Orchard Irrigation: Irrigation and Rainfall
In the Intermountain West, rainfall contributes a small fraction of the in-season water requirements of the crop. Therefore, regular irrigation is needed to supply orchard water needs. This irrigation water can be supplied by flood, furrow, impact sprinklers, drip lines or micro-sprinklers.
Whichever irrigation system you utilize, it is important to calibrate your system so that you know precisely how much water is being applied. With sprinklers and micro-sprinklers, the simplest way to do this is to place catch cans in multiple locations in your planting and collect water for a set period of time. The amount of water collected over time will give you an application rate (inches or gallons per hour), and differences in water collected among the catch cans will tell you how uniform the application is within your planting.
When trying to determine application uniformity, it is best to measure output at both ends of your irrigation system. Also, if your planting is on a slope, you should measure output at the highest and lowest points of your field. Elevation differences and the distance the water travels through the irrigation lines both affect water pressure, and consequently the flow rate at the nozzle. If you have trickle irrigation, you can place catch cans under the emitters and determine flow rate for each emitter. Flow rate from each emitter and emitter spacing can be used to calculate rate per area.
The efficiency of your system is a measure of how much you have to over water the wettest spots in the orchard to get adequate water to the dry spots. Efficiency is related to the uniformity of application and to the amount of evaporation that occurs before the water can move into the soil. A well-designed micro-sprinkler or drip system can be 70 to 90% efficient. Overhead sprinkler systems are typically 60 to 75% efficient, while flood and furrow irrigation is typically 30 to 50% efficient.
Tools: Several apps that estimate water use and help schedule irrigation have been developed that will run on a smartphone, tablet, or laptop. Washington State University’s Irrigation Scheduler Mobile (Peters and Hill) is designed for orchards and is simple and user-friendly (must make an account to use). It connects to networks in ID, MT, CO, and WY among other Western states.