Blaney–Criddle Calculator
Enter the daytime-hour factor p and the mean temperature T to compute the Blaney–Criddle reference evapotranspiration ET = p × (0.46T + 8.13) (mm/day), estimating crop water need from temperature and daylight only.
Input Data
Results
At a glance:Enter the daytime-hour factor p and the mean temperature T to compute the Blaney–Criddle reference evapotranspiration ET = p × (0.46T + 8.13) (mm/day), estimating crop water need from temperature and daylight only.
Formula
ET = p × (0.46 × Tmean + 8.13) (mm/day).
$$ET = p\,(0.46\,T_{mean} + 8.13)$$$$ET_c = ET \times K_c$$How to Use
- Enter the daytime-hour factor p (decimal, from latitude/month table).
- Enter the mean temperature Tmean (°C).
- The calculator returns the reference ET.
Blaney–Criddle evapotranspiration example
| Item | Value | Note |
|---|---|---|
| Daytime-hour factor p | 0.28 | from latitude/month table |
| Mean temp Tmean | 25 °C | monthly average |
| Temperature term | 19.63 | = 0.46×25 + 8.13 |
| Reference ET | ≈ 5.50 mm/day | = 0.28 × 19.63 |
| ETc at Kc 0.9 | ≈ 4.95 mm/day | = ET × Kc |
FAQ
What is the daytime-hour factor p and how do I look it up?
p is the percentage of the period's (usually monthly) average daily daylight hours out of total annual daylight, reflecting how long that period's days are and thus how much solar energy is available for evapotranspiration. It depends only on latitude and month and is read from standard tables (p for each latitude and month). High-latitude summers have long days and large p; near the equator p stays near 12 hours with little variation. Enter it as a decimal (e.g. a table value 0.28). Wrong latitude or month causes systematic error.
How is Blaney–Criddle different from Penman–Monteith?
Blaney–Criddle uses only temperature and daylight (latitude), with minimal data needs; it suits areas lacking full weather observations and monthly-scale estimates, but ignores wind and humidity, so short-term and extreme-climate errors are larger. Penman–Monteith (FAO-56) is a physical equation combining energy balance and aerodynamics, needing temperature, radiation, wind, and humidity — the most accurate and FAO's standard for reference ET. Prefer Penman–Monteith when full weather data exist; use Blaney–Criddle only for a first estimate when data are limited.
Why can temperature alone estimate evapotranspiration?
Because temperature and day length are the two main drivers of evapotranspiration and correlate fairly well with other factors (radiation, etc.). High temperature means more energy and greater air capacity to hold water vapor (saturation vapor pressure), boosting transpiration; the daylight factor captures day length and sunshine duration, indirectly reflecting available energy. Blaney–Criddle linearly combines the two with empirical coefficients to approximate ET. This works well on time-averaged (monthly) and typical climates, but accuracy drops in extreme conditions — dry and windy or humid and calm — because wind and humidity are not directly included.
Is the computed ET the crop water need?
No. Blaney–Criddle computes the reference evapotranspiration ET — the ET of a standard reference crop (short grass), representing the climate's evaporation drive. The actual crop water need is crop evapotranspiration ETc = ET × Kc, where Kc varies by crop and growth stage (small at seedling, large at peak, falling at maturity). So to get a specific crop's need at a stage, compute ET with this tool, then multiply by the corresponding Kc. Irrigation amount also needs to subtract effective rainfall and account for irrigation efficiency.
What time scale is Blaney–Criddle suitable for?
FAO recommends Blaney–Criddle for monthly or longer scales, because it is an empirical average relationship that represents the ET drivers better when time-averaged; daily use is less accurate because it omits that day's wind, humidity, and cloud changes. Irrigation planning usually works in monthly ET, then multiplies by days in the month for monthly total ET and by Kc for monthly need. For fine daily scheduling with full weather data, switch to Penman–Monteith.
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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.