Water use by irrigated cotton in Sudan. 3, Bowen ratios and advective energy.
Wet and dry bulb temperatures and run of wind were measured over irrigated Gossypium herbaceum cv. BAR XL 1 at various heights and distances from the leading edge of a field in the Sudan Gezira lying downwind from a bare fallow area. These were used in conjunction with radiation balance data to estimate the vertical and horitzontal fluxes of water vapour. Estimates of vertical flux under near-neutral conditions using the Bowen ratio method did not agree well with those obtained using the Thornthwaite-Holzman formula; the former method was preferred on theoretical grounds. Horizontal fluxes below the effective level of measurement of vertical flux were obtained from run-of-wind and the cross-wind humidity gradient at various heights. Cross-wind variation in the temperature and humidity profiles was small at >40 cm above the crop. In a check on the estimated income and expenditure of energy over a complete irrigation cycle, a discrepancy of 5-15% was found, depending on fetch and crop development. Mean Bowen ratios were calculated to illustrate the variation in temperature and humidity above the crop with fetch, stage in irrigation cycle and crop development. Estimates of total evaporation suggested that irrigation water was unevenly distributed over the field. Evaporation rates early in an irrigation cycle were smaller than in the middle of a cycle. Evaporation was less than the potential rate towards the end of irrigation cycles in the first half of the season, probably because soil water was limiting. The evaporation rate from an actively growing crop not short of water was often up to 1.8 times as great as the rate at which net radiant energy became available to the crop if both local and general advection were important; where general advection alone was important this figure reached 1.5. Local advection was not very noticeable beyond 200 m from the leading edge whereas general advection was noticeable over the whole field. RB.