Spatial heterogeneity of surface energy exchange in an alpine catchment

This paper presents results from a study investigating temporal and spatial controls on surface energy exchange in a large alpine catchment. In naturally complex environments, spatial variability in surface energy exchange results from changes in topographic characteristics such as elevation, slope angle and aspect as well as changes in the nature of the surface, including roughness, vegetation type, moisture content and thermal conductivity. Observational data were collected using eddy covariance and Bowen ratio energy balance systems over a variety of surfaces in the Tekapo catchment located in the Southern Alps of New Zealand between November 1997 and April 1999. Research was conducted as part of the Lake Tekapo Experiment (LTEX) to investigate boundary layer characteristics and airflow in complex terrain.

Considerable variability in both radiation and energy budgets was found to be controlled most strongly by surface cover, topography and soil moisture. Surface radiation budget modelling was conducted for the entire catchment and found to compare reasonably with observational data, with small overestimation of both incoming fluxes. The ratios of energy partitioning into sensible, latent and ground heat fluxes were evaluated for each major surface type from observational data and extrapolated over the catchment based on surface cover classification from IRS-1C satellite imagery. This method yields surface energy flux maps at a spatial resolution realistic for the scale of landscape heterogeneity based on direct measurement. The representativeness of point measurement for similar surfaces throughout the catchment, including problems of advection, is assessed. Finally, the implications of heterogeneity in surface energy exchange for boundary layer characteristics and development of local airflow are discussed.