## 1 Introduction  
Evapotranspiration (ET), which typically includes soil evaporation (E) and plant transpiration (T), is a fundamental component of the global water cycle [1]. E is the physical process through which water is transferred from the soil surface to the atmosphere in the form of vapor [2]. T is the biological process by which water absorbed by plant roots is transported through the plant and ultimately released into the atmosphere via stomata [3]. ET represents a continuous and dynamic process that relies on soil as a vital water reservoir. It is intricately linked to various environmental and biophysical processes occurring at the soil-plant-atmosphere interface [4]. With global warming, T and E are projected to be increased, which results in a great stress on water resources in the future [5]. The efficient utilization of limited water resources is particularly important for agriculture, especially in areas with water scarcity [6; 7; 8]. Rather than the non-productive E, T directly reflects the utilization of water by crops; thus, an accurate estimation of T is crucial for evaluating water use efficiency. The goal of agricultural research is to maximize T and minimize E [9]. However, the key indicator of cropland water consumption, namely the ratio of transpiration to evapotranspiration (T/ET), is still poorly understood. Estimating T/ET, or partitioning ET, has always remained challenging. Many published studies have been devoted to partitioning ET through a variety of direct or indirect measurements [9; 10]. At the site and field scales, the most reliable methods to measure T and E are sap flow sensors and lysimeters, respectively [11; 12; 13]. In addition, the isotope technique is also used to partition E and T based on the theory of evaporative fractionation of hydroxide isotopes [14; 15]. However, these methods are often costly, laborious, and spatially limited. Extrapolating the findings to the ecosystem scale continues to pose a substantial scientific challenge [16]. With the development of remote sensing techniques, regional and global ET estimation has become feasible, and many remote sensing-driven ET models have been developed to estimate global ET [17; 18; 19]. For example, [PERSON] et al. [20] developed and improved a global evapotranspiration algorithm for Moderate Resolution Imaging Spectroradiometer ET production. Similarly, based on the Penman-Monteith equation, [PERSON] et al. [21] established a water-carbon coupled model (i.e., PML-V2) to estimate ET and gross primary productivity (GPP). In addition, the Food and Agriculture Organization (FAO) 56 model [22; 23], the Global Land Evaporation Amsterdam Model (GLEAM) [24], and the Priestly-Taylor Jet Propulsion Laboratory (FT-JPL) model [25] are also widely used in estimating ET. Although these models have demonstrated high accuracy in ET estimates, their reliability in partitioning ET components has yet to be validated. The eddy covariance technique has been widely applied since it is recognized as a standardized tool for measuring ecosystem carbon and water fluxes [26]. However, this technique only observes ET, instead of T or E. To obtain T at the ecosystem scale, various ET partitioning methods based on eddy covariance measurements have been developed recently [9; 27; 28]. [PERSON] et al. proposed an ET partitioning method based on the flux-variance similarity theory [29]. This method relies on high-frequency eddy covariance data and leaf-level water use efficiency (WUE) which are difficult to obtain. Based on the concept of the underlying water use efficiency (uWUE), [PERSON] et al. (hereafter, Z16) developed an easy and applicable ET partitioning method [30]. However, a major limitation of this method is that the assumption of T \(\approx\) ET at some periods makes it unsuitable for water-limited and perennially wet ecosystems. Moreover, based on leaf and ecosystem WUE, [PERSON] et al. (hereafter, Y22) developed an ET partitioning method that applied the unified stomatal conductance model to calculate T/ET [31].