Evapotranspiration (ET) Rate Calculator

Enter your average temperature, relative humidity, wind speed, and crop coefficient to calculate daily evapotranspiration, crop water demand, and weekly irrigation requirements.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Average Temperature (°C)
Input the mean daily air temperature at 2 meters height. This is a primary driver for reference ET.
2
Provide Crop Coefficient (Kc)
Enter the crop coefficient (Kc) that relates crop ET to reference ET. Typical values: Lawn 0.7–0.9, Maize 1.0–1.2.
3
Specify Relative Humidity (%)
Input the average daily relative humidity. Lower humidity increases the atmospheric demand for water.
4
Set Wind Speed (m/s)
Enter the average daily wind speed at 2 meters height. Higher wind speeds accelerate ET rates.
5
Review ET Rates and Water Volume
Examine the daily crop ET rate, reference ET, daily water loss in inches, and weekly water volume estimates.

Example Calculation

An agronomist needs to estimate the daily evapotranspiration rate for a crop with a coefficient of 0.85, given an average temperature of 25°C, 50% relative humidity, and 2 m/s wind speed.

Average Temperature (°C)

Crop Coefficient (Kc)

0.85

Relative Humidity (%)

Wind Speed (m/s)

Results

0.32 mm/day

Tips

Consider Diurnal Temperature Range

The Hargreaves-Samani equation, used here, can be more accurate if you input the actual diurnal temperature range (difference between daily max and min) instead of assuming a default. A wider range typically indicates higher atmospheric demand for water.

Factor in Cloud Cover and Solar Radiation

While not direct inputs here, cloud cover significantly impacts solar radiation, a major driver of ET. On cloudy days, ET rates can be 10-20% lower than on clear, sunny days, even with similar temperatures, due to reduced energy for evaporation.

Validate with Soil Moisture Monitoring

Always cross-reference calculated ET rates with actual soil moisture levels using probes or sensors. This provides real-time feedback on whether the irrigation schedule derived from ET calculations is effectively meeting crop water needs and preventing stress.

Estimating Daily Crop Water Loss with the Evapotranspiration (ET) Rate Calculator

The Evapotranspiration (ET) Rate Calculator provides a precise estimate of daily crop water use by integrating key meteorological variables. This tool calculates both the reference ET (ET₀) and the specific crop ET (ETc), giving growers critical data for efficient irrigation. For a crop with a coefficient of 0.85, experiencing an average temperature of 25°C, 50% relative humidity, and 2 m/s wind speed, the daily crop ET rate is approximately 0.32 mm/day.

The Science of Plant Water Dynamics

Understanding the science of plant water dynamics, particularly evapotranspiration, is fundamental to modern agriculture. Plants continuously draw water from the soil through their roots and release it as vapor through their leaves (transpiration), a process essential for nutrient transport and temperature regulation. Simultaneously, water evaporates directly from the soil surface. This combined water loss, known as evapotranspiration, is heavily influenced by atmospheric conditions. By quantifying this dynamic, growers can tailor irrigation schedules to plant needs, preventing water stress or waste and optimizing yields, especially crucial when facing variable climate patterns.

The Hargreaves-Samani Method for ET₀ Calculation

The Evapotranspiration (ET) Rate Calculator employs a modified Hargreaves-Samani equation for estimating reference evapotranspiration (ET₀), which is then adjusted by the crop coefficient (Kc) to find the crop ET (ETc). This method is widely used due to its reliance on readily available temperature data, with adjustments for humidity and wind to enhance accuracy.

The simplified logic involves:

Base ET₀ Calculation: A base reference ET is calculated using average daily temperature and an assumed diurnal temperature range.
Humidity Adjustment: This base ET₀ is then adjusted by a factor that increases ET as relative humidity decreases, reflecting drier air's higher evaporative demand.
Wind Speed Adjustment: A further adjustment is applied based on wind speed, as higher winds increase the rate of vapor removal from the plant canopy and soil surface.
Crop ET Calculation: The adjusted ET₀ is multiplied by the crop coefficient (Kc) to derive the specific crop evapotranspiration (ETc).

ET₀ = 0.0023 × (Avg Temp °C + 17.8) × √(Diurnal Temp Range) × Humidity Factor × Wind Factor
Crop ET (ETc) = ET₀ × Crop Coefficient (Kc)

This model provides a robust estimate for daily crop water needs.

