Water Use by Irrigated Cotton in Sudan. I. Reflection of Short-Wave Radiation

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Abstract
The reflection of short-wave radiation by irrigated cotton was measured in every 1/2 hr. of daylight during Oct.-Dec. 1965, the major part of the growing season. As the crop developed and the declination of the sun changed the mean values per day of the reflection coefficient increased from 0.15 to 0.23. When the soil was wetter, after rain or irrigation, the values were relatively small, and they rose during the intervals between irrigations, though this effect became smaller as the leaf area Index of the crop increased. The mean values of the coefficient during the day ranged from 0.36 to 0.18; they were below 0.20 from 09.00 to 14.30 hr. This pattern appeared to depend on solar altitude. Wilting during the afternoon in the latter 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 emphasized because the rows were aligned from north to south.

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