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Irrigation - Evapotranspiration

As the name suggests, Evapotranspiration (“ETo”) is a combination of Evaporation and Transpiration. The UN’s FAO gives a great definition: While “Evaporation is the process whereby liquid water is converted to water vapour (vaporization) and removed from the evaporating surface, …. Transpiration consists of the vaporization of liquid water contained in plant tissues and the vapour removal to the atmosphere.”

Unfortunately nearly all water taken up through the roots of the plant is lost by transpiration and only very little is used within the plant.

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It is very obvious that water that is lost through both evaporation and transpiration somehow needs to be replaced, be that through rain, be that through irrigation. But calculating the rate of water loss is not easy, as many factors need to be considered.

As crops develop the ratio between evaporation and transpiration changes dramatically. While in an early crop stage, when the crop is still small, most water is lost through evaporation, in a fully developed crop almost all water is lost through transpiration. Apparently the weather also plays a key role in this process, as temperature, solar radiation and the wind all heavily impact on evapotranspiration.

And not all crops are the same. Alfalfa, corn, potatoes, apples and olives will all certainly behave very differently. Not to talk about the soil and the methods used to protect water from evaporating, such as permanent crop covers between rows of trees or mulching in other crops.

Sounds complicated? ADCON provides an answer – with monitoring equipment and software that let you determine with perfect accuracy how much water you need to feed to your crop to compensate for ETo!

No matter which formula you use to calculate ETo, weather data is a key parameter for all of them. ADCON has decided to rely on the most popular formula of all, the “modified Penman-Monteith” formula, which is also at the core of the “FAO Irrigation and Drainage Paper 56”.

The Penman-Monteith method of computing ETo is based on two main equations: an energy equation (solar radiation) and an aerodynamic equation (wind and air humidity). A typical ETo station is therefore composed of the following components:

  • A753 addWAVE RTU with solar panel
  • TR1 Combi sensor for air temperature and relative humidity
  • Pyranometer, e.g. SP-Lite or LP02
  • Wind speed sensor, e.g. Vento1
  • Rain gauge, e.g. the RG1/200
  • Pole set

If the station's data is also to be used for computing disease models, a leaf wetness sensor is usually added.

As outlined previously crops have very different rates of transpiration. To make this comparable the model initially computes what’s called the “Reference Evapotranspiration ETo”, which is defined as the evapotranspiration rate of a well irrigated, growing grass surface, and is expressed in mm (l/m²).

To arrive at the evapotranspiration rate of a specific crop, ETo needs to be multiplied with a so-called crop-coefficient. For every crop there is a table listing the respective crop coefficients for each growth stage the plant completes during its development. This crop specific evapotranspiration, also referred to as ETc, is defined by FAO as “The crop evapotranspiration under standard conditions … is the evapotranspiration from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions.“

addVANTAGE Pro will take care of that, fully automatically, and in real time! As weather data comes in, addVANTAGE Pro will calculate both ETo and ETc for a large number of crops already included in the software, such as corn, wheat, soybeans, potatoes, tomatoes, grapes, and many many more.

ETo and direct soil moisture monitoring
For a long time it was either or. Nowadays the technologies more and more complement each other, and soil moisture and other sensors (like dendrometers) are being used to fine-tune ETo and get better site specific data.

Especially for deep rooting crops direct soil moisture monitoring is very much recommended, while for many field crops, covering literally thousands of acres of farm land, such as Alfalfa or corn, ETo often is the better, sufficient and more economical alternative.


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