Alfieri, J G

Land surface characteristics, such as land use type and vegetation density, are key controls on land- atmosphere exchange processes. As such, it should be possible to link spatial variations in airborne flux measurements collected during the 2002 International H2O Project (IHOP_2002) with surface heterogeneity apparent in Landsat- and MODIS-derived products. The variability in the turbulent fluxes for the IHOP_2002 flights was typically 24 W m-2 and 57 W m-2, respectively, for the sensible and latent heat flux. Heterogeneity in the land surface was characterized using the 2001 National Land Cover Dataset (NLCD 2001) and MODIS land use maps, and the MODIS-derived lead area index (LAI), green vegetation fraction, and surface temperature. By calculating the Horst and Weil flux footprint for each 4-km segment along the flight track of the IHOP_2002 flights, both the source area and relative contribution of each land use type within the footprint to the measured flux was determined, assuming the flux contribution was a function of land use type alone. By overlaying the footprint atop the satellite-derived products, the spatial variability in the measured fluxes was linked to spatial heterogeneity in the surface characteristics. For example, although the fraction of wetlands was small, they had a disproportionately large influence on the latent heat flux. Contrary to our assumptions, it was found that fluxes varied even for a given land use type due to variations the range of vegetation characteristics contained in a single land-use category. This work has important implication both for understanding how to best represent fine-scale, subgrid variability in land surface and numerical weather models and for accurately modeling land-atmosphere processes and local, regional and continental scales.