## 1 Introduction  
Arid areas, including hyper-arid, arid, and semiarid regions, are of particular importance to humans since they support more than one-fifth of the global population [1]. The ecosystems in the arid area are fragile and sensitive to climate change and anthropogenic activities [2; 3; 4; 5] and play a dominant role in the interannual variability of global net primary productivity (NPP) [6]. The Tianshan North Slope in Xinjiang province of China, located in the inland of Eurasia and far away from the oceans, is one of the driest regions in the world. This region has experienced a faster warming trend with a rate of 0.40 \({}^{\circ}\)C warming per decade [7], which is much higher than the global mean rate of 0.27 \({}^{\circ}\)C per decade [8]. A wetter trend is also detected by observations during the last several decades [9; 10]. These climate changes play a very important role in the carbon and water flux dynamics of the arid ecosystems [11; 12; 13; 14], and have drawn great concern about the sustainable development of the arid ecosystems. Due to the development of the industry and the intensive human activities, the observations from Mauna Loa show that the atmospheric CO\({}_{2}\) concentration has increased by 29.3% since 1959 and exceeded 400 ppm in 2015 [15]. Moreover, previous experimental and modeling studies have shown that arid ecosystems benefit from the increase of atmospheric CO\({}_{2}\) concentration, which is called the CO\({}_{2}\) fertilization effect [2; 16; 17]. However, different vegetation types have different responses to these changes resulting from the temporal and spatial heterogeneity of the climate change and the unique characteristics of each vegetation type [12; 18]. Therefore, the dominant factor differs for each vegetation type. For example, by scenario simulations, [PERSON] et al. [17] quantitatively explored the responses of the arid ecosystems in Central Asia to the changes in climate and CO\({}_{2}\) concentrations by scenario simulations, and they found that the NPP of the natural ecosystems was sensitive to precipitation change, while the NPP of cropland was sensitive to the CO\({}_{2}\) fertilization effect. The carbon and water fluxes are key functions of the ecosystem, and are widely used to measure the status of the ecosystem [19; 20; 21; 22; 23; 24]. The carbon flux, NPP, which is the net carbon retained by the vegetation in the ecosystem, is a balance between the carbon uptake from the photosynthetic function and the carbon losses caused by the plant respiration. The water flux, evapotranspiration (ET), is the sum of the evaporation of the soil, evaporation of the canopy interception, and the transpiration of vegetation. The NPP is the key component in the global carbon cycle while the ET is the major component of the global hydrological cycle [21; 23]. On one hand, both NPP and ET are vulnerable to the changes in the climate and the atmospheric CO\({}_{2}\) concentration and the anthropogenic activities. Based on model simulations, [PERSON] et al. [12] found that the annual NPP of grassland in arid Central Asia followed a decreasing trend (\(-0.21\) g C m\({}^{-2}\) year\({}^{-1}\)) from 1979 to 2011, resulting from the increase of annual mean temperature and the decline of precipitation. During the same period, the ET of grassland experienced significant declines of 1.47-2.72 mm per decade due to the climate change [25]. On the other hand, the two indicators coexist in the photosynthesis and respiration processes and are controlled by the status of the stomas [26; 27; 28]. It is therefore necessary to explore the responses of the ecosystem to the climate change and CO\({}_{2}\) fertilization effect simultaneously, especially for the future projections. Additionally, it is of significant importance for the sustainable development of the arid ecosystems and the policy making for climate adaption and mitigation to study the NPP and ET dynamics of different arid ecosystems under multiple representative concentration pathway (RCP) scenarios.