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Calculating Daily Water Loss for a Maize Crop

Let's consider an agronomist monitoring a maize (corn) crop in a field. The average daily temperature is 25°C, the crop coefficient (Kc) for this growth stage is 0.85, relative humidity is 50%, and the average wind speed is 2 m/s. We'll assume a diurnal temperature range of 15°C for the Hargreaves-Samani formula.

Calculate Base ET₀:
- et0Base = 0.0023 × (25 + 17.8) × √(15) ≈ 0.0023 × 42.8 × 3.873 ≈ 0.3807 mm/day
Apply Humidity Factor:
- humidityFactor = 1 + (50 - 50) / 200 = 1 (no adjustment needed at 50% RH)
Apply Wind Speed Factor:
- windFactor = 1 + (2 - 2) × 0.04 = 1 (no adjustment needed at 2 m/s wind)
Calculate Adjusted Reference ET (ET₀):
- ET₀ = 0.3807 mm/day × 1 × 1 = 0.3807 mm/day
Calculate Crop ET (ETc):
- ETc = ET₀ × Kc = 0.3807 mm/day × 0.85 ≈ 0.3236 mm/day

Therefore, the estimated daily crop evapotranspiration rate for this maize crop is approximately 0.32 mm/day.

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Climate Factors Affecting Crop Water Demand

Crop water demand, as measured by evapotranspiration, is profoundly influenced by a complex interplay of climatic factors.

Temperature: Higher air temperatures increase the energy available for evaporation and transpiration, directly leading to higher ET rates. For every 10°C increase in temperature, ET rates can rise by approximately 15-20%.
Relative Humidity: Lower relative humidity creates a steeper vapor pressure deficit between the plant and the atmosphere, accelerating water loss from the leaves. Conversely, high humidity reduces this gradient, slowing ET. A drop from 80% to 40% relative humidity can increase ET by 10-15%.
Wind Speed: Wind physically removes humid air from around the plant canopy, replacing it with drier air, which drives further transpiration and evaporation. Increased wind speeds can elevate ET rates by 5-10%, particularly when combined with high temperatures and low humidity.
Solar Radiation: Although not a direct input in this simplified model, solar radiation is the primary energy source for the evaporation process. Higher solar radiation leads to more intense ET.

These factors combine to create the atmospheric demand for water, dictating how much water crops will consume daily.

Comparing Evapotranspiration Models

Several models exist for calculating evapotranspiration, each with varying levels of complexity and data requirements. The Hargreaves-Samani equation, used in this calculator, is a temperature-based method known for its simplicity and utility in regions where comprehensive meteorological data is scarce. It primarily relies on air temperature and an assumed or estimated diurnal temperature range, making it accessible for many users.

In contrast, the Penman-Monteith equation, as standardized by the Food and Agriculture Organization (FAO-56), is considered the most accurate and widely recommended method. It is a combination equation that integrates a broader set of meteorological inputs, including solar radiation, air temperature, humidity, and wind speed. While more data-intensive, Penman-Monteith provides a more physically based and globally applicable estimate, especially crucial for precise irrigation scheduling in diverse climates. The choice between models often depends on data availability and the required level of accuracy for a specific agricultural application.

Frequently Asked Questions

What is reference evapotranspiration (ET₀) and how is it calculated?

Reference evapotranspiration (ET₀) is the rate of water loss from a hypothetical reference crop (like grass or alfalfa) under ideal growing conditions, primarily driven by atmospheric factors. It's calculated using empirical equations that integrate meteorological data such as air temperature, relative humidity, and wind speed. The Hargreaves-Samani equation, for instance, uses temperature and radiation to estimate ET₀, which then serves as a baseline for determining specific crop water needs.

How does relative humidity affect crop water use?

Relative humidity significantly affects crop water use because it dictates the 'drying power' of the air. Lower relative humidity means the air is drier and can hold more moisture, thus increasing the rate at which water evaporates from the soil and transpires from plant leaves. Conversely, high humidity reduces the vapor pressure deficit between the plant and the atmosphere, thereby decreasing evapotranspiration rates. A drop from 70% to 30% humidity can increase ET by 15-25%.

Why is wind speed an important factor in evapotranspiration?

Wind speed is a crucial factor in evapotranspiration because it removes humid air from above the plant canopy, replacing it with drier air. This continuous exchange maintains a steep vapor pressure gradient between the leaf surface and the atmosphere, thereby accelerating the rate of transpiration from plants and evaporation from the soil. Higher wind speeds can increase ET rates by 10-20% compared to calm conditions, especially when combined with high temperatures and low humidity.