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Measuring Evapotranspiration & Crop Coefficient: A Complete Guide

  • By Arable

Water and agriculture are inextricably linked. One significant variable that can affect crop production is evapotranspiration (ET). Without understanding the importance of this variable in crop production, yields can be significantly impacted. In fact, in dry seasons and periods affected by water scarcity, it can make or break a crop. But when evapotranspiration is measured and analyzed efficiently, irrigation management plans can benefit in many ways. In this article, we will explore the use of evapotranspiration and crop coefficient (KcNDVI). We will also explore ways to measure these variables and a few specific examples.

What is Evapotranspiration?

Evapotranspiration (ET) is the measurement of the amount of water a plant loses in a day. It is the combined loss of water from the processes of evaporation (the movement of water from surfaces or water bodies to the air) and transpiration (the loss of water vapor through the plant’s stomata to the atmosphere).

Since the actual amount of water lost through transpiration depends on the plant species and the plant’s growth stage, a more precise field measurement that takes the canopy cover into account is canopy evapotranspiration (ETc).

There are many applications for evapotranspiration, such as irrigation scheduling, plant stress monitoring, water use efficiency, and crop protection.

Why Do We Measure Evapotranspiration?

The tendency to overwater a field is very common. However, it has many associated risks: disease inoculation, nutrient leaching, and soil erosion. Moreover, agricultural water use efficiency (WUE) is an increasingly important concept as droughts, the surge in atmospheric CO2, and denser plantings demand higher water intake and groundwater depletion.

By measuring evapotranspiration and monitoring a field’s ETc, we can appropriately budget irrigation inputs based on our management plans, such as replacing only the water lost since the last irrigation or adding only what we can determine a plant needs at a given time. This is the most efficient and sustainable approach to irrigation management and is crucial to anyone who needs to comply with irrigation regulations such as California’s Sustainable Groundwater Management Act (SGMA).

Measuring Evapotranspiration

Determining how much water a specific field is losing in real-time can be difficult, but it’s not impossible. While there are a few different methods for measuring evapotranspiration, you must consider plant health, phenotyping, and other environmental conditions:

Crop Phenology and Growth Stage Measurement

  • Normalized Difference Vegetation Index (NDVI)
  • Air and canopy temperature (T)
  • Crop coefficient (KcNDVI)

Environmental Conditions

  • Relative humidity (RH)
  • Saturated vapor pressure (esat)
  • Actual vapor pressure (ea)
  • Vapor pressure deficit (esat – ea)
  • Net radiation (Rn)
  • Precipitation (Precip)

The Arable approach includes a three-step process leveraging crop coefficient measurements.

  1. We derive the field evapotranspiration (ETf), which is akin to reference evapotranspiration (ETo) or the hypothetical evapotranspiration under a grass reference surface. ETf is a baseline (not species-specific) evapotranspiration rate based on your field’s actual weather conditions over a homogeneous area. Having in-field weather data is critical to calculating an accurate ETf value since it quantifies the evaporation power of the atmosphere. But using it for irrigation is risky because it can change based on crop characteristics and physiology.
  2. To get around this, we measure the Normalized Difference Vegetation Index (NDVI), which quantifies the health and stage of the crop’s growth, to calculate the crop coefficient (KcNDVI) via the linear regression method developed by Kamble et al. NDVI is a measure of the “greenness” of a plant-based on the canopy reflectance of light. The Kc depends on the species and changes throughout the growing season.
    KcNDVI = 1.457 x NDVI – 0.1725
  3. Finally, we multiply your field’s ETf by your plants’ Kc to get an ETc value unique to the plants in your field. You can then use this value to devise a precise irrigation plan.
    ETc = ETf x KcNDVI



Irrigation Management & Evapotranspiration Measuring Methods

As any grower knows, there are many different ways to approach irrigation management, and Arable offers a method that is unique in two ways.

First, it uses the dynamic NDVI to calculate KcNDVI I as discussed above, so you don’t rely on pre-established KcNDVI tables.

Second, it calculates a hyperlocal ETf based on weather conditions around the Arable Mark in your field instead of using a remote weather station. This provides a more precise value of evapotranspiration, highly representative of the conditions in your management area.

By using ETc, the Mark can help you determine the first step in irrigation planning: crop water demand (CWD). This is also known as the irrigation water requirement. Knowing exactly how much water your crop requires will improve your irrigation WUE and help you make evidence-based management choices by calculating precise, real-world needs.

Other considerations about your irrigation system, such as soil type, field size, flow rate, and efficiency, will dictate the exact timing and amount of water applied. By starting with CWD, you can ensure you are not over- or underestimating the amount of water needed to keep your plants healthy.

CWD = PrecipETc

Example Use Cases

Available in the Arable data export, CWD is calculated by subtracting the amount of water lost to ETc from the amount added by precipitation (Precip). In the example shown above, a Mark in Australia reported a total input of 0.46” of rain over the past week. The same Mark reported an ETc of 0.19”. Since precipitation—the input—exceeds the amount of water lost to evapotranspiration, we know it is not necessary to irrigate at this time.


On the other hand, a field in California reported precipitation of 0.3” and an ETc of 0.68” for the same week. There is a water deficit of -0.38” of water (CWD= 0.3” Precip – 0.68” ETc), which means that the field needs 0.38 acre-inches of water to replenish the losses from that week. At this point, you have an exact amount of water that needs to be added back into the field.

Putting It Into Practice

Your next steps for irrigation scheduling might include calculating in-system inefficiencies and determining timing. These are based on your specific setups, such as your irrigation system, number of lines per row, and flow rate.

At Arable, we specialize in providing solutions that help harness data to drive sustainable and profitable outcomes. Armed with a rich, in-field climate and plant dataset, you can build an evidence-based schedule that best enables you to define and achieve goals at each stage of the growing season.


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