Evapotranspiration (ET) Calculator

Enter your reference ET, crop coefficient, growing season length, and irrigation efficiency to calculate daily crop ET, seasonal water demand, and a weekly ETc breakdown.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Reference ET (ETo)
Input the daily reference evapotranspiration for a standard crop like grass or alfalfa, typically sourced from local weather stations (e.g., CIMIS data).
2
Provide Crop Coefficient (Kc)
Enter the FAO-56 crop coefficient specific to the crop and its current growth stage. Mid-season values usually range from 0.7 to 1.25.
3
Specify Growing Season Length
Input the total number of days the crop will be actively growing, from planting to harvest.
4
Set Irrigation Efficiency
Enter the application efficiency of your irrigation system (e.g., drip ~90%, sprinkler ~75%, furrow ~60%).
5
Review Water Requirements
Examine the daily crop ET, total seasonal water needs, and the gross irrigation requirement.

Example Calculation

An agricultural manager needs to calculate the seasonal water requirements for a crop with a daily reference ET of 0.25 in/day, a crop coefficient of 0.95, and a 120-day growing season, using an 80% efficient irrigation system.

Reference ET (ETo) (in/day)

0.25

Crop Coefficient (Kc)

0.95

Growing Season Length (days)

120

Irrigation Efficiency (%)

Results

0.24 in/day

Tips

Adjust Crop Coefficient for Growth Stages

The crop coefficient (Kc) changes significantly throughout the growing season. Use a lower Kc for early growth stages (e.g., 0.3-0.5), a higher Kc during mid-season (e.g., 0.8-1.2), and a declining Kc towards harvest to accurately reflect water demand.

Monitor Local Weather Station Data Regularly

Reference ET (ETo) is highly variable based on daily weather conditions. Consistently check data from nearby weather stations or agricultural information systems (like CIMIS in California) to ensure your ETo input is current and accurate for precise irrigation scheduling.

Consider Soil Type and Water Holding Capacity

While this calculator focuses on plant water use, your soil's water holding capacity dictates how frequently irrigation is needed. Sandy soils require more frequent, smaller applications, while clay soils can hold more water, allowing for less frequent, larger applications to match ETc.

Calculating Crop Water Needs with the Evapotranspiration (ET) Calculator

The Evapotranspiration (ET) Calculator is an essential tool for precision agriculture, enabling growers to determine the exact water requirements of their crops. By combining daily reference ET, crop coefficients, and irrigation efficiency, it provides critical data for optimizing water use throughout the growing season. For instance, a crop with a daily reference ET of 0.25 in/day, a crop coefficient of 0.95, and a 120-day growing season, irrigated with 80% efficiency, will have a daily crop ET of approximately 0.24 in/day.

The Critical Role of Evapotranspiration in Agriculture

Evapotranspiration (ET) is a cornerstone of effective irrigation management in agriculture. It quantifies the total water consumption by crops, encompassing both the water transpired through plant leaves and evaporated from the soil surface. Understanding ET is critical for preventing both over-irrigation, which wastes valuable water, leaches nutrients, and can lead to waterlogging, and under-irrigation, which causes crop stress, reduces yields, and impacts overall farm profitability. Accurate ET calculations allow growers to implement precise irrigation schedules, ensuring optimal crop health and maximizing water use efficiency, especially vital in regions facing increasing water scarcity.

Dissecting the Crop Evapotranspiration Formula

The Evapotranspiration (ET) Calculator utilizes a standard methodology to determine crop water requirements. The core of the calculation involves deriving the crop evapotranspiration (ETc) from the reference evapotranspiration (ETo) and the crop coefficient (Kc), then adjusting for the length of the growing season and irrigation efficiency to find the total gross water requirement.

The formulas used are:

Crop ET (ETc) per day = Reference ET (ETo) × Crop Coefficient (Kc)

Total Seasonal ETc = Crop ET (ETc) per day × Growing Season Length (days)

Gross Water Requirement = Total Seasonal ETc / (Irrigation Efficiency / 100)

This sequence allows for a comprehensive assessment of how much water needs to be applied to the field to meet the crop's needs, accounting for system losses.

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Calculating Water Needs for a 120-Day Crop Cycle

Let's consider an agricultural scenario where a farm manager needs to determine the irrigation requirements for a specific crop over its entire growing season.

