# Evapotranspiration in Dynamic of Highland Barley Using PML ET Product in Tibet
## 1 Introduction  
Terrestrial evapotranspiration (ET), comprising the transpiration from vegetation (Ec), the evaporation from soil (Es), and the evaporation of the intercepted precipitation by vegetation canopy (Ei) [1], is a critical process of water and carbon cycle, as well as the most important component of energy balance for soil-plant-atmosphere continuum [2]. ET and its components play key roles in linking ecosystem functioning, climate feedbacks, and water resources [3]. Crop water consumption by ET is a critical parameter for water and energy exchanges in the agriculture continuum [4]. Understanding the dynamic and characteristics of crop ET components is of great significance in the estimation of water requirements for agricultural irrigation [5], evaluation of water use efficiency [6], and development of optimal irrigation schedule [7]. Several techniques have been developed to measure ET components of crops. Many previous studies used the micro lysimeter and sap flow, respectively, to measure the Es and Ec of crops such as wheat [4], maize [8; 9], and tomato [10], the results suggested that sap flow was a useful tool to measure Ec of crops at an instantaneous interval [8]. As far as short-stalk, closed-system planting crops such as wheat and rice were concerned, Ei was often measured with classic water wiping method [4], while it was usually calculated using the water balance method for maize, in which throughfall was collected with rainfall collectors, and stemflow was collected with collars attached around the maize stems, and then stemflow in collars was drained to collectors using a slot with a transfer hose [9]. [PERSON] and [PERSON] [2] explored the isotope tracing technique coupled with the water balance method for partitioning ET of winter wheat under different irrigation and fertilization treatments during growing seasons. Some studies used the weighing lysimeter and eddy covariance system to measure the total ET, which was later partitioned into ET components using the energy balance method [10; 11; 12]. [PERSON] et al. [13] proposed a simple and practical method to estimate Es and Ec of crops; this method combined with an eddy covariance system was later used by [PERSON] et al. [14] to separate Es and Ec of maize cropland in a dry, semihumid climate regime. However, these techniques were not widely applied mainly due to the limitations of being time consuming, having costly equipment, and being prone to measuring manipulation [10]. Therefore, great efforts have been made for partitioning ET by means of modeling approaches, with the main focus on the improvements of the surface conductance model based on the Penman-Monteith (PM) model. [PERSON] and [PERSON] [15] modified the PM model and developed the Shuttleworth-Wallace (SW) model based on the energy balance theory and the physical process of Es and Ec. [PERSON] et al. [16; 17] introduced Ball-Berry stomatal conductance model to modify the SW model and suggested an improved SW model. [PERSON] et al. [18] and [PERSON] et al. [19] used a surface conductance model to modify the PM model and proposed the Penman-Monteith-Leuning (PML) model. Introducing a water carbon coupled canopy conductance model, [PERSON] et al. [20] revised the PML model and developed the PML-V2 model, which can estimate ET components and gross primary productivity (GPP) simultaneously. This model was further improved by [PERSON] et al. [1; 21] by incorporating the vapor pressure deficit constraint to GPP. [PERSON] et al. [22] considered Es and Ec as a function of crop coefficient and reference evapotranspiration and presented a dual crop coefficient model. [PERSON] et al. [10] reported a modified Priestley-Taylor model considering the effects of leaf senescence and plant temperature constraint on transpiration to estimate Es and Ec of tomato. Due to the difference in climate, geomorphology, soil, hydrological regimes, and agricultural management practices, ET components and the proportion of components for crop varied greatly from site to site [3].