The Radiation Balance

Solar radiation (short wave) is the energy source for photosynthesis, warming, and evaporation in agricultural systems. Its value can be calculated as a function of latitude, time of year, and cloud cover. Fifty percent of solar radiation is available to photosynthesis and is called Photosynthetically Active Radiation (PAR), although only a very small fraction is actually used in this process. Net radiation is obtained by discounting the reflected solar radiation (which depends on the albedo) and longwave losses that depend on air temperature, humidity, and cloud cover. Plants intercept all types of the radiation fluxes. Radiation interception is modulated by leaf angle distribution that varies with leaf area index (LAI) and plant type. The fraction of radiation intercepted by trees can be calculated assuming simple geometrical forms (e.g., spheroids).