Evaporative Flux from Wheat and Fallow in a Semiarid Climate

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Abstract
Daily evaporative flux and accompanying energy balance components were measured for land in summer fallow and in wheat (Triticum aestivum L.) located side by side in a typical strip‐crop farming area. The study was conducted on a Williams loam (fine‐loamy, mixed Typic Argiborolls) 11 km northwest of Sidney, Mont. Two precision‐weighing lysimeters were used, each situated in the middle of two adjacent 180‐ by 180‐m fields. The two fields, including the lysimeters, were alternately cropped and fallowed. ‘Olaf’ spring wheat was grown in 1978 and 1979 and ‘Roughrider’ winter wheat in 1979–1980. The three cropping seasons differed in that 33 cm of rain fell during the 1978 season and about 9 cm fell in each of the 1979 and 1980 seasons. However, because the root‐zone soil profile was near field capacity in the spring of 1979, the yield of about 4,000 kg/ha was comparable to that in 1978. In all 3 years, cropped and noncropped lysimeters lost almost identical amounts of water up to the time that tillering was completed. Thereafter, evapotranspiration from cropped surfaces was much higher than evaporation from noncropped surfaces. The fallowed lysimeter gained 12 cm of water in 1978 and lost 7 and 2.5 cm in 1979 and 1980, respectively. The cropped lysimeter lost 20 cm in 1978, 42 cm in 1979, and 14 cm in 1980. Rates of water use by the wheat were highest during the heading growth stage in 1978 and during flowering in 1979 and 1980. Differences in the growth stage duration among years seemed to have no relationship to degree days. As indicated by both energy balance and daily ratios of evaporative flux (LE) to net radiation (Rn), sensible heat transfer to the crop strongly influenced evapotranspiration, except in 1980 when the crop was stressed most of the season and contributed sensible heat downwind from the cropped area. For a crop to be energy efficient and also have water available for growth, water conservation practices must be followed consistently, and the crop must be protected as much as possible from stress conditions.

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