An Experimental Study of Net Radiation, Its Components and Prediction1

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Abstract
Accurate estimates of net radiation may be preferable to a direct measurement with a net pyrradiometer, due to sensor cost and maintenance requirements, for certain long‐term applications such as irrigation scheduling. Thus, the objective of this study was to investigate methods of estimating net radiation from readily available meteorological measurements and in the process, to study each component of net radiation.Simultaneous measurements of incoming and outgoing solar and longwave radiation, net radiation, air temperture, and vapor pressure were made over a well watered alfalfa (Medicago sativa L.) crop at the Scottsbluff Agricultural Laboratory of the Univ. of Nebraska Panhandle Station. Air temperature, used in the Stefan‐Boltzmann equation, was a good predictor of daily outgoing longwave radiation. Calculation of incoming longwave radiation using three values of atmospheric emisivity (Idso and Jackson, 1969; Brutsaert, 1975; Idso, 1980) overestimated measurements. Two procedures to calculate daily values of net radiation are outlined and tested on independent data. One method uses two emisivities, one for clear sky and one for cloudy conditions. The second method uses only one emissivity and is more straight forward. Calculation of net radiation from incoming solar radiation, air temperature, and vapor pressure by both methods compares quite well to measured values, r = 0.98 for each method.

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