2.3 Micro- vs. Overhead in the Container Nursery


In a study of a container nursery production system, trickle irrigation systems were shown to use less water than overhead watering systems.

Trickle irrigation applies water to individual containers near the growing substrate surface where it infiltrates quickly. The Coverage Patterns figure illustrates the difference in areas watered. Overhead watering means covering all the ground area in a field to water the containers. In the study, summarized in the Trickle vs. Overhead Irrigation Requirements table, a combination of five different container sizes and spacing arrangements were used.

On a per acre basis, the trickle system used 4,000 to 10,000 gal of water instead of 36,000 gal for the overhead system. This is a considerable savings in water. Both methods applied 1 inch of water onto the substrate surface of the container. Roadways were ignored and only container-occupied areas were considered.

Why is there such a large difference? One acre-inch of water (one inch of water on one acre) is 27,160 gal. However, there are evaporative losses in delivering water. Water is lost in the air and from falling on the ground between the plants.

On hot, dry days the evaporation of water delivered by overhead sprinkler may be 25% to 30%, giving an application efficiency of 70% to 75%. This means that 75% of the water reaches the plant root zone. At 75%, the sprinklers throw out 36,200 gal/acre to get 27,160 gal into the ground. Evaporation is greatly reduced with trickle irrigation so the application efficiency may be 90% to 95%.

In both irrigation methods, about 10% of the water may drain through the bottom of the container to remove salts (this is called the Leaching Fraction). Clocks and solenoid valves should be used to control the watering process, as well as a moisture sensor. Water conservation requires checking the soil or substrate moisture and adjusting the water application to plant needs.

Trickle systems vary in the amount of water required because container surface areas and spacings vary. However, in this study all containers received 1 inch of water for the calculations.

The Trickle vs. Overhead Irrigation Requirements table shows the ratio of the container surface area to the area of the ground allotted to the container by its spacing. Landscape trees and shrubs were in the containers so the spacing was wide enough to allow for growth. The water used for the trickle systems ranged from 4,020 to 9,920 gallons per acre, a considerable savings from the 36,200 gal used for overhead watering.

The pumping station and pipe size requirements are also smaller for a trickle system than for overhead with relative cost savings. However, a sand filter was required because surface pond water was used at the nursery where the study was conducted.

This study shows that the water needed for one acre with overhead sprinklers could water three or four acres by trickle irrigation. Longer term perennial crops work best for trickle because labor is needed to set up the emitters in the containers.