Blaney-Criddle ET & Water Use from Temperature
Estimates crop ET
Estimate crop water use (ETc) from temperature and daylight when full weather data is missing — get reference ET0, weekly use and the daily water volume for your field.
Estimate crop water use
Next: replace ~5.5 mm/day; get the p factor from a latitude/month table and the Kc for the crop stage; cross-check with a pan or ET₀ tool where possible.
Blaney-Criddle is a temperature-based estimate (less accurate than Penman-Monteith); best for monthly planning, not daily scheduling.
Blaney-Criddle ET — key facts
- ET0
- p × (0.46 × Tmean + 8.13)
- ETc
- ET0 × Kc
- p factor
- ≈ 0.20–0.32
- Needs only
- temperature + daylight
- 1 mm over 1 ha
- = 10 m³ (10,000 L)
- Best for
- monthly / seasonal planning
- Kc
- crop and stage dependent
- Privacy
- Runs in your browser; nothing uploaded
Crop water use when all you have is a thermometer
The best evapotranspiration equations need humidity, wind and radiation — data many farms simply don't have. Blaney-Criddle was built for exactly that gap: it estimates crop water use from mean temperature and the length of the day. Warmer air and longer daylight mean more evaporative demand, and the simple relationship ET0 = p × (0.46 × Tmean + 8.13) captures it well enough to plan irrigation when nothing else is available.
This tool computes reference ET0 and crop ETc in mm/day, weekly water use, and the daily volume in m³ for your field. Set the daylight p factor for your latitude and month and the crop-stage Kc, and you have a monthly water plan from a thermometer alone. Pair it with the Pan Evaporation, Reference ET₀ and Irrigation Scheduling tools to refine it as more data becomes available.
No weather station
Temperature and daylight are all you need.
Plan the season
Monthly ET for budgeting irrigation water.
Scale to your crop
Stage Kc turns ET0 into real crop use.
Size each irrigation
Daily and weekly volume in m³ for the field.
Frequently Asked Questions
What is the Blaney-Criddle method?+
Blaney-Criddle estimates crop water use from just temperature and daylight hours, making it useful where full weather data is missing. It links evapotranspiration to mean air temperature and the share of annual daytime hours that fall in the month, so a thermometer and a latitude table are enough to schedule irrigation.
How is ET0 calculated?+
The formula is ET0 = p × (0.46 × Tmean + 8.13), where Tmean is the mean daily temperature in °C and p is the mean daily percentage of annual daytime hours. The result is reference evapotranspiration in mm/day, which is then scaled by a crop coefficient to get the crop's actual use.
What is the p factor?+
p is the mean daily percentage of annual daytime hours for your latitude and month. It captures how long the sun is up: longer summer days mean a higher p and more evaporative demand. It typically ranges from about 0.20 to 0.32 and is read from a standard latitude-by-month table.
How do I get ETc from ET0?+
ETc = ET0 × Kc, where Kc is the crop coefficient for the current growth stage. Kc is low at the seedling stage, peaks at mid-season full canopy, and falls at maturity. Using the right stage Kc is the main thing that turns a reference figure into your crop's real water use.
When should I use Blaney-Criddle?+
Use it when you only have temperature data and lack the humidity, wind and radiation needed for Penman-Monteith. It's a practical planning tool for monthly or seasonal water needs. Where you have more data, the Penman-Monteith or Hargreaves approaches give a more accurate reference ET.
How accurate is the temperature-only estimate?+
Because it ignores humidity and wind, Blaney-Criddle can over- or under-estimate in very dry, windy or humid places. It works well for monthly planning in inland temperate climates and is a reasonable first estimate elsewhere. Calibrate against local experience where you can.
How do I turn mm/day into water for my field?+
One millimetre over one square metre is one litre, so 1 mm over a hectare (10,000 m²) is 10 m³. Enter your area and the tool reports the daily and weekly volume to apply, so you can size pumping and irrigation from the ETc estimate.
What temperature should I enter?+
Use the mean daily temperature for the period — the average of the daily maximum and minimum, or a monthly mean. Blaney-Criddle is built around mean temperature, so a representative monthly average gives a sensible planning figure for the season ahead.
How often should I irrigate using this?+
Multiply the daily ETc by the days between irrigations to get the depth to apply each time, then check it against your soil's available water and allowable depletion. Light soils need shorter intervals; heavy soils hold more and can go longer between waterings.
Is this an exact recommendation?+
It's a planning estimate using the standard Blaney-Criddle relationship. Real water use shifts with humidity, wind, crop variety and canopy. Use it to plan monthly and seasonal needs, and confirm against local data or the Pan Evaporation and Reference ET₀ tools where possible.