HR: 1340h
AN: B13D-1520    [Abstracts]
TI: Remote sensing of heat fluxes using SEBAL: Comparison between Landsat and MODIS
AU: * Zhang, X
EM: zhang@aero.und.edu
AF: University of North Dakota, Department of Earth System Science and Policy, 4149 University Ave Stop 9011, Grand Forks, ND 582029011, United States
AU: Berhane, T
EM: tedros.berhane@und.edu
AF: University of North Dakota, Department of Earth System Science and Policy, 4149 University Ave Stop 9011, Grand Forks, ND 582029011, United States
AU: Hill, M
AF: University of North Dakota, Department of Earth System Science and Policy, 4149 University Ave Stop 9011, Grand Forks, ND 582029011, United States
AU: Rundquist, B
AF: University of North Dakota, Department of Geography, 221 Centennial Drive Stop 9020, Grand Forks, ND 582029020, United States
AB: Instantaneous heat fluxes were estimated using data obtained from Landsat 5 TM (Thematic Mapper), Landsat 7 ETM+ (Enhanced Thematic Mapper Plus) and Terra MODIS (Moderate Resolution Imaging Spectroradiometer) using Surface Energy Balance Algorithm for Land (SEBAL) model for cloud-free days. The modeled results were compared with measurements of net radiation (both incoming and outgoing short and longwave), soil, sensible and latent heat fluxes by two flux towers located in Brookings, SD and Fort Peck, MT. Flux tower data were 30 minutes averages at every half an hour and the contributing area of the air within the period was estimated for each satellite pass by taking into accounts the factors of observation height, atmospheric stability, and surface roughness as well as wind speed and directions (Hsieh et al. 2000). We found that footprints (considering 90% contributing areas) were normally larger than the size of one Landsat pixel (30 m) but smaller than that of one MODIS pixel (1 km). Therefore for Landsat the data were average for pixels within the concurrent footprint and for MODIS the data for the particular pixel covering the flux tower is used. The correlation coefficients between the modeled and the observed net radiation values for Landsat and MODIS were found to be 0.70 and 0.66 respectively. Relatively, comparisons were better at Brookings than at Fort Peck site for both sensors. This could be because the former site has a relatively flat topography and larger fetch than the latter, minimizing the possible effects of terrain heterogeneity on incoming and outgoing solar radiation modeling. Poor correlation was found for soil heat flux between satellites estimate and in-situ observations. In addition, the correlation coefficient for sensible heat flux was found to be 0.62 for Landsat. However, for MODIS, the correlation was only 0.11. On the other hand, the comparisons for latent heat flux showed improvement with correlation coefficients being 0.62 and 0.37 for Landsat and MODIS respectively. In SEBAL, cold pixels are used to estimate air temperature, which is used in computation for both net radiation and sensible heat flux. The uncertainties associated with this assumption cancelled out somehow in deriving latent heat flux. SEBAL performed better in modeling the heat fluxes with Landsat data. It is probably due to the scaling issue in comparison as the footprint areas of the flux towers have always been significantly less than a single MODIS pixel. By simulating MODIS observation using Landsat, we found the correlation coefficients for the aggregated Landsat pixels decreased from 0.62 to 0.25 with an increase of RMSE from 50.5 to 68.3 Wm^-2. This suggested that poor performance of MODIS estimate of heat fluxes as compared to the flux tower measurements is due to heterogeneity of the surface within the field of view of MODIS sensor.
DE: 0480 Remote sensing
DE: 1719 Hydrology
SC: Biogeosciences [B]
MN: 2007 Fall Meeting