Spatially explicit regionalization of airborne flux measurements using environmental response functions

[摘要] The goal of this study is to characterize the sensible (H) and latent (LE) heat exchangefor different land covers in the heterogeneous steppe landscape of the Xilin River catchment,Inner Mongolia, China.Eddy-covariance flux measurements at 50–100 m above ground wereconducted in July 2009 using a weight-shift microlight aircraft. Wavelet decomposition of theturbulence data enables a spatial discretization of 90 m of the flux measurements. Fora total of 8446 flux observations during 12 flights, MODIS land surface temperature (LST) andenhanced vegetation index (EVI) in each flux footprint are determined. Boosted regression treesare then used to infer an environmental response function (ERF) between all flux observations(H, LE) and biophysical (LST, EVI) and meteorological drivers. Numerical tests show that ERFpredictions covering the entire Xilin River catchment (≈3670 km²) are accurateto ≤18% (1 σ). The predictions are then summarized for each land cover type, providingindividual estimates of source strength (36 W m⁻² < H < 364 W m⁻²,46 W m⁻² < LE < 425 W m⁻²) and spatial variability(11 W m⁻²< σ_H < 169 W m⁻², 14 W m⁻² < σ_LE <152 W m⁻²) to a precision of ≤5%.Lastly, ERF predictions of land coverspecific Bowen ratios are compared between subsequent flights at different locations in the XilinRiver catchment. Agreement of the land cover specific Bowen ratios to within 12 ± 9%emphasizes the robustness of the presented approach. This study indicates the potential of ERFsfor (i) extending airborne flux measurements to the catchment scale, (ii) assessing the spatialrepresentativeness of long-term tower flux measurements, and (iii) designing, constraining andevaluating flux algorithms for remote sensing and numerical modelling applications.

[学科分类] 地球化学与岩石