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Norman J. Rosenberg and Shashi B. Verma

Abstract

The rate of evapotranspiration by irrigated alfalfa at Mead, NE (41°09′N; 96°30′W, elevation 354 m) reached record levels in 1976. Evapotranspiration was measured with precision weighing lysimeters in a field 1.9 ha in size. Evapotranspiration ranged during the growing season from 4.75 to 14.22 mm day−1 and exceeded 10 mm day−1 on one-third of the days studied. On each day of study the ratio of latent heat flux density (LE) to the sum of the net radiation and soil heat flux densities (Rn + S) was such that LE/(Rn + S) > 1, indicating the occurrence of significant sensible heat advection. On clear days during mid-summer the net radiation provides energy sufficient for evaporation of no more than 7 mm day−1. Sensible heat advection provided the remaining energy consumed in evapotranspiration. The unusually strong sensible heat advection likely was due to the generally dry condition of surrounding regions during the drought of 1976.

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Shashi B. Verma and Jack E. Cermak

Abstract

Distribution of local mass-transfer coefficients over saturated corrugated surfaces was measured in a wind tunnel. Wavy (sinusoidal) surfaces were used to represent field corrugations (furrows). Visualization techniques, in conjunction with wall-pressure distribution measurements, were used to investigate the air flow pattern near the wave surface.The mass-transfer rates from various locations on a corrugated surface are intimately related with the flow structure inside the furrows. The buildup of high humidity in the vortices formed in the furrows tends to reduce the moisture transfer. These vortices are significantly more effective in reducing the evaporation loss from the bottom of deeper corrugations as compared with shallower ones. This substantiates that furrows, if deep enough, can markedly decrease moisture loss from soil.

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Raymond P. Motha, Shashi B. Verma, and Norman J. Rosenberg

Abstract

Thermal inversions induced by regional advection dominate the daytime climate of the central Great Plains during much of the growing season. The influence of these inversions on the turbulent transfer of momentum, sensible heat and water vapor was investigated through detailed observations over an alfalfa crop. The standard deviations of fluctuations in vertical velocity, air temperature and vapor pressure as well as the correlation coefficients for momentum, sensible heat and water vapor were found to be similar under both advective and lapse conditions. Results indicate that turbulent mixing is effectively maintained under advective conditions to transport large quantities of sensible heat to the crop surface and water vapor away from it.

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T. Grayson Redford Jr., Shashi B. Verma, and Norman J. Rosenberg

Abstract

Turbulent fluctuations of vertical wind and fluxes of momentum, sensible heat and latent heat measured with a drag anemometer are compared to like data measured with other instruments. Means of the measured parameters agreed well with energy balance computations of the heat fluxes and profile measurements of the momentum flux. Drag anemometer measurements of turbulent fluxes generally exceeded those obtained with a propeller anemometer, run concurrently. Spectral analysis indicates that the propeller anemometer did not respond well at high frequencies, causing an underestimation of the fluxes and vertical wind fluctuations. The drag anemometer appears to respond well up to 5 Hz.

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Shashi B. Verma, Norman J. Rosenberg, and Blaine L. Blad

Abstract

Results are presented of micrometeorological measurements made over alfalfa and soybeans under conditions of sensible heat advection at Mead, Neb. The sensible heat advection phenomenon reported here is of a regional rather than a local nature. The exchange coefficient for sensible heat (KH) is found to be generally greater than the exchange coefficient for water vapor (KW). This result contradicts the usual assumption of equality of KH and KW under nonadvection (lapse or unstable) conditions when the net transfer of both sensible heat and water vapor are away from the earth's surface. Under advective conditions, however, heat and water vapor are transferred in opposite directions. Our results are supported by Warhaft's (1976) recently published theoretical analysis in which he concludes that the greatest departure of KH/KW from unity will occur when temperature and humidity gradients are of opposite sign.

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T. Grayson Redford Jr., Shashi B. Verma, and Norman J. Rosenberg

Abstract

Simultaneous measurements of humidity fluctuations over a crop made with a specially modified Lyman-alpha hygrometer and a fine-wire thermocouple psychrometer are compared. Standard deviations of the two sets of data are comparable except occasionally when wind speeds were low. The psychrometer appears to underestimate the vertical flux of water vapor due to its slow response. Analysis of humidity spectra and moisture flux cospectra shows that the Lyman-alpha hygrometer is superior to the psychrometer in response at high frequencies and low wind speeds.

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Raymond P. Motha, Shashi B. Verma, and Norman J. Rosenberg

Abstract

Spectra of vertical and horizontal velocity, air temperature and humidity fluctuations were analyzed from measurements made over a well-watered alfalfa crop under conditions of sensible heat advection. Vertical velocity and air temperature spectra as well as cospectra of momentum, sensible heat and water vapor were found to be dependent on atmospheric thermal stratification. These spectra and cospectra were shifted toward higher frequencies under advective as compared to lapse conditions. These results indicate that the predominant eddy sizes are relatively smaller under advective conditions. Humidity spectra, on the other hand, were independent of daytime thermal stratification conditions but were influenced by conditions of the crop. The humidity spectra over a transpiring crop were shifted toward lower frequencies as compared to those over a less well-developed crop canopy.

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Shashi B. Verma, Norman J. Rosenberg, and Blaine L. Blad

Abstract

No abstract available.

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Shashi B. Verma, Norman J. Rosenberg, and Blaine L. Blad

Abstract

No abstract available.

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Thomas W. Brakke, Shashi B. Verma, and Norman J. Rosenberg

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 (A loc and A reg, 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. A reg was greatest on days with strong winds; A loc was independent of wind speed. The dryer the air, the greater the advection of sensible heat.

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