Evapotranspiration Calculator

TL;DR: To calculate evapotranspiration, enter your air temperature in degrees Celsius and your relative humidity as a percentage into our free calculator, which instantly applies the formula ET = 0.0023 × (T + 17.8) × √(Tmax - Tmin) × √(RH) to estimate the daily water loss from soil and plants in millimeters per day.

What Is the Evapotranspiration Calculator?

The Evapotranspiration Calculator is a free online tool designed to estimate the combined water loss from evaporation (from soil and water surfaces) and transpiration (from plant leaves). It takes two crucial environmental inputs—air temperature and relative humidity—and converts them into an actionable metric: the estimated water loss in millimeters per day. This is a simplified, yet powerful, method for understanding how much water your crops, garden, or landscape actually needs on a given day.

This tool is indispensable for a wide range of professionals and hobbyists. Farmers use evapotranspiration (ET) data to schedule irrigation, ensuring they don't overwater (which wastes water and leaches nutrients) or underwater (which stresses plants and reduces yield). Landscape managers and golf course superintendents rely on ET rates to maintain healthy turf while conserving water. Even home gardeners can benefit by using this calculator to adjust their watering schedule during hot, dry, or windy periods. The practical value lies in transforming raw weather data into a direct irrigation recommendation, saving both water and money.

How to Use the Calculator

Using this calculator is a straightforward process that requires only two pieces of environmental data. Follow the steps below to get your result instantly.

1. Enter Air Temperature: Locate the input field labeled 'Temperature'. Enter the current air temperature in degrees Celsius (°C). Ensure you are using a standard thermometer reading from a shaded, well-ventilated area (typically 1.5 to 2 meters above the ground) for the most accurate result.
2. Enter Relative Humidity: Find the input field labeled 'Relative Humidity'. Enter the relative humidity value as a percentage (%). This is the amount of water vapor in the air relative to the maximum it can hold at that temperature. You can obtain this from a hygrometer, a weather station, or a local weather report.
3. Initiate Calculation: Once both fields contain valid numeric values, click the 'Calculate' button. The calculator will process your inputs.
4. Read the Output: The result will be displayed, showing the estimated evapotranspiration rate in millimeters per day (mm/day).

Formula and Calculation Method

The calculator operates using a modified version of the empirical Hargreaves equation, which has been adapted to estimate evapotranspiration using only temperature and humidity. While the original Hargreaves equation uses the difference between maximum and minimum temperature (Tmax - Tmin) as a proxy for solar radiation, this version recognizes the significant impact of humidity on the atmospheric demand for water. Here is the formula used in our calculator:

ET = 0.0023 × (T + 17.8) × √(Tmax - Tmin) × √(RH)

Where:

• T is the daily average air temperature in degrees Celsius (°C).
• Tmax is the maximum daily air temperature (°C).
• Tmin is the minimum daily air temperature (°C).
• RH is the relative humidity as a percentage (%).
• ET is the estimated evapotranspiration in millimeters per day (mm/day).

In our simplified calculator, we assume that the user inputs an average temperature (T), and that the difference between the day's maximum and minimum temperature is a constant seasonal factor. The term √(Tmax - Tmin) is a proxy for the amount of incoming solar radiation. To make the calculation user-friendly, we incorporate this, but the primary drivers of the calculation are your input temperature and humidity values.

Worked Example:

Let’s calculate evapotranspiration for a typical summer day. Suppose the air temperature is 28°C, and the relative humidity is 55%.

Let's assume a daily temperature range (Tmax - Tmin) of 10°C, which is typical for a cloudless summer day.

1. Start by adding the temperature to 17.8: 28 + 17.8 = 45.8
2. Find the square root of the temperature range: √10 = 3.16
3. Find the square root of the relative humidity as a percentage: √55 = 7.42
4. Multiply all components together with the constant: ET = 0.0023 × 45.8 × 3.16 × 7.42
5. ET = 0.0023 × 45.8 = 0.10534
6. 0.10534 × 3.16 = 0.3328
7. 0.3328 × 7.42 = 2.47 mm/day

Therefore, on this specific day, the water loss due to evaporation and transpiration is estimated to be approximately 2.47 millimeters per day. This means your crops will need roughly 2.47 liters of water per square meter of soil to replace what is lost.

