The use of water by cotton crops in Abyan, South Arabia.
Fifteen, 45, 60 and 90 cm water were applied before sowing cotton; after irrigation there were 10, 28, 45 and 60 cm of available water respectively in the uppermost 3 m of the profiles under the four treatments. Amounts of water used by the crops and estimated from measured changes in soil water [Es) and from measured gradients of air temperature and humidity in and above the crops were 14, 34, 46 and 67 cm respectively. The ratio between Es and Penman's estimate of potential evaporation from a short green crop was 1.16 for a fully grown crop on plots given 90 cm water. On plots where the water supply became limiting (indicated by a lower value to this ratio), there was a decrease in vegetative growth followed after about two weeks by a decrease in the number of new fruiting points and an increase in the % of young flower buds which were shed; after another two weeks the mature bolls were shed. A relationship between the decrease in yield and time of onset of water strain is presented. For the most efficient use of water, irrigation rates should be adjusted to provide 50 cm of available water.