Calculator
Example data table
| Crop | Stage | Typical Kc range | Notes |
|---|---|---|---|
| Leafy greens | Initial | 0.30–0.50 | Low canopy coverage early on |
| Tomato | Mid-season | 1.00–1.20 | High transpiration at full canopy |
| Maize | Late-season | 0.60–0.90 | Declines as crop matures |
| Orchard | Development | 0.60–0.95 | Depends on ground cover and spacing |
These are general ranges for planning. Local climate, cultivar, and management can shift Kc.
Formula used
- Crop coefficient Kc = ETc ÷ ETo
- Crop evapotranspiration ETc = Kc × ETo
- Period total ETc_period(mm) = ETc(mm/day) × days
- Gross depth Gross(mm) = Net(mm) ÷ efficiency
- Volume Volume(m³) = Gross(mm) × Area(m²) × 0.001
Use matching time windows: daily with daily, weekly with weekly.
How to use this calculator
- Select a mode: compute Kc, compute ETc, or estimate irrigation needs.
- Enter ETo from a station or trusted dataset for the same period.
- Provide ETc if solving for Kc, or provide Kc if solving for ETc.
- For irrigation volume, also enter area and efficiency.
- Press Submit to see results above the form, then export.
Professional notes
ETo inputs and timing
Use ETo from a nearby weather station, preferably computed with a consistent method across your season. Match the averaging window to your crop observation window; daily ETo pairs with daily ETc, while weekly ETo pairs with weekly ETc. When ETo is biased high, Kc will appear too low, and irrigation plans may be under-sized. If station exposure differs from your field, adjust expectations and document the source.
Kc as a scaling factor
The coefficient is dimensionless and scales reference demand to crop demand. Kc rises as canopy closes, peaks around full cover, then typically declines during senescence or harvest preparation. Use stage hints as a quick check, not as a replacement for measured or locally published coefficients. Row spacing, pruning, and mulches can move Kc within the same nominal stage.
ETc interpretation for scheduling
ETc represents water lost through evaporation and transpiration under well-watered conditions. Multiply ETc by days to estimate the period depth, then compare that value with rainfall and soil storage. For drip systems, consider frequent, smaller applications; for sprinklers, account for wind drift and distribution uniformity. In sandy soils, smaller intervals help reduce deep percolation losses.
Efficiency and field losses
Efficiency converts a net crop requirement into a gross applied depth. Use lower values when you expect runoff, deep percolation, leaks, or poor coverage, and higher values for well-managed drip. A 10% efficiency improvement reduces gross water demand by about the same proportion across the period. Track pressure, emitter clogging, and uniformity tests to keep your assumption realistic.
Units, area, and records
Depth is reported in millimeters; volume uses the area in square meters and a millimeter-to-meter conversion. Log ETo source, stage, and notes each time you calculate; those details support audits and seasonal tuning. Re-check coefficients after major weather shifts, canopy changes, or irrigation hardware adjustments. Over time, compare calculated depth with soil moisture readings to refine decisions. Keep a simple weekly log to spot trends and prevent stress early.
FAQs
1) What does Kc represent?
Kc scales reference evapotranspiration to crop evapotranspiration for the same climate conditions. It reflects canopy cover, crop height, and surface wetness.
2) Can I use Kc values from another region?
You can start with published values, but verify locally. Differences in humidity, wind, planting density, mulches, and management can shift the effective coefficient.
3) Why does my calculated Kc seem too high?
Common causes include underestimated ETo, ETc measured after irrigation or rainfall, or atypical microclimates. Confirm units and ensure both ET values cover the same timeframe.
4) How should I choose irrigation efficiency?
Use realistic field performance. Well-managed drip can be high, while sprinklers in wind or uneven terrain are lower. If unsure, start conservative and refine using observed runoff and soil moisture.
5) Does ETc include rainfall?
No. ETc is atmospheric demand under well-watered conditions. When scheduling, subtract effective rainfall and consider soil water storage before deciding how much to apply.
6) How often should I update Kc?
Update when growth stage changes, canopy expands, or management shifts. Weekly checks are common, and after heat waves or pruning events, recalculating helps keep irrigation aligned with demand.