Solar and net radiation, available energy and its influence on evapotranspiration from grass.

Abstract

From studies on radiation and energy balance in the Netherlands it is concluded that reliable values for net radiation over 24-hr periods can be obtained by computation, if differences in the weather during day and night are taken into account. A comparison is given of measured net radiation above short grass with net radiation computed from air temperature, water vapour pressure in the air and sunshine duration. Variations in the nature of a cropped surface, as a consequence of variations in conditions of such a surface, may influence the radiation and energy balance. However, if soil moisture becomes less available for plant roots, soil moisture content has a much more important influence on the proportion of the available energy used in heating the soil, the air and in evapotranspiration than the type and nature of the soil cover. For daily and 24-hr periods with similar atmospheric conditions, a linear relationship between total global radiation and net radiation can be expected. The ratio between evapotranspiration from short grass under optimal water supply and the evaporation from a wet surface (or thin water layer) varied greatly for short periods of time under different conditions of radiation and temperature. These variations could be due to plant or weather factors influencing the stomatal apertures, although variations in stability of the lower air layers may have had some influence. Variations in intensity of potential and of maximum evapotranspiration rates and differences in the diurnal variations, as compared with the diurnal variation of net radiation, are shown. A decrease of actual evapotranspiration from crops as soil moisture becomes less available and, of course, the potential evapotranspiration rate, strongly depend on the net radiation gain. F. s.- R.B. (Abstract retrieved from CAB Abstracts by CABI’s permission)