Crop Water Requirement (Kc)
Enter crop coefficient Kc, reference evapotranspiration ET₀ and stage days to compute daily crop evapotranspiration ETc and cumulative stage water requirement (mm and m³/ha) for irrigation planning.
Input Data
Results
At a glance:Crop water requirement is the water a crop consumes to maintain normal growth under a given climate and growth stage, expressed as evapotranspiration ETc. It is the product of reference evapotranspiration ET₀ and crop coefficient Kc: ETc = Kc × ET₀. ET₀ is the evapotranspiration potential of a hypothetical standard grass surface under local climate (mm/day), depending only on weather; Kc condenses the water-use difference of a specific crop and stage relative to the standard grass into a single coefficient. Over a crop's life Kc changes in four phases: early (just emerged, bare soil evaporation dominates) ~0.3–0.5; rising to a peak at mid-season canopy closure, often 1.0–1.2; then declining in maturity and senescence as leaves age. Multiplying a stage's daily ETc by the stage days gives the cumulative stage water requirement (mm); multiplying by 10 converts to volume per hectare (1 mm over 1 ha = 10 m³). Estimating cumulative water by stage helps farmers schedule irrigation timing and volume precisely, budget water cost, and compare with effective rainfall to decide supplemental irrigation — the most basic step in agricultural water management.
Formula
Daily crop water need: ETc = Kc × ET₀ (mm/day).
Cumulative stage water: cumulative = ETc × days (mm).
Volume per hectare: V(m³/ha) = cumulative(mm) × 10 (1 mm/ha = 10 m³).
$$ET_c = K_c \times ET_0$$$$W_{stage} = ET_c \times n_{days}$$$$V_{(m^3/ha)} = 10 \times W_{stage}$$How to Use
- Look up the stage crop coefficient Kc from the FAO-56 tables.
- Enter average daily reference evapotranspiration ET₀ (mm/day) and the stage days.
- The tool returns daily ETc, cumulative stage water (mm) and volume (m³/ha).
Approximate crop coefficient Kc by growth stage (common crops)
| Growth stage | Kc range | Note |
|---|---|---|
| Early (emergence) | 0.3–0.5 | Bare soil, mostly surface evaporation |
| Development | 0.5–0.9 | Canopy expanding, use rising |
| Mid-season (canopy closed) | 1.0–1.2 | Fullest canopy, highest use |
| Late (maturity) | 0.4–0.9 | Leaves aging, use declining |
Ranges are approximate and vary by crop, variety and climate. Kc is roughly stable within a stage but must be computed stage-by-stage and summed across stages.
Case Studies
30-day mid-season water need of maize
Maize at mid-season (canopy closed): Kc = 1.15, local average ET₀ = 5 mm/day, stage ≈ 30 days.
Daily ETc = 1.15 × 5 = 5.75 mm/day; cumulative = 5.75 × 30 = 172.5 mm.
Volume = 172.5 × 10 = 1725 m³/ha, i.e. about 1725 t/ha over these 30 days (net irrigation = this minus effective rainfall).
Supplemental irrigation after subtracting rainfall
Continuing the example, suppose effective rainfall over the 30 days totals 100 mm.
Net water depth to irrigate = 172.5 − 100 = 72.5 mm; volume = 72.5 × 10 = 725 m³/ha.
Divide this net need by irrigation efficiency (see the Irrigation Water calculator) for the gross irrigation volume, showing Kc cumulative water is the starting point of scheduling.
FAQ
Where do I look up the crop coefficient Kc?
The authoritative source is FAO Irrigation and Drainage Paper 56 (FAO-56), which lists by crop a few representative values: early, mid and late. Most field crops have mid-season Kc ~1.0–1.2, vegetables slightly lower, fruit trees depending on canopy cover. Local agricultural experiment-station data are even better. After getting the stage Kc values, pair each with that stage's ET₀ and days, compute separately and sum for whole-season cumulative water.
How do I obtain ET₀?
ET₀ depends only on weather. Estimate it with FAO-56 Penman-Monteith (needs air temperature, humidity, wind, sunshine — most accurate) or the Hargreaves method (only max/min temperature, for sparse data); this site has both calculators. You can also get local daily ET₀ reference values from the meteorological or agricultural office. Enter the stage daily average.
Why compute by growth stage instead of one average Kc?
Because Kc changes greatly over a crop's life: 0.3–0.5 early, up to 1.0–1.2 mid, then down again. Using one season-average Kc underestimates the mid-season peak and overestimates early/late needs, making scheduling inaccurate. Stage-by-stage Kc × ET₀ × days summed together matches the crop's actual water-use rhythm.
Is the cumulative water the amount I should irrigate?
No. Cumulative water is the crop's total consumption, part of which comes from effective rainfall. The net depth to irrigate = cumulative water − effective rainfall; divide by irrigation-system efficiency for the actual gross volume pumped. This site's Irrigation Water calculator continues that step. So this tool gives the starting number for planning, not the final intake.
Why is 1 mm of water equal to 10 m³/ha?
1 hectare = 10000 m², and 1 mm depth = 0.001 m, so volume = 0.001 × 10000 = 10 m³. Hence cumulative water (mm) × 10 is the volume per hectare. This conversion lets a depth in mm map directly to tonnes or cubic metres, convenient for estimating water cost and sourcing.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.