 3.14 or pi).

Example: A tree has a 10 foot diameter crown, measured from drip-line to drip-line. Plant Area = 10 x 10 x 0.7854 = 78.54 square feet.

For shrubs planted in a grid, multiply plant spacing to get plant area per shrub. For example, if shrubs are planted on a 4' by 4' spacing, 4 x 4 = 16 square feet per shrub.

## Plant Factor

This is a factor that helps correct for the fact that not all plants use water at the same rate under the same conditions. The following are examples of some research-derived plant factors.

• 1.00 Ground covers, flower beds, evergreens, some perennials, small shrubs (under 4' tall), vines
• 0.85 Apples, cherries, walnuts
• 0.75 Pecans, peaches, plums, pears, apricots, almonds
• 0.70 Native plants in semi-arid areas, some ornamental plants, large shrubs (over 4' tall)
• 0.40 Established low-water use native or other low water-use plants

## Potential Evapotranspiration

Potential evapotranspiration (PET) for Moab = 0.35-0.40 inches per day; for most of the rest of Utah (slightly cooler and moister climate) PET = 0.25-0.30 inches per day. Your local Extension office may know the PET, even on a week-by-week basis.

## Drip System Efficiency

Most drip systems are 85 to 90% efficient; 85 to 90% of the water applied is actually used by the plant. This figure is expressed in decimal form in the calculation (85% = 0.85)

## Sample Water Use Calculation

You want to calculate the water needed (maximum daily PET) for a mature, broadleaved shade tree that's not very drought tolerant with a 10 foot diameter crown (measures 10 feet from drip-line to drip-line). The PET for the area is 0.25 inches per day, and the drip system is 90% efficient.

Gallons per day = 0.623 x 78.54 sqare foot plant area x 0.80 plant factor x 0.25 PET / 0.90 drip system efficiency = 10.9 gallons per day used by this tree. Described in a less technical manner, your tree will use a certain amount of water in a day based on its size, its drought hardiness, and the day's weather. However, you have to give it a little bit extra because the irrigation system loses some. The size of your tree is proportional to the area under the canopy, so you multiply the amount of water needed per square foot by the area under the canopy, then convert that to gallons needed per day, which is 10.8 gallons per day.

## Emitter Spacing and Numbers

Soil texture determines how much area is wetted by each emitter:
Sandy texture –  5 to 21 square feet
Loam texture –  21 to 65 square feet
Clay texture –  65 to 161 square feet

To calculate the number of emitters needed for each plant:

Number of Emitters = Area per Plant x % of Area to be Wetted /Area Wetted per Emitter

For example, you have a 15 foot diameter (crown diameter) tree growing on loam soil. You want to wet 60% of the root area. The plant area is 15' x 15' x 0.7854 = 176.71 square feet. Area wetted per emitter is 21 square feet (loam soil, conservative figure).

Number of Emitters = 176.71 square feet x 0.60 / 21 square feet per emitter = 5 Emitters Needed

These 5 emitters would be distributed evenly under the tree's crown.

## Final Considerations

Once you have calculated the amount of water and the number of emitters needed, you will need to decide on an emitter size (flow per unit of time) on a watering schedule (how long you irrigate and how often) that will deliver the needed amount of water. Factors like the infiltration rate of your soil and the timing and duration of your water availability will need to be considered.