The Core Principles of SEBAL

SEBAL stands for Surface Energy Balance Algorithm for Land

It is a model that calculates evapotranspiration (ET) values, crucial for understanding crop water use, from thermal measurements taken from satellites by making use of the energy balance found on the Earth’s surface.
SEBAL measures energy exchange at the Earth's surface, in the form of temperature, instead of tracking water itself. Energy is required to evaporate each unit of water into water vapour and the energy used is expressed as temperature.

This creates a direct physical link between energy balance and water consumption: ET requires energy, and energy is expressed as temperature, therefore a change in temperature is a change in energy and thus represents a change in ET.

The surface energy balance equation:

Information from satellites means SEBAL has the values for Rn, G and H and therefore can solve for λET.

The energy used for ET (λET) translates into actual ET by dividing λET by the latent heat of vaporisation (λ), where λ has specific values depending on the water temperature.

  • Rn = net radiation = the total energy reaching the surface from the Sun plus outgoing radiation from the Earth's surface

  • G = soil heat flux = the energy used to heat the soil           

  • H = sensible heat flux = the energy used for heating the air

  • λET = latent heat flux = the energy used for evapotranspiration

ET is related to crop water consumption through processes that happen within the crops themselves.

Write your awesome label here.

As covered in Irrigation Principles, plant stomata open and close to intake CO₂ and release water vapour and O₂. Plant stomata and roots communicate to ensure the right amount of water vapour is being released in relation to the water uptake of the roots, as to not leave the plant in a deficit. Therefore, if there is not enough water uptake by the roots to balance out the amount leaving the plant as water vapour, the roots will send a signal to the stomata to close to prevent a water deficit. 

Write your awesome label here.

However, the crop is still being subjected to the radiation from the sun, and this energy – normally used to drive evaporation, turning water from the roots into water vapour leaving the plant – has nowhere to go. This results in the temperature of the crop’s surface increasing, which is what is identified by the thermal infrared satellites and is fed into the SEBAL model.

Write your awesome label here.

You can think of it similar to the mechanics of getting rid of an illness in the human body: when you’re ill, your body increases in temperature as it’s trying to fight off the sickness, but then, when you’re sweating (the equivalent of the plant releasing water vapour due to returned water at the root zone) you begin to cool down and the illness has passed.

This is why a cool field is a good sign, and a hot one is likely a sign of crop stress. 
It is also how we can directly correlate ET measurements to the water consumption of the root zone: root zone water intake must be in equilibrium with water vapour release which is in turn linked to the temperature of the field and our thermal measurements.

The SEBAL model uses temperature to determine water stress at the root zone therefore if stress is recorded due to a lack of moisture in the root zone, a moisture alert is given. 

Hot surfaces + high H + low ET = water stressed crop

However, if the stress is due to any other factor, this cannot be determined from satellites, and the system will give a crop alert, but the cause will not be water availability.

Scenarios like poor soil, when the plants are just beginning to grow or the field has just been harvested will also mean the field will be flagged due to its hotter temperatures, but this is not due to water availability and is something that needs to be verified on the ground

Want to learn more? Click below to read about another linked topic

Help us
Improve

Your feedback helps us make our Academy more useful. Tell us about your experience and what you'd like to see next.

Share With
Your Network

Know someone who could benefit?
Share the IrriWatch Academy with your colleagues and network.