# Partitioning Evapotranspirationiration in a Cotton Field under
## 1 Introduction  
Evapotranspiration (_ET_) is an important component of the Soil-Plant-Atmosphere Continuum (_SPAC_). It is also a link between the land surface water cycle and the biochemical cycle [1, 2, 3] and an important link between the land surface water cycle and the energy cycle [4]. For farmland ecosystems, most agricultural water is consumed in the form of _ET_[5]. The _ET_ of farmland consists of transpiration (_T_) and evaporation (_E_); \(T\) is closely related to crop growth, but \(E\) is ineffective evaporation. Especially in an arid region, more than 90% of precipitation returns to the atmosphere through _ET_[6, 7]. Therefore, accurate estimation of _ET_ components from farmland in an arid region is necessary for improving agricultural water use efficiency and promoting the development of water-saving agriculture. At present, the partition methods for _ET_ of farmland mainly focus on Stable Isotope Method, Direct Observation Method, and Model Method [8, 9, 10, 11]. The instrument and equipment of Stable Isotope Method are expensive, the experiment is complex, and the workload is too large, so ordinary laboratories cannot use it for analysis [12], and this method is not suitable for agricultural and forest ecosystems when \(E\) is small [13]. The Direct Observation Method needs the Eddy Covariance (_EC_) system, the lysimeter, andthe liquid-flow meter at the same time to determine the \(ET\) components directly, but the lysimeter cannot measure \(ET\) during irrigation and rainfall [14], and the liquid-flow meter cannot monitor the plants at the seedling stage [15], so the Direct Observation Method cannot partition the \(ET\) components continuously and completely. The accuracy of the Model Method depends on the number of parameters and the size of the experimental scale [16; 17]. If the observed parameters are less or the experimental scale is not appropriate, the calculation results of the Model Method will also have a large deviation. With the rapid development and extensive application of the \(EC\) system in farmland \(ET\), rich research experience has been accumulated. Many scholars partitioned the \(ET\) components of different farmland systems from the perspective of water-carbon flux coupling with the help of \(EC\) system. [PERSON] et al. [18; 19] in 2014 showed that \(ET\) was related to Gross Primary Productivity (\(GPP\)) and Vapor Pressure Deficit (\(VPD\)), proposed the concept of an underlying water use efficiency (\(uWUE\)), and argued that the relationship between \(GPP\)-\(VPD\)-\(5\) and \(ET\) was nearly linear when \(E\) was small and \(T\) took up a large portion of \(ET\) during the growing period. The effect of \(E\) on the relationship between \(GPP\)-\(VPD\)-\(5\) and \(ET\) resulted in variations in \(uWUE\) both at half-hourly and daily time scales. Since the variation in \(uWUE\) was attributed to the changes in \(T\)/\(ET\), \(uWUE\) could be used for \(ET\) partitioning when the variations in \(uWUE\) are related to the relative portion of \(ET\) as \(T\). Because \(uWUE\) was obtained by combining the direct effect of vapor pressure deficit (\(VPD\)) on transpiration and the response of stomatal conductance to \(VPD\). Therefore, it was recognized as a more representative indicator of physiological regulation of water-carbon exchange than Water Use Efficiency (\(WUE\)) in plants. On this basis, [PERSON] et al. [20] in 2016 used \(uWUE\) to partition \(ET\) components by combining water-carbon fluxes and meteorological data, which could realize continuous estimation of \(T\)/\(ET\) at different time scales. It can be seen that using \(uWUE\) to partition \(ET\) components has the advantages of simplicity and ease of use, and \(ET\) components are in the same source area. In the same year, [PERSON] et al. [21] estimated the \(T\)/\(ET\) of two coniferous forests in the United States using the uWUE and found that the results of the \(uWUE\) were consistent with the Stable Isotope Method.