## 1 Introduction  
As a link between water balance and surface energy balance [1], evapotranspiration is an important index reflecting climate variation and water circulation [2, 3]. Reference evapotranspiration (\(ET_{0}\)) represents the maximum amount of evapotranspiration from a hypothetical reference surface under specific climatic conditions, given an adequate supply of water [4]. \(ET_{0}\) plays a critical role in irrigation management [5] and is also an important parameter for agriculture water demand estimation and water balance analysis [6]. The Penman-Monteith (P-M) method is a fairly reliable method to estimate \(ET_{0}\), which considers radiative and aerodynamic terms [7]. It focuses on the meteorological factors controlling evaporation process and is widely used in numerous research [8, 9]. Understanding the spatial-temporal changes in \(ET_{0}\) and the main causes can provide considerable understanding into the impact of climatic variation on water resources and hydrological cycles [10]. Extensive research has explored the spatial variation characteristics of \(ET_{0}\) and the impact of climate factors on \(ET_{0}\) in the late years [11]. It reported that pan-evaporation and \(ET_{0}\) both have shown a steady downward trend in many regions of the world over the past several decades, including China, India, New Zealand and Italy [12, 13, 14, 15], which is called 'evaporation paradox' [16]. The reasons for decreasing \(ET_{0}\) have been widely discussed, global dimming and wind stilling were usually employed to explain the changes. For example, [PERSON] et al. [17] analyzed the entire trend in \(ET_{0}\) in Jing-Jin-Ji region, finding that the decrease of wind speed and sunshine hours offset the influence of increased air temperature and led to a reduction in \(ET_{0}\) before 1992. [PERSON] et al. [18] found downward tends in \(ET_{0}\) in the Yellow River and the reduction of wind speed was the major driving force for decreasing trends. However, in recent years, increased \(ET_{0}\) has been found in some regions, such as India and China [19]. Investigating long term change in \(ET_{0}\) over India's humid tropical Narmada River watershed, [PERSON] et al. [20] found an increasing trend in \(ET_{0}\). [PERSON] et al. [21] found that annual pan evaporation in China has increased since 1986. The changes in climatic variables such as air temperature, wind speed may explain the increase of \(ET_{0}\). The increase of pan evaporation in China is mainly owing to the increase of vapor pressure deficit [21]. [PERSON] et al. [22] investigated the \(ET_{0}\) trends in the Northwest China from 1960 to 2010, finding the \(ET_{0}\) began to increase in 1994, mainly caused by the increasing atmospheric temperature and wind speed. Since there are many factors affecting \(ET_{0}\), the temporal and spatial differences between its changes and the changes of climate factors are complex and diverse. In addition, it is still uncertain how much \(ET_{0}\) changes are caused by a certain factor. Therefore, it is necessary to further study the variation of \(ET_{0}\) and its possible causes. The Tibetan Plateau (TP), as the 'third pole' of the earth, is very sensitive to global change due to its unique climate conditions and natural environment [23]. The TP is rich in lakes, glaciers, and wetlands and is the main source area of several major rivers in Asia. The management of affluent water resources on the plateau are not only a major issue related to China, but also the neighboring countries located in the lower reaches of the international rivers originating from the plateau. The management of abundant water \(ET_{0}\) is the key constituent of the hydrologic cycle and has a great impact on regional water resources. Several studies have focused on the variation of \(ET_{0}\) over the TP under the background of climatic change [3; 24; 25]. [PERSON] et al.