The observation of evapotranspiration (ET) is an important but challenging task and is closely related to currently popular research topics such as surface energy balance closure, scale, and remote sensing product validation issues, etc. In the HiWATER project, a flux observation matrix was established and 22 eddy covariance system (EC) sets, 21 automatic weather stations (AWS), and 4 large aperture scintillometer (LAS) groups were designed. The objective was to capture the 3-D dynamic characteristics of water and heat interactions between the surface and atmosphere, to better understand the ET process under heterogeneous surface, to reveal the spatial heterogeneity of ET and its influence mechanism, to develop the scaling method of ET, and to provide ground truth value at satellite pixel scale for the development and validation of the ET remote sensing estimation model.
Two nested matrices, one large experimental area (30 km× 30 km) and one kernel experimental area (5.5 km x 5.5 km), were established in an artificial oasis-riparian ecosystem-wetland-desert compound in the Zhangye area, which is located in the midstream of the Heihe River Basin. The large experimental area contained one superstation (within the oasis, cropland) and four ordinary stations (around the oasis), with underlying Gobi (Bajitan Gobi station), desert (Shenshawo sandy desert station), wetland surfaces (Zhangye wetland station), and desert steppe (Huazhaizi desert steppe station). The superstation was equipped with one meteorological element gradient observation system and two layers of EC observations. The four ordinary stations were equipped with one AWS and one EC observation. The 5.5 km x 5.5 km kernel experimental area was located in the oasis, and 17 elementary sampling plots were divided according to the distribution of, for example, crop structure, windbreak, resident area, soil moisture, and irrigation status. In each plot, one EC system and one AWS were installed. There were also four LAS groups (eight sets with two sets in each group) installed in 3 x 3 and 2 x 1 Moderate Resolution Imaging Spectroradiometer (MODIS) pixels within the kernel experimental area. Moreover, there were also a stable isotope and mobile observation measurement to separate the soil evaporation and vegetation transpiration, the shelter-forest transpiration measured by a thermal dissipation sap flow probe, and area-averaged soil moisture measured by two sets of cosmic-ray soil moisture observation systems at the superstation pixel.