UTILITY OF REMOTE SENSING BASED TWO-SOURCE ENERGY BALANCE MODEL UNDER LOW AND HIGH VEGETATION COVER CONDITIONS

Authors: Li, Fuqin; Kustas, William - Bill; Prueger, John; NEALE, CHRISTOPHER - UTAH STATE UNIV; Jackson, Thomas

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2005
Publication Date: 12/5/2005
Citation: Li, F., Kustas, W., Prueger, J.H., Neale, C., Jackson, T. 2005. Utility of remote sensing based two-source energy balance model under low and high vegetation cover conditions. Journal of Hydrometeorology. 6:878-891.

Interpretive Summary: Land surface temperature is a key boundary condition in many remote sensing-based land surface modeling schemes. Land surface temperatures derived from high resolution Landsat TM/ETM scenes and aircraft imagery, along with tower-based meteorological data and Normalized Difference Water Index (NDWI)/Optimized Soil Adjusted Vegetation Index (OSAVI) from Landsat/aircraft, provided inputs for the two-source energy balance model during the Soil Moisture Atmosphere Coupling Experiment (SMACEX). The tower based flux observations collected during SMACEX were used to evaluate model results. In this investigation, two resistance network formulations used in a two-source model for parameterizing soil and canopy energy exchanges are evaluated for a wide range of soybean and corn crop cover and soil moisture conditions during the experiment. The parallel resistance formulation does not consider interaction between the soil and canopy systems whereas the series resistance algorithms provide interaction via the computation of a within-air canopy temperature. The results indicated that both the parallel and series resistance formulations produced similar good estimates, with root-mean-square-differences (RMSD) values ranging from approximately 20 to 50 W m-2 for net radiation and latent heat fluxes, respectively. Although both series and parallel versions gave similar results, the parallel resistance formulation was more sensitive to model parameter specification, particularly in accounting for effects of vegetation clumping due to row crop planting on flux partitioning. A sensitivity and model stability analysis for a key model input variable, fractional vegetation cover, also show that the parallel resistance network is more sensitive to the errors vegetation cover estimates. Furthermore, the parallel resistance scheme is only able to simultaneously balance the radiative temperature and convective heat fluxes between the soil and canopy components for a much narrower range in vegetation cover fraction compared to the series resistance network. This result appears to be related to the modulating effects of the canopy-air temperature parameterization used in the series resistance scheme.

Technical Abstract: Two resistance network formulations used in a two-source model for parameterizing soil and canopy energy exchanges are evaluated for a wide range of soybean and corn crop cover and soil moisture conditions during the Soil Moisture Atmosphere Coupling Experiment (SMACEX). The parallel resistance formulation does not consider interaction between the soil and canopy fluxes whereas the series resistance algorithms provide interaction via the computation of a within-air canopy temperature. Land surface temperatures were derived from high resolution Landsat TM/ETM scenes and aircraft imagery. These data, along with tower-based meteorological data, provided inputs for the two-source energy balance model. Comparison of local model output with tower-based flux observations indicated that both the parallel and series resistance formulations produced basically similar estimates with root-mean-square-differences (RMSD) values ranging from approximately 20 to 50 Wm-2 for net radiation and latent heat fluxes, respectively. The largest relative difference in percentage (mean-absolute-percent-difference, MAPD) was for sensible heat flux, which was ~ 35 %, followed by a MAPD ~ 25% for soil heat flux, ~ 10% for latent heat flux and MAPD < 5 % for net radiation. Although both series and parallel versions gave similar results, the parallel resistance formulation was found to be more sensitive to model parameter specification, particularly in accounting for effects of vegetation clumping due to row crop planting on flux partitioning. A sensitivity and model stability analysis for a key model input variable, fractional vegetation cover, also show that the parallel resistance network is more sensitive to the errors vegetation cover estimates. Furthermore, it is shown that for a much narrower range in vegetation cover fraction, compared to the series resistance network, is the parallel resistance scheme able to achieve a balance in both the radiative temperature and convective heat fluxes between the soil and canopy components. This result appears to be related to the moderating effects of the air temperature in the canopy air space computed in the series resistance scheme, which represents the effective source height for turbulent energy exchange across the soil-canopy-atmosphere system.