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 was found to be at least 20-30 percent greater than the sum of sensible, latent and soil heat fluxes. We hypothesis that a point measurement of net radiation in an open-canopied forest ecosystem might result in an overestimate of the available energy in the eddy flux source area. To test this hypothesis we measured surface temperature and reflection coefficients of soils and plant species in a 100 by 100 m area, and developed geometrical models of the net radiation. Measured radiative surface temperature of shrubs, grasses, and trees was found to vary considerably from 55-60 C (sunlit bare soil) to 24-29 C (trees and grasses). Reflection coefficient varied only from 0.09 to 0.12 between components. A net radiometer located a few meters above tree height will receive upwelling radiation from the sides of the surrounding trees and records a larger fraction of radiation from vegetation than from hot bare soil. Tree distribution and size information of the 100 by 100 m area was used to generate the view of the lower dome of a net radiometer using a 3D ray tracing model.
Preliminary results suggest that the long-wave radiation received by the lower net radiometer dome was lower than the spatially integrated upward flux based on fractional ground cover of the vegetation and soil, but was strongly dependent on the local vegetation distribution around the net radiometer. Spatial variation in the modeled upward long-wave radiation between different locations (above tree top, close to tree, or in a clearing) decreased with increased distance from the surface. Variation in the fraction of sunlit and shaded foliage and soil viewed by a net radiometer may bias estimates of the short-wave upwelling radiation.