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The determination of area-averaged evapotranspiration (ET) at the\udsatellite pixel scale/model grid scale over a heterogeneous land surface\udplays a significant role in developing and improving the parameterization\udschemes of the remote sensing based ET estimation models and general\udhydro-meteorological models. The Heihe Watershed Allied Telemetry\udExperimental Research (HiWATER) flux matrix provided a unique opportunity to\udbuild an aggregation scheme for area-averaged fluxes. On the basis of the\udHiWATER flux matrix dataset and high-resolution land-cover map, this study\udfocused on estimating the area-averaged ET over a heterogeneous landscape\udwith footprint analysis and multivariate regression. The procedure is as\udfollows. Firstly, quality control and uncertainty estimation for the data of\udthe flux matrix, including 17 eddy-covariance (EC) sites and four groups of\udlarge-aperture scintillometers (LASs), were carefully done. Secondly, the\udrepresentativeness of each EC site was quantitatively evaluated; footprint\udanalysis was also performed for each LAS path. Thirdly, based on the\udhigh-resolution land-cover map derived from aircraft remote sensing, a flux\udaggregation method was established combining footprint analysis and\udmultiple-linear regression. Then, the area-averaged sensible heat fluxes\udobtained from the EC flux matrix were validated by the LAS measurements.\udFinally, the area-averaged ET of the kernel experimental area of HiWATER was\udestimated. Compared with the formerly used and rather simple approaches, such\udas the arithmetic average and area-weighted methods, the present scheme is\udnot only with a much better database, but also has a solid grounding in\udphysics and mathematics in the integration of area-averaged fluxes over a\udheterogeneous surface. Results from this study, both instantaneous and daily\udET at the satellite pixel scale, can be used for the validation of relevant\udremote sensing models and land surface process models. Furthermore, this work\udwill be extended to the water balance study of the whole Heihe River basin

hydrology and earth system sciences

Area-averaged evapotranspiration over a heterogeneous land surface: aggregation of multi-point EC flux measurements with a high-resolution land-cover map and footprint analysis