EARS Holding B.V.

Energy Balance Data Software

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Actual evapotranspiration is derived through the earth surface energy balance. Hourly thermal infrared and visual data are pre-processed to noon and midnight composites. By means of calibration procedures the data are subsequently converted to converted to planetary temperature and albedo. By means of atmospheric correction procedures these are subsequently converted to actual surface temperature and albedo. In addition the boundary layer air temperature (Ta) is determined from the noon and midnight surface temperature using an analytic model of the daily temperature cycle. The air temperature at observation height (1.5 m) is then obtained by mixing the surface and boundary layer air temperature.

Radiation

Incoming global radiation (Ig) is derived from the time of the year, longitude, latitude, atmospheric optical depth and cloudiness. Then the net radiation (In) is obtained with:

               In = (1-A).Ig+Ld-Lu

The downward and upward longwave radiation fluxes (Ld, Lu) are derived from the boundary layer and surface temperature, respectively.

Sensible heat flux

The daily sensible heat flux (H) is found by multiplying the turbulent heat transfer coefficient with  the surface-boundary layer temperature difference:

       H = a.(T-Ta)

The heat transfer coefficient a is a function of height, taken from a DEM. This is based on  decreasing specific mass and increasing aerodynamic roughness with height.

Evapotranspiration

The daily latent energy flux (LE) is found as the net radiation minus the sensible heat flux (H), minus a correction for photosynthetic light use (E):

       LE = In - H - E

The latent heat flux is converted into the actual evapotranspiration in mm/day. The figure above shows the actual evapotransiration in the Nile basin during the year 2006, expressed in mm water. The potential evapo-transpiration may be determined from the net radiation using a Makkink type formula. Also the Thornthwaite estimation method has been implemented.

Snow accumulation and melting

Given the precipitation and surface temperature data, snow accumulation may be mapped in a post-processing step. In spring snowmelt is simulated on the basis the available latent energy after accounting for potential evaporation/sublimation.

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