Abstract
Abstract. The determination of area-averaged evapotranspiration (ET) at the satellite pixel scale/model grid scale over a heterogeneous land surface plays a significant role in developing and improving the parameterization schemes of the remote sensing based ET estimation models and general hydro-meteorological models. The Heihe Watershed Allied Telemetry Experimental Research (HiWATER) flux matrix provided a unique opportunity to build an aggregation scheme for area-averaged fluxes. On the basis of the HiWATER flux matrix dataset and high-resolution land-cover map, this study focused on estimating the area-averaged ET over a heterogeneous landscape with footprint analysis and multivariate regression. The procedure is as follows. Firstly, quality control and uncertainty estimation for the data of the flux matrix, including 17 eddy-covariance (EC) sites and four groups of large-aperture scintillometers (LASs), were carefully done. Secondly, the representativeness of each EC site was quantitatively evaluated; footprint analysis was also performed for each LAS path. Thirdly, based on the high-resolution land-cover map derived from aircraft remote sensing, a flux aggregation method was established combining footprint analysis and multiple-linear regression. Then, the area-averaged sensible heat fluxes obtained from the EC flux matrix were validated by the LAS measurements. Finally, the area-averaged ET of the kernel experimental area of HiWATER was estimated. Compared with the formerly used and rather simple approaches, such as the arithmetic average and area-weighted methods, the present scheme is not only with a much better database, but also has a solid grounding in physics and mathematics in the integration of area-averaged fluxes over a heterogeneous surface. Results from this study, both instantaneous and daily ET at the satellite pixel scale, can be used for the validation of relevant remote sensing models and land surface process models. Furthermore, this work will be extended to the water balance study of the whole Heihe River basin.
Highlights
Land surface evapotranspiration (ET) is a key component in the regional water circulation, but is essential in the surface energy balance and land surface process
On the basis of an accurate high-resolution land-cover classification map and multi-point ground-based flux measurements from 16 EC systems and four groups of large-aperture scintillometers (LASs) systems during the intensive observation period of Heihe Watershed Allied Telemetry Experimental Research (HiWATER), a flux aggregation method for determining area-averaged flux was established through the combination of footprint analysis and multiple-linear regression
The method was applied to estimate the area-averaged surface fluxes over a heterogeneous surface from multi-point EC flux measurements, and its results were verified by the LAS measurements
Summary
Land surface evapotranspiration (ET) is a key component in the regional water circulation, but is essential in the surface energy balance and land surface process. F. Xu et al.: Aggregation of multi-point EC flux measurements land surface and atmosphere (Mengelkamp et al, 2006). The flux tower group can quantify the turbulent exchange of energy and mass between the atmosphere and a variety of surface types (Sellers et al, 1995), and these local point measurements need to be aggregated to provide meaningful area-averaged fluxes (André et al, 1986). Given the EC network’s high price and the requirement for their continuous maintenance, the largeaperture scintillometer (LAS) is a useful alternative method for direct measurements of area-averaged sensible heat fluxes on the scale of 1–5 km (Ezzahar et al, 2009b; Ezzahar and Chehbouni, 2009)
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