By Jeffrey Knight
In order to maximize a drip irrigation system’s performance, you must set proper run times. To achieve this, you should first ask yourself these very important questions: How much water does the plant material actually need, and how much water is currently being applied?
To uncover your target run time, follow these simple steps to quickly and easily make adjustments, and move on to the next job site.
Step 1: Consider your plant material
One method of determining the amount of water required by a plant is to consult the experts from which you purchased your plant materials, as they should have this information readily available.
Another method is to find evapotranspiration (ET) data posted online through universities and weather stations. Typically this ET data is based on a reference crop, such as turfgrass, over a period of time (daily, monthly or annually).
Many people make the mistake of planting plant material that requires vastly different amounts of water, with the intention of watering them with the same drip system. In order to avoid the problems that result from this type of scenario, either separate the plants into different drip zones and adjust the watering times accordingly, or avoid mixing these types of plant life.
Step 2: Determine your application rate
Once you have determined how much water your plants actually need, evaluate your drip system to find out if the amount of water being applied matches the actual plant water requirement. To discover the amount of water being applied — also known as the application rate — of the drip system, try this simple formula:
Application Rate (in/hr) = GPH x 1.604 / irrigated area (in square feet)
Let’s try it out. You have a small shrub that has two drip emitters installed on it. Each dripper puts out one gallon of water per hour (1 gph). The total irrigated area of the shrub is four square feet. What is the application rate of the shrub? [Note: 1.604 is a constant that converts the GPH flow rate in a given area (in square feet) to inches per hour.]
2 GPH x 1.604 / 4 = 0.80 in/hr
Let’s do another. Sixty feet of drip line is used to irrigate an entire planting bed. The drip line has one gallon per hour emitters pre-installed every 18 inches, for a total of 40 emitters on the line. The planting bed covers an area of 75 square feet. What is the application rate?
40 GPH x 1.604 / 75 = 0.86 in/hr
Easy, right? Now you can figure out the appropriate run times for the system (and impress your friends at cocktail parties).
Step 3: Adjust run times
Once you know how much water your plants require, and the rate at which your drip system is applying water, it’s easy to figure out a basic run time for your drip irrigation system.
Run Time = (Inches of Water Required / Application Rate) x 60
For example, the expert at the nursery told you the shrub you bought requires 0.3 inches of water each week during its peak watering month. You have already calculated the application rate of the drip system to be 0.80 in/hr. How long do you run the system in order to apply the weekly demand of water?
(0.30 inches / 0.80 in/hr) x 60 = 23 minutes of runtime for the week
There you have it! You will need to run the drip system for approximately 23 minutes a week in order to give the shrub the proper amount of water. From here, you will need to make visual observations of the system to control run-off, ensure that the quality of each plant is as high as possible, and make any necessary adjustments to the system accordingly.
Keep in mind, if a shrub that requires 0.3 inches of water per week is installed in the same irrigated area as a shrub that requires 0.8 inches of water per week, you are going to have issues. If left alone, you will have to over water one plant or under water the other. To fix it, you can:
A) Use plants with a similar watering requirement (one of the plants above will have to go).
B) Separate the watering of the two plants in separate drip zones, and adjust the watering times accordingly.
C) Install more drippers on the plant that requires more water so its needs are met during the 23-minute runtime (in this case you would need to increase its application rate to 2 in/hr).
Follow these simple steps on your next drip irrigation project, and you’ll be calculating run times in no time.
Jeffrey Knight, CIC, CID, CLIA, serves as educator and national trainer for Ewing Irrigation Products.