An Integrated 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.
These values are then used in our system:
1. The ET values identify specific fields or areas where combinations of crop yield and water usage are suboptimal per unit of crop production

2. The system assesses the difference between current levels of water productivity and their optimal values given the local socio-agroecological context

3. Using the framework, Hydrosat helps determine the underlying causes of low water productivity and suggests the actions required to bring all levels back to optimum
A breakdown of the SEBAL process

SEBAL in Steps

  • Albedo is calculated from visible and NIR satellite reflectance bands
    - This is important as a surface with high albedo (e.g. snow or deserts) reflects most solar radiation meaning there is less energy available to heat the surface and a surface with low albedo (e.g. forests or dark soils) absorb more solar radiation so will be warmer
  • NDVI is calculated from Red and NIR bands to understand vegetation cover
  • Surface temperature is received from the TIR band
  • Rn is computed using albedo, NDVI and solar radiation
  • G is estimated from Rn and NDVI
    - NDVI is used for this calculation as in vegetated areas, most energy goes into H or λET whereas in bare soils, more energy goes into G
  • H is calculated using surface temperature and windspeed
    - In dry conditions, most energy is used for H whereas in wet conditions λET dominates
  • ET is calculated as the residual (Rn – G – H) 
  • ET is scaled from instantaneous to daily or seasonal values 
    - This takes the ET value from a single measurement in time, to values relevant for irrigation management

The surface energy balance equation:

  • 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

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