Water use by irrigated cotton in Sudan. 1. Reflection of short-wave radiation.
Reflection of short-wave radiation by irrigated cotton in the Sudan Gezira was measured every 0.5 h in the daytime during Oct.-Dec. 1965, the major part of the growing season. As the crop developed and the declination of the sun changed, the mean daily reflection coefficient values increased from 0.15 to 0.23. When the soil was wet, after rain or irrigation, the values were relatively small, and they rose during the intervals between irrigations, though this effect declined as crop LAI increased. Mean values of the coefficient during the day ranged from 0.18 to 0.36; they were below 0.20 from 9 a.m. to 2.30 p.m. This pattern appeared to depend on solar altitude. Wilting during the afternoon, in the later part of the season, tended to decrease the coefficient, probably because the leaves tended to move from a horizontal to a vertical position as the day advanced and internal water deficits increased, so that more of the dark soil surface became 'visible' to the instruments. The diurnal variation was pronounced because the rows were aligned N.-S.-R.B.