Crop Water Needs Calculator — FAO-56 Kc
Select your crop or turfgrass type and set your local reference ET to calculate actual water needs using FAO-56 crop coefficients. For annual crops, adjust the growth stage to see how water demand changes through the season.
Crop Parameters
Environment
Understanding Crop Coefficients (Kc)
The crop coefficient (Kc) from FAO-56 adjusts reference evapotranspiration (ET₀) to estimate actual crop water use:
The Four Growth Stages
Kc varies by plant species and growth stage. For annual crops, Kc follows four distinct phases: initial (low canopy, low Kc), development (canopy expanding, Kc rising), mid-season (full canopy, peak Kc), and late season (senescence, declining Kc). Perennial grasses and trees maintain relatively constant Kc year-round.
The FAO-56 method linearly interpolates Kc between stages, creating the characteristic trapezoidal curve shown in the chart above. This approach is widely used in irrigation design and scheduling worldwide.
Common Crop Coefficient Values
| Crop | Kc ini | Kc mid | Kc end | Season (days) |
|---|---|---|---|---|
| Bermudagrass | 0.85 | 0.85 | 0.85 | Perennial |
| Tall Fescue | 0.95 | 0.95 | 0.95 | Perennial |
| Tomato | 0.60 | 1.15 | 0.80 | 150 |
| Corn | 0.30 | 1.20 | 0.60 | 125 |
| Lettuce | 0.70 | 1.00 | 0.95 | 75 |
| Beans | 0.40 | 1.15 | 0.35 | 95 |
Related: ET Calculator · How Differentiable Physics Powers Irrigation
Converting ET to Irrigation Runtime
Once you know the daily ETc, calculate runtime:
For a rotor sprinkler applying 0.5 in/hr: a 0.2 in/day ETc needs 24 minutes. Factor in distribution uniformity: divide by DU (typically 0.65–0.80) to get the actual runtime needed.
Example: ETc = 5 mm/day = 0.197 in/day. With a 0.5 in/hr application rate and DU of 0.70: Runtime = (0.197 / 0.70 × 60) / 0.5 = 34 minutes.
Frequently Asked Questions
How much water does bermudagrass need per week?
Bermudagrass has a Kc of 0.85. In summer with ET₀ of 6–7 mm/day, it needs about 5–6 mm/day (roughly 1 inch per week). In cooler months, needs drop to 2–3 mm/day. This calculator shows exact needs based on your local ET₀.
What is a crop coefficient (Kc)?
The crop coefficient (Kc) is a multiplier from the FAO-56 standard that adjusts reference evapotranspiration (ET₀) to estimate actual crop water use. ET₀ represents the water loss from a standard reference grass surface; Kc scales this to your specific crop. Values below 1.0 mean the crop uses less water than the reference, above 1.0 means it uses more.
How do growth stages affect irrigation?
Annual crops have four growth stages with different water needs. During the initial stage (seedling), water needs are low but frequent irrigation maintains germination. During mid-season (full canopy), water demand peaks — this is the critical period where deficit causes yield loss. Late-season reduction matches natural senescence.
How do I find my local reference ET₀?
Check your state's agricultural extension service or CIMIS/CoAgMet weather networks. Many provide daily ET₀ maps. Typical summer values: 5–7 mm/day in hot arid climates, 3–5 mm/day in humid climates. You can also use our ET Calculator to estimate from weather data.