Crops and Their Environment

Abstract

The primary energy source for almost all ecosystems is solar energy, which reaches the Earth continuously. At the boundaries of the Earth’s atmosphere, the sun’s radiation efficiency is approximately 1,360 W m⁻². This value is nearly constant and is, therefore, termed the solar constant. The portion of the radiation that reaches the Earth’s surface, the so-called insolation (incoming solar radiation), is lower, because approximately half of the solar energy is lost by absorption and reflection in the atmosphere. Another factor that determines the insolation of a particular site is the angle at which the sun’s rays meet the Earth. Therefore, the insolation is significantly higher at the equator than at the poles. The worldwide highest values (>250 W m⁻² annual average) are reached in tropical and subtropical region rarely covered by clouds (e.g. the Sahara desert). The mean annual insolation in the temperate latitudes is approximately half of that value and amounts to 127 W m⁻². In Stuttgart-Hohenheim in Germany, for example, the value is highest in July (230 W m⁻²) and lowest in December (50 W m⁻²).