Variation of net radiation over heterogeneous surfaces: measurements and simulation in a juniper–sagebrush ecosystem

As part of a larger study of carbon dioxide and energy exchange, energy components in an open-canopied juniper–sagebrush ecosystem located in the semi-arid region of Eastern Oregon were measured with the eddy covariance technique. Daytime net radiation averaged 20–30 % greater than the sum of sensible, latent and soil heat fluxes. On cloudless days several days after a rain event the imbalance was 200–250 W m−2. At such times, differences between the surface radiation temperatures of soil and foliage were large, and we investigated whether such differences may generate systematic errors in the measurement of net radiation. A point measurement of net radiation above an open-canopied forest ecosystem is uncertain, because vegetation structure around the measurement location can be highly variable. Depending on location, various fractions of the upwelling radiation from the soil are intercepted by vegetation and do not reach the radiometer. To determine the magnitude of this uncertainty, we measured tree locations and dimensions, and surface radiation temperature (Tr) and shortwave reflection coefficients () of soils and vegetation in a 100 by 100 m area. Geometrical models generated by ray tracing and rendering software were used to calculate the upwelling radiation that would reach radiometers placed at random locations above the surface. In summer, under cloudless skies the measured radiative surface temperatures of soil and vegetation varied considerably, from a mean of 56C for sunlit soil to 25C for shaded soil, and 27–29C for sunlit and shaded vegetation (trees and shrubs)