How to Calculate Crop Water Requirements

Why Crop Water Requirements Matter

Knowing exactly how much water your crops need is the foundation of efficient irrigation management. Over-irrigating wastes water, increases energy costs, leaches nutrients below the root zone, and can promote root diseases. Under-irrigating causes crop stress, reduces yields, and in severe cases kills plants outright. The difference between a profitable season and a disappointing one often comes down to applying the right amount of water at the right time.

The standard method for calculating crop water requirements starts with reference evapotranspiration (ET₀) and adjusts it using a crop-specific coefficient. This approach, developed by the Food and Agriculture Organization of the United Nations (FAO), has been the global standard since 1998 and is used by irrigation engineers, extension agents, and farmers worldwide.

Key Concepts

Reference evapotranspiration (ET₀) represents the water demand of a standard reference grass crop under specific weather conditions. It accounts for solar radiation, temperature, humidity, and wind speed using the Penman-Monteith equation from FAO Irrigation and Drainage Paper No. 56. ET₀ is expressed in millimeters per day (mm/day) and varies with weather conditions and time of year.

Crop coefficient (Kc) adjusts ET₀ for a specific crop and growth stage. A newly planted corn field might have a Kc of 0.30 (initial stage), rising to 1.15 at mid-season when the canopy fully covers the ground, then declining to 0.60 as the crop matures. FAO-56 publishes Kc values for dozens of crops.

Crop evapotranspiration (ETc) is the actual water need of your crop: ETc = ET₀ × Kc. This is the net amount of water that must be replaced through irrigation and/or rainfall.

Worked Example: Corn in Central Kansas

Suppose you are growing corn in central Kansas during July. The weather station reports conditions that produce a reference ET₀ of 6.5 mm/day. Your corn is at mid-season growth stage, so the crop coefficient Kc is 1.15 (from FAO-56 tables).

First, calculate the actual crop water use:

• ETc = ET₀ × Kc = 6.5 × 1.15 = 7.48 mm/day
• Weekly water need = 7.48 × 7 = 52.3 mm per week (about 2.06 inches)

Next, account for irrigation system efficiency. A center pivot system typically operates at about 80% efficiency (some water is lost to evaporation and wind drift):

• Gross irrigation = 52.3 / 0.80 = 65.4 mm per week (about 2.57 inches)

For a 10-acre field, this translates to roughly 177,000 gallons per week. If you receive 15 mm of effective rainfall during that week, you can reduce your irrigation to about 50.4 mm gross application.

Tips and Common Mistakes

• Do not use peak Kc all season. Crop coefficients change with growth stage. Using mid-season Kc during initial growth overestimates water needs by 3-4 times.
• Account for effective rainfall. Not all rainfall reaches the root zone. Light showers under 5 mm often evaporate before infiltrating. Use the USDA-SCS method to estimate how much rainfall actually offsets irrigation.
• Check ET₀ frequently during heat waves. A single day above 40°C (104°F) can push ET₀ above 10 mm/day, more than double a typical summer value. Monitor conditions and adjust your irrigation schedule accordingly.
• Consider soil water holding capacity. Sandy soils hold less water than clay loams, so you may need to irrigate more frequently with smaller applications even if the total weekly volume is the same.