Irrigation Scheduling: When and How Much to Water

Why Irrigation Scheduling Matters

Irrigation scheduling answers two critical questions: when to irrigate and how much water to apply. Getting these answers right saves water, reduces pumping costs, and maximizes crop yields. Research from university extension services consistently shows that farmers who schedule irrigation based on soil moisture data use 15-25% less water than those who irrigate on a fixed calendar, often with equal or better yields.

The key principle is simple: let the soil tell you when it needs water. Soil acts as a reservoir, holding water between field capacity (fully charged) and wilting point (empty). Your job is to refill the reservoir before the crop runs out of easily available water.

Key Concepts

Field capacity (FC) is the maximum water a soil can hold against gravity after excess drainage stops, typically 24-48 hours after saturation. Sandy soils hold about 1.0 inch per foot of depth, while clay loams hold 2.0 inches or more per foot.

Permanent wilting point (PWP) is the moisture level at which plants can no longer extract water from the soil. The difference between FC and PWP is the available water capacity (AWC), which is the total water reservoir your crops can access.

Management allowable depletion (MAD) is the percentage of available water you allow the crop to use before irrigating. Most crops perform best when MAD is set to 50%, meaning you irrigate when half the available water is gone. Sensitive crops like lettuce may need a lower MAD of 30%, while drought-tolerant crops like grain sorghum can handle 60% depletion without yield loss.

Worked Example: Tomatoes on Sandy Loam

You are growing tomatoes on a sandy loam soil in California's Central Valley. Here are your field parameters:

• Soil texture: Sandy loam (AWC = 1.5 inches per foot)
• Effective root depth: 24 inches (2 feet)
• MAD for tomatoes: 50%
• Daily crop water use (ETc): 0.25 inches/day

First, calculate the total root zone water storage:

• Root zone AWC = 1.5 in/ft × 2 ft = 3.0 inches

Next, determine the allowable depletion before irrigating:

• Allowable depletion = 3.0 × 0.50 = 1.5 inches

Now calculate the irrigation interval:

• Days between irrigations = 1.5 / 0.25 = 6 days

Finally, determine the application depth, accounting for 80% system efficiency:

• Gross application = 1.5 / 0.80 = 1.875 inches per irrigation

In practice, you would irrigate every 6 days with about 1.9 inches of water. After rain, check soil moisture and delay irrigation if the soil has recharged significantly.

Tips and Common Mistakes

• Check soil moisture before irrigating. A simple hand-feel test or a tensiometer reading can confirm whether your field actually needs water. Many farmers irrigate too early out of habit.
• Adjust for rainfall. After a 0.5-inch rain, you can likely delay your next irrigation by 2 days. Use the USDA-SCS effective rainfall method to estimate how much of each rain event actually reaches the root zone.
• Avoid deep percolation. Applying more water than the root zone can hold pushes water (and dissolved fertilizer) below the roots. Split heavy applications into two lighter ones if your soil drains quickly.
• Update ETc as the season progresses. Crop water use changes with growth stage and weather. A tomato plant uses 0.10 inches/day after transplant but 0.30 inches/day at peak fruit set.