Orchard Irrigation:  Case Study

The following is an example of how to calculate water needs for a mature peach orchard just prior to fruit harvest.  The orchard is on a deep sandy loam soil with row middles planted to grass cover. 

Water use (expenses)

• ET_r values are 2.10 inches per week (weather station data). 
• Crop coefficient is 0.98 (Growth stage = 130, from Figure 10.3).
• ET_crop  = ET_r × K_crop
• ET_crop  = 2.10 inches/week × 0.98 = 2.06 inches/week

Soil storage capacity (potential bank balance)

• The total storage capacity for readily available water over the 2 foot effective rooting depth is 1.5 inches (Sandy loam soil, Found in Available water holding capacity for different soil textures table).
• 1.5 inches ÷ 2.06 inches per week = 0.73 weeks or 5.1 days between irrigations.

Restated, soil moisture in the root zone will go from capacity to plant stress levels in 5.1 days. To recharge the soil profile, you will need to add 1.5 inches of water. Assuming a microsprinkler irrigation system with an efficiency of 80%, 1.9 acre inches of water application will be required per acre for each watering.

Irrigation efficiency

• To apply the necessary volume of water you must factor in your system’s efficiency. For drip systems efficiency is assumed to be 95%, less for other types of systems. Using the example above you need to apply 1.6 inches of water per application. For micro-sprinklers with an efficiency closer to 80% you would apply 1.9 inches (i.e. 1.5 inches divided by 0.8).

Irrigation duration

• To determine the duration of an irrigation event you need to know the application rate of your system. You can determine the application rate for your specific system using online calculators such as WSU Irrigation Calculator. The calculator uses the emitter flow rate, distance between emitters and lines to determine the application rate of your system. Using the provided example, if the application rate is determined to be 0.262 in/hr you would need to run your irrigation system for 6.1 hours to apply 1.6 inches of water.  

While this example uses a sandy loam soil, storage capacity will decrease in sandier soils. The result is less storage capacity and the need for more frequent, shorter duration, irrigation sessions. Conversely, in heavier soils, irrigation frequency will decrease, but growers should monitor for adequate water infiltration and adjust irrigation sessions to avoid surface pooling of water.

Summary

Good irrigation management requires:

1. An understanding of the soil-plant-water relationship.
2. A properly designed and maintained irrigation system, and a knowledge of the efficiency of the system.
3. Proper timing based on:
   1. Soil water holding capacity.
   2. Weather and its effects on crop demand.
   3. Stage of crop growth.

Each of these components requires a commitment to proper management. Proper irrigation management will provide the most efficient use of water, and will optimize orchard yields in balance with long term orchard health and productivity.