Using Remote Sensing for Calculating Crop Evapotranspiration (ETc)

Tal Shilo, Ph.D., Chief Agronomist,  Manna Irrigation,
April 2022

If we could measure exactly how much water is getting out of the field and back to the atmosphere, we could easily replenish this amount to the soil, so the plants will get exactly what they need. One of the most popular practices of irrigation is giving back the crop the amount of water it lost since last irrigation. To do that, we need to know what the Field Water Consumption (ETc) is. In this post we will discuss some basic agronomic terms, what is ETc? how is it calculated? And what remote sensing has to do with it?

Let’s start from the basics – back to biology class

The water consumption of a field called evapotranspiration or ET in short.
Evapotranspiration is the process describing the outflow of water from a given field. It combines two inseparable processes:

  • Transpiration – the process where water is taken up by the roots of the crop and evaporates through the leaves.
  • Evaporation – the process where water evaporates from the topsoil layers.

But every crop is transpiring a different amount of water. Evapotranspiration of a given crop in a specific field is called ETc.

This is a dynamic value that depends on the vegetation level, (i.e., the leaf area or crop cover), as well as the meteorological conditions in the field.

Crop evapotranspiration is highly dynamic through the season. In field crops, for instance, at the beginning of season, most of the Evapotranspiration originates from soil evaporation. Later crop transpiration is the dominant outflow of water.

So how do we calculate field water consumption?

ETc parameter is seldom measured because it is operationally difficult. Therefore, ETc is estimated by using the ET of a reference crop, with consideration of the crop stage and size.

To find the ETc (field water consumption) we follow a simple equation: ETc = ETo x Kc*

ET0 stands for the evapotranspiration of a reference crop, usually grass and is mostly calculated according to the weather conditions using the Penman-Monteith equation. This is a meteorological value, taken either from weather stations or from the virtual hyper-local weather service.

Kc stands for the crop coefficient, and is the outcome of the plant’s attributes, and crop’s management by the grower, measured with scientific tools in non-stress conditions.  Each crop in a specific growth stage has a factor called Kc.

How do you find out a crop’s Kc? Growers usually can get it from protocol and published tables. These protocols, in many cases are outdated and generic, not necessarily relevant to the specific crop and location of your field.

Manna does away with this traditional approach, by observing the field’s ACTUAL Kc, using high-resolution, high-frequent satellite images.

Kc Calculation based on Remote Sensing

Analyzing the reflection of different light wavelengths of a cultivated crop area can provide rich ‘intelligence’ about the phenology and health of the crop. Using high-resolution satellite images the Manna model transforms vegetation into the actual crop coefficient. Unlike the protocols, Manna’s Kc value is specific to the monitored field and calculated daily for every pixel in the field.

The actual ETc value based on satellite image analysis was always part of the Manna irrigation recommendation model. For growers’ convenience, who sometimes need to take into consideration operational or governmental water use limitations, we recently added the ETc value to our dashboard and irrigation scheduling page. Now you can use this valuable data for your own decision-making process.

To test it for yourself, start a free trial

*The equation originated from FAO56