Two Improvements of an Operational Two-Layer Model for Terrestrial Surface Heat Flux Retrieval

Abstract

In order to make the prediction of land surface heat fluxes more robust, two improvements were made to an operational two-layer model proposed previously by Zhang. These improvements are: 1) a surface energy balance method is used to determine the theoretical boundary lines (namely ‘true wet/cool edge’ and ‘true dry/warm edge’ in the trapezoid) in the scatter plot for the surface temperature versus the fractional vegetation cover in mixed pixels; 2) a new assumption that the slope of the Tm – f curves is mainly controlled by soil water content is introduced. The variables required by the improved method include near surface vapor pressure, air temperature, surface resistance, aerodynamic resistance, fractional vegetation cover, surface temperature and net radiation. The model predictions from the improved model were assessed in this study by in situ measurements, which show that the total latent heat flux from the soil and vegetation are in close agreement with the in situ measurement with an RMSE (Root Mean Square Error) ranging from 30 w/m2∼50 w/m2, which is consistent with the site scale measurement of latent heat flux. Because soil evaporation and vegetation transpiration are not measured separately from the field site, in situ measured CO2 flux is used to examine the modeled λEveg. Similar trends of seasonal variations of vegetation were found for the canopy transpiration retrievals and in situ CO2 flux measurements. The above differences are mainly caused by 1) the scale disparity between the field measurement and the MODIS observation; 2) the non-closure problem of the surface energy balance from the surface fluxes observations themselves. The improved method was successfully used to predict the component surface heat fluxes from the soil and vegetation and it provides a promising approach to study the canopy transpiration and the soil evaporation quantitatively during the rapid growing season of winter wheat in northern China.

Comparative Assessment of Two Vegetation Fractional Cover Estimating Methods and Their Impacts on Modeling Urban Latent Heat Flux Using Landsat Imagery

“…LST vd and LST vw represent dry, full canopy green spaces and water-saturated soils, respectively. As described by previous studies [16,17,56], the Bowen ratio of each pixel can be retrieved through the linear interpolation between wet and dry edges of the trapezoid using the following equation:…”

“…In this study, we did not focus on the procedures used to identify extreme pixels. Following previous studies [16,17], LST sd and k dry were retrieved using LST values from representative bare land in the study area, and LST sw and k wet from the Landsat images were based on the LST values of several water districts in the study regions.…”

Using Simplified Thermal Inertia to Determine the Theoretical Dry Line in Feature Space for Evapotranspiration Retrieval

“…The detailed description of the three procedures can be found in Zhang (2008) [20].We focus on how to determine the theoretical dry line in this paper. For pixels on the theoretical dry line, ET is assumed to be 0.…”

“…Then, the available energy can be separated into soil evaporation and vegetation transpiration. A detailed introduction can be found in the literature [12,20].…”

“…Formulas are provided in Equations (31) and (32) [12,20]. Then, the available energy can be separated into soil evaporation and vegetation transpiration.…”