Measurement and Modeling of Surface Energy Fluxes within Rangeland Ecosystems.

Intermingling of vegetation communities in mountainous rangeland and forest ecosystems creates a challenge for measuring or modeling the energy and mass fluxes in these complex environments, as adequate fetch requirements and complex topography make interpretation of EC results difficult.  This study assesses surface energy balance measurements taken over sagebrush, aspen and the understory of grasses and forbs beneath the aspen canopy, and measurements were compared with simulations from a multi-layer canopy model.  Energy balance closure for the sagebrush community was quite good and surface energy fluxes were simulated quite well by the model.  Closure of the understory measurements was highly dependent on whether direct solar radiation was impinging on the net radiation sensor through gaps in the aspen canopy.  Upward sensible heat flux within the aspen canopy was unexpectedly observed during snowmelt; model simulations suggested that this was due to the influence of tree trunks absorbing solar radiation and releasing sensible heat upward to the atmosphere and downward to the snowpack.  Simulated turbulent fluxes above the aspen canopy were reasonable, but model modifications were necessary to capture the very stable conditions at the bottom of the aspen canopy, particularly during snowmelt.  Results indicate that reliable surface energy balance measurements can be taken in these complex environments, but care must be used in the interpretation of the results.