Variations in the Surface Energy Balance Over Sagebrush: Implications for Energy Balance Closure

Micrometeorological studies using the eddy covariance method often underestimate some component(s) of the surface energy balance, and hence fail to "close" the energy budget. Reasons cited for this include: a footprint "mismatch" between the turbulent flux measurements, net radiometers and soil heat flux plates, due to spatial variability within the measurement area. To investigate this we compared the net radiation, latent and sensible heat flux, soil heat flux, and soil moisture at a stationary site relative to a mobile site positioned for one week at 10 locations. Over an extensive sagebrush site in northeastern Colorado during June-August 2001, we found that the energy balanced closed to within 10% (half-hour basis) both at our mobile and stationary sites, located at distances of 16 and 32 m along N, S, E and W transects (one and two times the peak footprint of the mobile site). All of the energy balance components at the mobile site were within 5-10% (half-hour basis) of those measured at the stationary site regardless of the mobile site's position. We conclude that at this site, point measurements are representative of this ecosystem, and that our missing energy was not a result of spatial heterogeneity.