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

• 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 Global Radiation

Global radiation is the daily sum of incoming solar radiation that reaches the earth surface. It is mainly composed of wavelengths between 0.3 μm and 3 μm. Approximately half of the incoming radiation with wavelengths between 0.4 and 0.7 μm is Photosynthetically Active Radiation (PAR). Global radiation is the driving variable in the growth-determining CO2 assimilation process and thus crop growth models are sensitive to radiation data (van Diepen, 1992).

A major problem is the scarcity of measured global radiation. In cases where no direct observations are available it must be derived from sunshine duration, cloud cover and/or temperature, on the basis of relatively weak relationships. The global radiation calculation uses one of three formulae (Ångström, Supit, Hargreaves), depending on the availability of meteorological parameters. An important component in these formulae is the amount of Angot radiation which is the extra-terrestrial radiation integrated over the day at a certain latitude on a certain day. In fact, all of the three formulae estimate the fraction of Angot radiation actually received at the earth surface. The calculation of the Angot radiation and the three different formulae are described by Supit et al. (1994) and van der Goot (1998a).

The following hierarchical method is used to calculate global radiation (Supit and van Kappel, 1998). If observed global radiation is available it will be used. In the case sunshine duration is available, global radiation is calculated using the equation postulated by Ångström (1924) and modified by Prescott (1940). The two constants in this equation depend on the geographic location.

(2-1)

where:

Rg	: global radiation	[J m-2 d-1]
Ra	: Angot radiation	[J m-2 d-1]
n	: bright sunshine hours per day	[h]
L	: astronomical day length	[h]
Aa, Ba	: regression coefficients (Ångström)	[-]

When sunshine duration is not available but minimum and maximum temperature and cloud cover are known, the Supit formula is used, which is an extension of the Hargreaves formula (Supit, 1994). Again, the regression coefficients depend on the geographic location.

(2-2)

where:

Tmin, Tmax	: minimum and maximum daily temperature	[°C]
CC	: cloud cover in octets	[-]
As, Bs	: regression coefficients (Supit)	[-]
Cs	: regression coefficient (Supit)	[J m-2 d-1]

Finally, when only the minimum and maximum temperatures are known the equation of Hargreaves et al. (1985) is used. Again, the regression coefficients depend on the geographic location.

(2-3)

where:

Ah,	: regression coefficient (Hargreaves)	[-]
Bh	: regression coefficient (Hargreaves)	[J m-2 d-1]

The main problem with the application of these formulae is the quality of the regression constants. Studies by Supit (1994), Supit and van Kappel (1998) and van Kappel and Supit (1998) showed no relationship between latitude and the coefficients, although such a relation is frequently used to estimate these regression constants.

Supit and van Kappel (1998) and van Kappel and Supit (1998) have obtained sets of regression constants for the above mentioned formulae for as many weather stations as possible, with a geographic distribution that corresponds to the area of interest for the CGMS in Europe. As a result, a set of 256 reference stations has been identified for which a relevant set of measured radiation data and other parameters in the formulae exist. For these stations regression constants have been calculated based on measured radiation data for the three formulae mentioned above. They are stored in table SUPIT_REFERENCE_STATIONS.

The program SupitConstants (an additional program in addition to CGMS) uses this set of data, consisting of latitude, longitude, altitude and calculated regression constants, to derive the regression constants for all stations in the CGMS for Europe. Interpolation of the regression constants of the reference stations to other stations is based on a simple distance weighted average of the three nearest stations. More information is given by Kappel and Supit (1998). This process is carried out once, unless the set of reference stations changes or when new stations are added. Interpolated regression constants are written in the table SUPIT_CONSTANTS. After the regression constants have been established for all stations, global radiation can be calculated by the CGMS using any one of the above formulae. Finally, the CGMS writes the derived daily global radiation of every station in the table CALCULATED_WEATHER.