## 1 Introduction  
Water scarcity is one of the most serious global challenges [1; 2]. Evidence shows that the changing and uncertain climate in the future could lead to more uncertainty in water availability [3; 4]. This creates greater pressure on water resources and ecosystem management in arid and semi-arid regions where there is often poor infrastructure and a fragile ecological environment [2]. Thus, to obtain accurate information of water use is important for water resources and ecosystem management in these regions [2; 5]. Evapotranspiration (ET) is an essential component in catchment water balance since it is almost the only water-extracting constituent in arid and semi-arid regions [6; 7]. ET is the loss of water when mitigating vegetation stress responses and the key variable for linking ecosystem functioning, carbon and climate feedbacks [5]. Therefore, ET is commonly considered as a reference and indicator for water resources and ecosystem management. It is used for: the determination of irrigation requirements of crops, evaluation of the hydrological effects of afforestation and deforestation, and the estimation of the productivity and water-use efficiencies of ecosystems [5; 8; 9; 10]. A wide range of global and regional ET products have been developed in recent decades for monitoring water use and assisting in planning management. They can be broadly classified into three categories: remote sensing (RS)-based ET products using vegetation index-based data or land surface temperature (LST) (e.g., the Moderate Resolution Imaging Spectroradiometer (MODIS) ET, Advanced Very High Resolution Radiometer (AVHRR) ET, Priestly-Taylor Jet Propulsion Laboratory (PT-JPL) ET and Global Land Evaporation Amsterdam Model (GLEAM) ET), ET output from land surface models (LSMs) (e.g., Global Land Data Assimilation System (GLDAS) ET, Modern-Era Retrospective analysis for Research and Applications (MERRA) ET and European Centre for Medium-Range Weather Forecasts Re-analysis (ERA)-interim ET), and ET from a water budget with measured precipitation, runoff and total water storage change (e.g., TerraClimate ET and Gravity Recovery and Climate Experiment (GRACE)-inferred ET) [5; 11; 12]. These ET products have been used to a varying degree for irrigation water management [8], vegetation productivity estimation [9; 10], calibration and optimization of hydrological models and prediction of runoff in the ungauged basins [13; 14], and water management modes and water rights regimes [2; 15; 16]. However, these different ET products are of different spatial-temporal scales, and differ in accuracy or uncertainty [11; 17; 18]. As ET is the largest water-use component in the catchment water balance in arid and semi-arid regions, any variation of ET will cause corresponding errors in the estimation of runoff and soil water storage, thus also large errors in the determination of water demand of irrigated agriculture and in the estimation of productivity of water use in an ecosystem [19; 20]. To date, most hydrologic studies, as well as river basin management proposals, have tended to focus on supply elements e.g., precipitation, snow, runoff, water storage soil moisture and groundwater, but have largely ignored the demand side (e.g., ET) (Figure 1) [5]. This is mainly because of insufficient knowledge of ET at local to global scales. The ground observations for ET (eddy covariance towers, Bowen ratio systems, and weighing lysimeters) are poorly covered in many regions, furthermore there are uncertainties about the ET from a water budget perspective which accumulate from errors in other hydrological components, and uncertainties of the ET from land surface models and RS-based approaches due to differences in the structures, resolutions, and inputs of the models and approaches. However, the range of these errors caused by the variations of ET products is not yet clear. Without such knowledge, our capacity for accurate water resource management, economic development and sustainable ecosystems would be seriously compromised.