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Blaine L. Blad
and
Norman J. Rosenberg

Abstract

Most reports show good agreement between evapotranspiration (ET) rates estimated by the Bowen Ratio-Energy Balance (BREB) method and rates measured with lysimeters, although underestimation by the BREB model has occasionally been reported. This study was conducted to evaluate the performance of the BREB technique in the climatic conditions characteristic of the central Great Plains, a region where a significant proportion of the energy consumed by evapotranspiration is supplied from advected sensible heat.

Agreement between the BREB method and lysimetric measurements of ET is good during non-advective periods but during advective periods the BREB model underestimates ET by about 20%. Data collected in this study suggest that the difference is due primarily to an inequality of the exchange coefficients for beat (K h ) and water vapor (K w ). In the development of the BREB method these coefficients are assumed to be identical but our results indicate that the ratio K h /K w is greater than 1 for the stable conditions associated with the downward transport of sensible heat.

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Blaine L. Blad
and
Donald G. Baker

Abstract

This study is based upon net radiation measurements made with a Suomi-type ventilated net radiometer. The average annual net radiation was 36,247 ly for the study period 1 April 1964–31 March 1967. April–October showed positive net radiation totals, November–February negative net radiation totals, and March had positive net radiation totals in two of the three years studied.

Only fourteen days with all negative net radiation and two days with all positive net radiation occurred in the three-year period. Average daily time of positive net radiation ranged from 244 min in January to 749 min in June. Although positive net radiation exceeded negative net radiation an average of 36,247 ly per year, negative or null net radiation was recorded 63% of the time.

The ratio of net radiation to incoming solar radiation varied widely during the year. However, from May–August the ratio ranged only from 0.45–0.54 for any one month during the three years and averaged 0.50.

Monthly net radiation values calculated by the method described by Sellers were within 10% or less of the measured values for all but two months. Budyko's calculated annual value of ∼35,000 ly also agreed favorably with the average measured value.

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Sherwood B. Idso
and
Blaine L. Blad

Abstract

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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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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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John M. Norman
,
Blaine L. Blad
, and
Donald G. Baker

Abstract

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