|Kustas, William - Bill|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2009
Publication Date: 6/15/2009
Publication URL: http://handle.nal.usda.gov/10113/30028
Citation: Sanchez, J.M., Caselles, V., Niclos, R., Coll, C., Kustas, W.P. 2009. Estimating energy balance fluxes above a boreal forest from radiometric temperature observations. Agricultural and Forest Meteorology. 149:1037-1049. Interpretive Summary: An increasing number of studies on water and energy exchange between forests and the atmosphere have been conducted over the past few years in order to understand both forest functioning and the role of forests as sinks or sources of atmospheric water, energy and carbon. This is particularly the case for boreal forests which, constitute over 10% of the earth’s terrestrial surface. However, many studies have relied on micrometeorological measurements, which are local point-based observations, not readily scalable to the watershed or region. The main objective of this work is to test the recently proposed Simplified Two-Source Energy Balance (STSEB) model that uses remotely sensed (thermal) surface temperature for estimating energy and moisture exchange from the vegetation and soil/substrate components over a boreal forest site. The STSEB scheme affords the capability of mapping water, energy and heat exchange over large areas using thermal satellite data with most other inputs extracted from the combination of the multi-spectral information contained in a satellite image. The results of model output in comparison with local measurements suggests the STSEB has potential if from the satellite thermal observations, component soil and canopy temperatures can be reliably estimated and local atmospheric properties can be operationally defined.
Technical Abstract: The great areal extent of boreal forests confers these ecosystems potential to impact on the global surface-atmosphere energy exchange. A modeling approach, based on a simplified two-source energy balance model, was proposed to estimate energy balance fluxes above boreal forests using thermal infrared measurements. Half-hourly data from the Solar Induced Fluorescence Experiment, carried out in a Finnish boreal forest, was used to evaluate the performance of the model. Energy balance closure, determined by linear regression, found all fluxes to underestimate available energy by 9% (r2=0.94). Significance in the energy balance of the heat storage in the air and in the soil terms was also analyzed. Canopy temperatures, measured by a CIMEL Electronique CE 312 radiometer, together with ancillary meteorological variables and vegetation characteristics, were used to run the model. Comparison with ground measurements showed errors lower than ±15 W m-2 for the retrieval of net radiation, soil heat flux and storage heat flux, and about ±50 W m-2 for the sensible and latent heat fluxes. A sensitivity analysis of the approach to typical operational uncertainties in the required inputs was conducted showing the necessity of accurate measurements of the target radiometric surface temperature.