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Title: IMPACT OF USING DIFFERENT TIME-AVERAGED INPUTS FOR ESTIMATING SENSIBLE HEATFLUX OF RIPARIAN VEGETATION USING RADIOMETRIC SURFACE TEMPERATURE

Author
item Kustas, William - Bill
item Prueger, John
item HIPPS, LAWRENCE - UTAH STATE UNIVERSITY

Submitted to: Journal of Applied Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Reliable estimates of spatially distributed surface energy balance components are essential for accurate modeling of the hydrologic cycle and assessing vegetative conditions over a landscape. A riparian corridor along the Rio Grande river dominated by the Eurasian tamarisk or salt cedar (Tamarix spp.) is being studied to determine water and energy exchange rates from 10s of minutes to daily and seasonal time scales. Local estimates of the heat fluxes in the Tamarix were made using tower-based measurements. To extrapolate local estimates of riparian water and energy fluxes to large sections of the Rio Grande basin requires information concerning the spatial extent and condition of the riparian corridor. Remote sensing data can provide information related to vegetation cover and surface temperature which has been shown to reflect vegetation water use and energy exchange rates. Critical issues related to surface temperature-water use relations are being addressed. However, little attention has been given to the effect of using different time-averaged quantities in these formulations. This study evaluates the impact on heat flux estimation using relatively short time-averaged (1- min) surface temperatures measured from a fixed-head infrared radiometer with 1-, 10- and 30- min time averaged meteorological input data used in heat transfer formulations. The results indicate significant variations in computed heat flux estimates using different time-average quantities can be obtained. This has implications in the application of these formulations using remote sensing data from satellite platforms which provide essentially "instantaneous" surface temperature values and validation with flux tower measurements along riparian corridors.

Technical Abstract: A riparian corridor along the Rio Grande river dominated by the Eurasian tamarisk (Tamarix spp.) is being studied to determine water and energy exchange rates. Local estimates of the heat fluxes were made using eddy covariance instrumentation mounted on a 12 m tower. To extrapolate local estimates of riparian heat fluxes to large sections of the Rio Grande basin nrequires information concerning the spatial extent and condition of the riparian corridor. Remote sensing data provides a surface brightness temperature shown to reflect the relative partitioning of net radiation energy into sensible and latent heat fluxes from vegetated landscapes. Critical issues related to uncertainty in converting brightness temperature to a radiometric surface temperature and non uniqueness of radiometric-aerodynamic surface temperature relationship are being addressed. However, little attention has been given to the effect of using different time-averaged quantities in heat transfer formulations. This study evaluates the impact on sensible heat flux estimation using relatively short time-averaged (1- min) canopy temperatures measured from a fixed-head infrared radiometer with 1-, 10- and 30- min time averaged input data. The results indicate that with short time-averaged radiometric surface temperatures (essentially "instantaneous" from a satellite), significant variations in sensible heat flux estimates are likely to occur, even under near-constant net radiation conditions, due in part to natural perturbations in surface temperature. The resulting implications for computed heat flux estimates using data from remotely sensing platforms and validation with flux tower measurements along riparian corridors are discussed.