Aggregation of area-averaged evapotranspiration over the Ejina Oasis based on a flux matrix and footprint analysis

Abstract

The determination of area-averaged evapotranspiration (ET) over a heterogeneous land surface at the satellite pixel scale/model grid scale (103–104 m) is crucial to the evaluation of the remote sensing ET products and development of the parameterization schemes of general hydro-meteorological models. The Heihe Watershed Allied Telemetry Experimental Research (HiWATER) flux matrix of multi-site eddy-covariance (EC) and large aperture scintillometers (LASs) provided an unprecedented opportunity to establish an aggregation scheme for determining area-averaged surface fluxes. Based on the HiWATER flux matrix datasets and high-resolution land-cover map, a new flux aggregation method was developed by using footprint analysis and multiple-linear regression. The area-averaged sensible heat fluxes aggregated from the multi-site EC flux measurements using the developed method were compared with the LAS measured fluxes. The results showed that the new method produced reasonable estimates of sensible heat flux with a mean absolute percentage error (MAPE) value of around 20–30%. The new aggregation method also significantly outperformed other methods being applied commonly, such as the rather simple area-weighted method. The daily ET of various land-cover types and the area-averaged ET over the key area of the Populus euphratica forest reserve in the Ejina Oasis were separately estimated using the established disaggregation and aggregation schemes of the new method. The area-averaged ET over this key area ranged between 1 and 5 mm d−1 for the 68 days growing period in 2014. Results of this study can be used for the evaluation of relevant remote sensing models and land surface process models.