Blaney-Criddle evapotranspiration calculator

It applies the classic temperature-based relation and reports the result in millimetres per day, together with the intermediate temperature factor. This makes the calculation easy to inspect, repeat, and include in irrigation planning or preliminary water-balance work when a complete weather record is unavailable. The method is an approximation, so its result should be interpreted in light of local climate conditions and the quality of the selected period inputs.

Choose inputs that describe the same period

Start with a mean air temperature in degrees Celsius and a mean daily percentage of annual daytime hours that both refer to the same month or study period. The daytime percentage is normally obtained from a published latitude-and-month table; it is not the number of daylight hours and it is not a decimal fraction. Enter a percentage value such as 7.5 rather than 0.075. This distinction matters because the formula multiplies the percentage value directly by the temperature term. Use a representative mean temperature rather than a daily maximum, daily minimum, or instantaneous observation. If several years of data are available, a climatological monthly mean may be more appropriate for planning, while an observed period mean may be more appropriate for reviewing a particular season. The calculator requires the daytime percentage to be greater than zero because a zero or negative share of annual daytime hours has no valid meaning in this method. Keep the source and period of both inputs with your result so another person can reproduce the estimate and judge whether the assumptions fit the intended decision.

Understand the Blaney-Criddle calculation

The calculator evaluates the relation ET₀ = p × (0.46 × Tmean + 8). In this expression, Tmean is mean air temperature in degrees Celsius and p is the mean daily percentage of annual daytime hours. The parenthesized quantity is returned as the temperature factor, which gives you a transparent intermediate value for checking the arithmetic. The final reference evapotranspiration estimate is returned in millimetres per day. For example, warmer conditions increase the temperature factor, while a larger daytime percentage increases the result in direct proportion. The implementation accepts only finite numeric inputs, rejects a non-positive daytime percentage, and rounds reported values to six decimal places for stable automated comparisons. It does not fetch latitude tables, infer a month, convert actual daylight hours into p, or adjust for humidity, wind, sunshine, crop type, soil, or irrigation efficiency. Those exclusions are deliberate: the output is the direct Blaney-Criddle estimate from the two values supplied, without hidden corrections that would make the calculation harder to reproduce.

Use the estimate within its practical limits

Blaney-Criddle is useful when temperature records and a suitable daytime-percentage table are available but the richer meteorological observations required by more detailed reference evapotranspiration methods are not. Treat the result as a screening or planning estimate rather than a complete irrigation prescription. Local humidity, wind, radiation, elevation, and seasonal climate patterns can cause actual atmospheric demand to differ from a temperature-only estimate. Before using the number to schedule water applications, consider whether a locally calibrated coefficient, an agronomic standard, or a more complete method is required. Reference evapotranspiration is also not the same as crop evapotranspiration: crop-specific work commonly applies a crop coefficient and then accounts separately for rainfall, application efficiency, rooting conditions, and management constraints. The API costs $0.002 per request, while the browser calculation uses the same deterministic formula. That parity makes it practical to check an individual scenario interactively, then automate a larger workflow with confidence that the computation itself has not changed between channels.