Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 20, 2005
Publication Date: February 10, 2006
Citation: Kustas, W.P., Anderson, M.C., French, A.N., Vickers, D. 2006. Using a remote sensing field experiment to investigate flux-footprint relations and flux sampling distributions for tower and aircraft-based observations. Advances in Water Resources. 29:355-368.
Interpretive Summary: Remote sensing studies evaluating the effects of image resolution on reliably detecting land cover changes find that pixel sizes of less than 1x1 km are typically required. Associated with land use and land cover changes, there can be significant changes in surface energy balance and ET. Such changes can be monitored with remotely sensed surface temperature and vegetation cover, but these inputs must be at high enough resolutions where different land cover conditions can be distinguished. Traditional tower-based measurements provide only a small sample of the ET variability over a landscape. Aircraft-based ET observations have also been used since these measurements sample more of the spatial variability in fluxes and are more representative at landscape and regional scales. To date, however, comparisons with remote sensing-based model output have not given acceptable results. In this paper we explore the relationships between the spatial variations in aircraft-flux measurements collected over an agricultural region in the Southern Great Plains and flux maps created by a multi-resolution remote sensing-based energy balance model. More specifically, we examine the utility of the high resolution output (30 m) from the remote sensing based model to help determine the distribution of fluxes sampled by tower and aircraft-based flux systems, and how well they represent the variation in fluxes over the landscape. The high resolution flux map generated by the model is then used to explore the spatial correlations with land surface flux variability computed from the aircraft transects. This analysis indicated that the source-area for heat was estimated to be much closer to the aircraft flight line than for water vapor. This has major implications in interpreting and validating land surface model output with tower and aircraft measurements.
During a remote sensing field experiment conducted in the Southern Great Plains in 1997 (SGP97), tower and aircraft-based flux observations were collected over one of the main study sites in central Oklahoma. This is an agricultural region and contains primarily grassland/pasture and winter wheat, which was recently harvested leaving a significant number of fields either as wheat stubble or plowed bare soil. Multi-spectral data obtained by aircraft provided high resolution (30 m) spatially-distributed vegetation cover and surface temperature information over the study area. The spatial variations in these surface states strongly affect the partitioning of surface fluxes between sensible and latent heat. These data, together with coarser resolution (5 km) satellite data, are used in a remote sensing-based energy balance modeling system that disaggregates flux estimates from 5 km to 30-m resolution. The resulting high-resolution flux maps provide a means for evaluating whether tower and aircraft-based flux measurements sample a full range in flux conditions for this landscape. In addition, this remote sensing-based modeling system can be used to investigate the influence of variability in these key surface states on tower and aircraft measurements through flux-footprint modeling. Under the light wind and unstable conditions that existed during the observations, highest correlation between aircraft and modeled estimated heat and water vapor fluxes were obtained using different flux-footprint estimates. More specifically, the source-area for heat was estimated to be much closer to the aircraft flight line than for water vapor.