Practical Examples

Understanding the numbers is one thing, but applying them is what makes this calculator valuable. Here we present three different scenarios to illustrate how the inputs affect the output and what that means for your irrigation strategy.

The table clearly highlights the driving forces behind evapotranspiration. As temperature rises and humidity falls, the atmospheric demand for water increases dramatically, leading to a much higher ET rate. This translates directly to a greater need for irrigation on hot, dry days, as the soil and plants are losing water at a faster clip.

Tips for Accurate Results

To ensure you are getting the most reliable estimate from this calculator, it is critical to avoid common mistakes. The precision of your result depends not only on the calculator's formula but also on the quality and context of your inputs.

• Always Verify Units: The most common pitfall is entering Fahrenheit temperatures into a Celsius calculator. Double-check that your thermometer or weather station is set to °C. If it's in Fahrenheit, convert it first (Subtract 32, multiply by 5, then divide by 9). The calculator is not unit-agnostic; accurate inputs are essential.
• Avoid Rounding Intermediate Results: Do not round the temperature or humidity inputs to whole numbers unless that is the true precision of your measurement. For example, entering 25.9°C instead of 26°C provides a more precise output. The difference may seem small, but it can accumulate over time when scheduling irrigation.
• Check the Validity Range: This simplified formula is best suited for moderate conditions, typically temperatures between 5°C and 40°C and relative humidity between 20% and 90%. Using it for extreme conditions (e.g., sub-zero temperatures or 100% humidity) may yield non-physical or misleading results, as the plant physiological processes change outside these parameters.
• Use Peak Data: For best results, use the maximum temperature of the day and the minimum humidity reading of the day, usually occurring in the mid-to-late afternoon. This "worst-case scenario" calculation ensures you are prepared for the peak water demand.

Frequently Asked Questions

Here are several common questions users have about evapotranspiration and our calculator, with clear and direct answers to help you make the most of this tool.

Q: Is the Evapotranspiration Calculator suitable for all types of crops?
A: No, this calculator provides a reference evapotranspiration (ET₀) rate. This is the potential water loss from a hypothetical, well-watered grass reference crop. Actual crop water use can be different. Specific crops like rice or bananas require more water, while grapes or olives require less. To get a specific crop's water requirement, you would multiply the ET₀ result from this calculator by a crop-specific coefficient (Kc). For instance, for corn in the mid-season, Kc might be around 1.2, so the actual water loss would be ET₀ × 1.2. This calculator is an excellent starting point, but for precision agriculture, you must factor in the Kc value for your specific plant.

Q: What if my weather data includes wind speed and solar radiation? Will this calculator use it?
A: No, this specific calculator only uses temperature and humidity to provide an estimate. It is a simplified model based on the Hargreaves equation. More sophisticated methods, like the FAO-approved Penman-Monteith equation, also require solar radiation, wind speed, and atmospheric pressure. While those methods are more accurate, they demand much more data. If you have access to those data points, you might need a more advanced calculator. This tool is designed to provide a "good enough" estimate for general planning when only basic weather data is available.

Q: How does the temperature range (Tmax - Tmin) affect the result, and why isn't it an input?
A: In the formula, the temperature range (Tmax - Tmin) serves as a critical proxy for solar radiation. A greater daily temperature swing indicates that the day was clear and sunny, leading to more solar energy available to drive evapotranspiration. A small temperature swing suggests cloudy, overcast conditions with less energy. In this simplified calculator, we've assumed a moderate, constant range to keep the tool easy to use. However, you should be aware that this is a limitation. For more precise results, you would want a calculator that lets you input specific Tmax and Tmin values to directly calculate the solar radiation proxy.