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Simplified reference evapotranspiration estimate for irrigation

At 25 °C, a 2.5 m/s wind and 18 MJ/m² of solar radiation per day, the estimate is close to 12 mm of water lost per day. Only temperature is required; it is a screening figure, not a full FAO-56 calculation. Free in your browser.

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Worked example

If you enter

Creativity: 25
Wind speed: 2.5
Solar radiation: 18

You get

Reference evapotranspiration mm day: 11.70045
Temperature c: 25
Wind speed m s: 2.5
Solar radiation mj m2 day: 18

This simplified reference evapotranspiration calculator turns three practical weather estimates into an approximate daily water-loss depth in millimetres. Enter average air temperature in degrees Celsius, wind speed at two metres in metres per second, and incoming solar radiation in megajoules per square metre per day. The result is intended for early irrigation planning, quick comparisons, and data-gap scenarios. It is transparent and repeatable, but it is not a replacement for a full FAO-56 Penman–Monteith calculation or a local agronomic recommendation.

Choose weather inputs that represent the same day and site

Start with the average air temperature for the day, expressed in degrees Celsius. This is the only required input because the calculator can use conservative planning defaults for the other two variables. For a more useful site-specific estimate, provide average wind speed measured or adjusted to two metres above the ground and total incoming solar radiation for the day in megajoules per square metre.

Keep all three values aligned to the same location and twenty-four-hour period. Mixing a midday temperature with a daily radiation total, or combining weather from stations with different exposure, creates a number that looks precise but describes no real day. If your wind sensor is installed much higher than two metres, adjust the reading using an accepted meteorological method before entering it.

Use measured solar radiation when available; otherwise, use a reputable nearby station or a documented forecast estimate. The defaults of 2 m/s wind and 15 MJ/m²/day radiation are neutral placeholders for exploration, not universal climate normals. Record whether each input was measured, forecast, or assumed so later irrigation decisions can be reviewed honestly.

Understand the simplified equation and its limits

The calculator applies ET0 = max(0, 0.0135 × (Tmean + 17.8) × Rs × (1 + 0.05 × u2)). Tmean is average daily temperature in degrees Celsius, Rs is incoming solar radiation in MJ/m²/day, and u2 is wind speed at two metres in m/s. The radiation-temperature core converts available solar energy into an approximate water depth, while the small linear wind factor represents faster removal of moist air above the reference surface.

The maximum with zero prevents an unphysical negative irrigation demand during very cold or dark conditions. This deliberately compact method does not calculate vapour-pressure deficit, net longwave radiation, soil heat flux, atmospheric pressure, or aerodynamic and surface resistance separately. Those terms belong in the full FAO-56 Penman–Monteith method and can matter greatly in humid, arid, coastal, high-elevation, or advective environments.

Treat the result as a screening estimate, not as a standards-compliant ET0 observation. The response includes the normalized inputs, radiation component, wind factor, formula, and method name so you can reproduce the arithmetic and distinguish it from more complete models in stored records.

Turn the estimate into a cautious irrigation plan

Reference evapotranspiration describes atmospheric demand over a standardized reference surface; it is not automatically the amount of water a particular crop needs. To move toward crop water use, multiply ET0 by a crop coefficient appropriate to the crop and growth stage. Then subtract effective rainfall and account for soil water already available in the root zone.

If you are calculating the gross amount to apply, divide the remaining net requirement by irrigation-system efficiency. These later steps can easily change the answer more than another decimal place in the simplified ET0 estimate. Compare results over several days and use rolling totals when scheduling irrigation, because a single unusually windy or cloudy day may not justify changing a practical schedule.

Check the estimate against a trusted local weather service, lysimeter, or FAO-56 calculation whenever water cost, crop stress, salinity, runoff, or regulatory reporting makes accuracy consequential. Avoid using this tool alone for safety-critical horticulture, pesticide timing, drought declarations, or legal water accounting. Its best role is consistent scenario comparison: use the same inputs and method across fields or forecast days, document assumptions, and revise the plan when measured weather and soil-moisture observations arrive.

What you can do with it

Build a preliminary irrigation schedule

Estimate daily atmospheric water demand from a short weather forecast before field measurements are available.

Compare weather scenarios

See how a hotter, windier, or sunnier planning day changes the same transparent approximation.

Fill a clearly marked data gap

Create a reproducible interim estimate while preserving the assumed wind and radiation values in the output.

FAQ

Is this the FAO-56 Penman–Monteith equation?

No. It is a simplified radiation-temperature estimate with an explicit wind adjustment. It does not use humidity, elevation, pressure, or net longwave radiation.

What happens if temperature is omitted?

Wind and solar radiation use documented defaults if omitted.

Why is the result reported in millimetres per day?

A millimetre over one square metre equals one litre of water, making the depth convenient for daily irrigation planning.

Can I use the result as crop water demand?

Not directly. Apply an appropriate crop coefficient, effective rainfall, soil-water information, and irrigation efficiency before planning an application.

When should I use a more complete method?

Use FAO-56 Penman–Monteith or validated local guidance when reliable humidity and station data exist, or when financial, agronomic, or regulatory consequences are significant.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/weather/evapotranspiration-estimate

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/weather/evapotranspiration-estimate \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"temperature":25}'

const res = await fetch("https://api.kit.forhosting.com/weather/evapotranspiration-estimate", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "temperature": 25
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/weather/evapotranspiration-estimate",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "temperature": 25
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/weather/evapotranspiration-estimate", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"temperature":25}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"temperature":25}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/weather/evapotranspiration-estimate", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "temperature": 25
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "weather.evapotranspiration_estimate",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical questions

What does the API call cost?

Each API request costs $0.002. The browser version can run the same deterministic calculation locally.

Read the full KIT documentation →

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