## 1 Introduction  
ET, a key process for connecting water, carbon, and energy across various ecosystems [1], consists of evaporation (E) and T from wet surfaces and vegetation through the stomata. Partitioning of ET into biological component T and non-biological component E is of great significance in order to improve our understanding of abiotic and biotic ecosystem processes, such as vegetation water use efficiency, interaction of water, and carbon cycles [2; 3; 4; 5; 6]. Terrestrial vegetation has significant impacts on ETpartitioning owing to the fact that transpiration accounts for the major portion of ET and is intrinsically linked to photosynthesis or plant productivity [7; 8]. Furthermore, how vegetation responds to climate change is complex, particularly under climate extremes such as drought, which makes the relationship between vegetation and ET partitioning more complicated [9; 10]. Previously, many efforts have been devoted to investigating the relationship between vegetation and ET partitioning on different spatial and temporal scales [11; 12; 13; 14]. It has been reported that the T/ET ratio fluctuates among different vegetation types [15] and normally increases from grass to shrub to trees. For instance, [PERSON] et al. [16] estimated the T/ET ratio in a grassland and shrub site within the Walnut Gulch Experimental Watershed in southeast Arizona, USA. The outcomes showed that T/ET is higher for the grass-dominated site compared to the shrub-dominated site. Furthermore, some studies also revealed that the T/ET ratio is higher for deep-rooted trees as compared to grasses. Apart from vegetation type, ET partitioning is also significantly associated with vegetation index. The majority of studies in the past stated that T/ET ratio is positively correlated to leaf area index (LAI). For instance, [PERSON] et al. [17] found that the expansion of LAI leads to the increase of transpiration and the decrease of soil evaporation. The isotopic analysis by [PERSON] et al. [12] revealed increments from 0.6 to 0.83 in the T/ET as the vegetation cover increases from 25% to 83%. [PERSON] et al. [18] found a linear relationship between T/ET and enhanced vegetation index (EVI). Several studies also found that the influence of LAI on ET partitioning is distinctive with different levels of LAI. [PERSON] et al. [19] illustrated that the increase of LAI led to an increase in the T/ET ratio; however, when LAI is over a specific range, the T/ET ratio is insensitive to the change in LAI. In a word, although it is reported that the T/ET ratio and vegetation have a positive correlation in the existing studies, whether it is universally true with regard to different vegetation types and different sites is understudied. ET partitioning can be commonly achieved by combining different in situ measurements [20], such as EC measurements [21; 22; 23; 24], Bowen ratio [25; 26], gap flow [27; 28], plant weighing lysimeters [29; 30; 31], soil chamber measurements [32; 33], CO\({}_{2}\) and isotopic labeling [34; 35; 36; 37; 38], however, there is still huge uncertainty in ET partitioning due to the uncertainty of stomata behavior and a lack of high-density observation [39]. Recently, EC measurement has demonstrated its potential for ET partitioning [40; 41; 42]. The EC-based approach has several advantages that make it suitable for ET partitioning. First, due to its high frequency, it is capable of capturing small eddies, contributing to the turbulent transfer [43]. Secondly, it measures continuously and could provide long-term measurements, which is valuable for ET partitioning on multiple time scales. Therefore, the present study aims to examine the relationship between vegetation and T/ET ratio among different sites and vegetation types. For this, five different sites situated in Northern China are selected for the comparison of ET partitioning. ET partitioning is conducted using an approach based on the concept of the underlying water use efficiency with eddy covariance measurements.