Home > Thematic description of CGMS level 1 > Acquisition, checks and processing daily station data > Calculation of Evapotranspiration

• Introduction
• Thematic description of CGMS level 1
• Goals and assumptions
• Acquisition, checks and processing daily station data
  • Data Acquisition and Checks
  • Calculation of Global Radiation
  • Calculation of Evapotranspiration
• Spatial Interpolation to regular climatic grid
• Alternative entry for grid weather data
• Thematic description of CGMS level 2
• Additional functionality in the CGMS software for yield forecast
• Overview of different CGMS versions
• CGMS 9.2 database
• CGMS User Interface
• References
• Annex A Using CGMS in batch mode
• Contacts
• Legal Notice

Thematic description of CGMS level 1

Calculation of Evapotranspiration

Daily meteorological station data received from GTS does not contain potential evapotranspiration. This parameter is calculated by the CGMS with the well-known Penman formula (Penman, 1948).

In general, the evapotranspiration from a water surface can be described by:

(2-4)

where:

E0	: evapotranspiration from a water surface	[mm d-1]
Rna	: net absorbed radiation	[mm d-1]
EA	: Evaporative demand	[mm d-1]
Δ	: Slope of the saturation vapour pressure curve	[mbar°C-1]
γ	: Psychrometric constant (0.67)	[mbar°C-1]

Evapotranspiration from a wet bare soil surface (ES0) and from a crop canopy (ET0) can also be calculated with the above formula. Only the albedo and surface roughness differs for these three types of evapotranspiration as explained below. The net absorbed radiation depends on incoming global radiation, net outgoing long-wave radiation, the latent heat and the reflection coefficient of the considered surface (albedo). For E0, ES0, and ET0 albedo values of 0.05, 0.15 and 0.20 are used respectively. The evaporative demand is determined by humidity, wind speed and surface roughness. For crop canopies (ET0) a surface roughness value of 1.0 is used and for a free water surface and for the wet bare soil (E0, ES0) value 0.5 is taken. For a more detailed description of the underlying formulae we refer to Supit et al. (1994) and van der Goot (1997). The calculated E0, ES0, and ET0 are stored in table CALCULATED_WEATHER.