Reference ET (ETo): 0.25 inches per day
Crop Coefficient (Kc): 0.95 (mid-season value)
Growing Season Length: 120 days
Irrigation Efficiency: 80%

Here’s how the calculations unfold:

Calculate Daily Crop ET (ETc):
- ETc = ETo × Kc = 0.25 in/day × 0.95 = 0.2375 in/day
Calculate Total Seasonal ETc:
- Total Seasonal ETc = 0.2375 in/day × 120 days = 28.5 inches
Calculate Gross Water Requirement:
- Gross Water Requirement = Total Seasonal ETc / (Irrigation Efficiency / 100)
- Gross Water Requirement = 28.5 inches / (80 / 100) = 28.5 inches / 0.80 = 35.625 inches

Therefore, this crop will require approximately 0.24 inches of water per day (rounded), totaling 35.63 inches of gross irrigation over its 120-day growing season to account for water losses.

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Water Management Strategies for Sustainable Agriculture

Sustainable water management is paramount in modern agriculture, particularly as global water resources face increasing pressure. Farmers employ a range of strategies to optimize water usage and minimize waste. Precision irrigation techniques, such as drip irrigation and micro-sprinklers, deliver water directly to the plant root zone, significantly reducing evaporation and runoff. These systems can achieve irrigation efficiencies of 90-95%, compared to 60-75% for traditional furrow or flood irrigation. Soil moisture sensors and weather-based scheduling (using ET data) further refine water application, ensuring crops receive water only when and where it's needed. Additionally, practices like mulching reduce soil evaporation, and selecting drought-resistant crop varieties or adopting no-till farming conserves soil moisture. These integrated approaches are crucial for meeting growing seasonal water demands while protecting finite water resources, especially in drought-prone regions like California's Central Valley.

Typical Evapotranspiration Rates by Crop Type

Evapotranspiration (ETc) rates vary significantly across different crop types and their growth stages, making generalized irrigation challenging. Understanding typical benchmarks is crucial for effective water management.

Corn (Maize): During its peak growth (mid-season), corn can have daily ETc rates ranging from 0.25 to 0.35 inches/day (6.35 to 8.9 mm/day), reflecting its high water demand for rapid biomass accumulation.
Wheat: For a cool-season grain like wheat, daily ETc during its active growth phase typically falls between 0.15 to 0.25 inches/day (3.8 to 6.35 mm/day), with lower rates in early and late stages.
Alfalfa: As a perennial forage crop, alfalfa has a deep root system and can have very high ETc rates, often reaching 0.35 to 0.40 inches/day (8.9 to 10.2 mm/day) during hot, dry summer conditions.
Vegetables (e.g., Tomatoes): Mid-season ETc for vegetables can range from 0.20 to 0.30 inches/day (5.1 to 7.6 mm/day), depending on canopy cover and local climate.

These rates are influenced by environmental factors like temperature, humidity, and wind, as well as the crop's specific genetics and management practices.

Frequently Asked Questions

What is evapotranspiration (ETc) and why is it important for crops?

Evapotranspiration (ETc), or crop evapotranspiration, is the total amount of water lost from a cropped field through evaporation from the soil surface and transpiration from the plant leaves. It's crucial for crops because it represents their actual water consumption. Accurately calculating ETc allows farmers to apply the precise amount of irrigation water needed, preventing both over-watering (which wastes resources and can lead to nutrient leaching) and under-watering (which stresses crops and reduces yields).

How does 'reference ET' differ from 'crop ET'?

'Reference ET' (ETo) is the evapotranspiration rate from a standardized reference surface, typically a hypothetical grass or alfalfa field, under optimal water conditions. It reflects the atmospheric demand for water, influenced by weather factors like temperature, humidity, and wind. 'Crop ET' (ETc) is the actual water use of a specific crop, calculated by multiplying ETo by a crop coefficient (Kc) that accounts for the crop's unique characteristics, growth stage, and ground cover.

What does 'irrigation efficiency' mean for water requirements?

'Irrigation efficiency' measures how effectively water applied to a field reaches the crop root zone and is used by the plant, rather than being lost to runoff, deep percolation, or evaporation. For example, an 80% irrigation efficiency means 20% of the applied water is lost. Therefore, to meet a crop's net water requirement, the gross irrigation amount must be increased to compensate for these losses. Drip irrigation systems typically have efficiencies of 90% or higher, while furrow irrigation can be as low as 60%.