Estimating daily global radiation from air temperature and rainfall measurements

Abstract

Radiation-mediated processes (e.g. photosynthesis, evapotranspiration) can be simulated on the basis of measured incoming radiation. Since irradiance is measured at very few weather stations it is often estimated from the duration of bright sunshine or from mean daily cloudiness. In the absence of both these variables, irradiance can be estimated by means of satellite images, spatial interpolation techniques or standard meteorological observations. In this paper, a model to estimate daily global radiation from air temperature and rainfall measurements is presented. The model first assesses days on w k c h the probabilities of clear and overcast skies are maximum and predicts, for these days, the mean sky transmittance. The mean sky transmittance can then be interpolated for any other day. In the model, station parameters are directly derived from latitude and ancillary information, so neither historical climatic records nor calibration parameters are required for the calculations. This simplifies to a large extent the estlrnation procedure. The model was tested for 22 weather stations in 11 countries by comparing estimated and measured irradiance data. Measured and estimated records were also separately used as input of a wheat growth simulation model (NWHEAT) and model results were compared. This comparison showed that when measured irradiance is substituted by estimates there is a very small devlance in the grain yield as calculated by the model.