Water use by irrigated cotton in Sudan. 2. Net radiation and soil heat flux.
Abstract
Radiation balance was measured from Oct.-Dec. 1965 in an irrigated cotton crop grown on ridges. During this time crop cover increased from about 10% to 95%. Net long-wave loss was affected equally by the radiating surface and the overlying airmass, decreased as crop cover increased and when irrigations were given early in the season, and was 120-160 cal/ cm2 per day less than net long-wave loss in temperate climates would have been at similar r.h. and air temperature. Net radiation was approximately proportional to incoming short-wave radiation because the effects of developing crop cover, decrease in solar altitude and change in the overlying airmass on reflected short-wave radiation and on net long-wave loss were approximately self-cancelling. Net radiation increased after irrigations before full crop cover was reached. The seasonal pattern of soil heat-flux was dominated by the increase in crop cover. Daytime downward flux into the ridge reached an average minimum value of 40 cal/cm2 when the crop was 2 months old; in the furrow it reached an average minimum value of 60 cal/cm2 when the crop was about 3.5 months old. There was an average minimum difference between daytime downward and night-time upward flux of about 25 cal/cm2, which probably was lost as latent heat by evaporation from cracks in the soil. Row orientation affected the diurnal pattern of energy available for evaporation and consequently the occurrence or absence of a midday decrease in evaporation from a well-watered crop. In the absence of measurements, net radiation can be calculated from a Brunt-type formula for which revised coefficients are given.-R.B.