The radiation balance of a tropical pasture, II. Net all-wave radiation

Abstract

Seasonal changes in net all-wave and long-wave radiation are discussed, as measured over three surfaces, one dominated by Townsville stylo, another by two annual grasses and a third consisting of bare soil. The short-wave reflection coefficient of these three surfaces has been discussed in Kalma and Badham (1972). Net long-wave loss during night-time varied between 3 and 12% and net long-wave loss over 24 h between 19 and 47% of total incident short-wave radiation. In general, wetting and rapid growth caused net long-wave loss, as a fraction of incident solar radiation, to decrease. Drought, maturing, lodging and senescence had the opposite effect. Differences between the three surfaces were related to cover, stand height, drought resistance and growth phase. A unique relationship was observed between daily atmospheric transmissivity and the ratio between daily totals of diffuse and total radiation. It is shown how daily net long-wave radiation, under all conditions, may be predicted from known daily values of atmospheric transmissivity and air temperature. Effective surface radiative temperatures, indirectly obtained from measurements with single-sided pyrradiometers, have been presented for the two vegetative surfaces for three representative days. Semi-empirical relations for estimating net all-wave radiation during daylight from incident total radiation were obtained from linear regressions computed, using hourly totals, for net all-wave radiation on incident total radiation for all three surfaces and six successive periods covering the experimental season. Three seasonal regressions for the individual surfaces are given, and are considered to have practical applicability in further studies. Finally, long-wave and short-wave components of the radiation balance are given as percentages of monthly and seasonal mean daily short-wave radiation income.