Local and Regional Components of Sensible Heat Advection

Thomas W. Brakke Department of Agricultural Engineering, University of Nebraska, Lincoln 68583

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Shashi B. Verma Department of Agricultural Engineering, University of Nebraska, Lincoln 68583

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Norman J. Rosenberg Department of Agricultural Engineering, University of Nebraska, Lincoln 68583

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Abstract

Detailed knowledge of the advection of sensible heat is necessary to understand the energy balance of the evaporating surface in many parts of the world. Sensible heat advection can result from regional and/or local sources. The local and regional components of sensible heat advection (Aloc and Areg, respectively) are identified and their magnitudes in a semi-arid to sub-humid zone are established in the work reported here. Measurements of dry- and wet-bulb air temperature, wind speed and net radiation were made above an irrigated alfalfa field with relatively dry surroundings upwind at Mead, NE. A modified Bowen ratio-energy balance method which incorporates horizontal gradients of air temperature and vapor pressure was used to compute evapotranspiration (ET) rates.

Sensible heat advection at the furthest upwind location in the irrigated field contributed from 15 to 50% of the energy consumed in ET on a daily basis. Areg was greatest on days with strong winds; Aloc was independent of wind speed. The dryer the air, the greater the advection of sensible heat.

Abstract

Detailed knowledge of the advection of sensible heat is necessary to understand the energy balance of the evaporating surface in many parts of the world. Sensible heat advection can result from regional and/or local sources. The local and regional components of sensible heat advection (Aloc and Areg, respectively) are identified and their magnitudes in a semi-arid to sub-humid zone are established in the work reported here. Measurements of dry- and wet-bulb air temperature, wind speed and net radiation were made above an irrigated alfalfa field with relatively dry surroundings upwind at Mead, NE. A modified Bowen ratio-energy balance method which incorporates horizontal gradients of air temperature and vapor pressure was used to compute evapotranspiration (ET) rates.

Sensible heat advection at the furthest upwind location in the irrigated field contributed from 15 to 50% of the energy consumed in ET on a daily basis. Areg was greatest on days with strong winds; Aloc was independent of wind speed. The dryer the air, the greater the advection of sensible heat.

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