## 1 Introduction  
The study of the evolution of the atmosphere  
## 1 Introduction  
The land surface ecological processes significantly affect the cycling of energy, mass and moisture between the soil, vegetation and atmosphere. These dynamics also make the land surface an important energy and moisture source, sink and reservoir, and hence contributed a lot to terrestrial and climate systems [1,2]. However, the sophistication of individual, and interactions between, hydrological, ecophysiological and energy cycles are hard to comprehend because of the related nonlinear feedback mechanisms, especially for landscape-scaled ecohydrological processes and the uncertainty of natural and anthropogenic disturbances [3]. The tight linkage between water and carbon dynamics is expressed in many hydrologically controlled ecological processes, for instance, respiration [4], photosynthesis [5], water use efficiency [6] and crop coefficient analyses [7]. Currently, there are many ecophysiological modeling researches published at multiple spatio-temporal resolutions and spans, including point scales [8,9], mesoscales [10,11] and global scales [12] at yearly [13] or minute time-steps [1], while few of them completely considered the soil water processes and the non-linear relationship between it and vegetation ecophysiological processes. Due to the limited understanding of water-carbon interactions, many researches (regardless of modeling or measurement) often regard them as an isolated process [2]. At mesocales, especially for the watershed scale, where the land surface ecological processes are highly sensitive to local water balance, topography is an important contributor for local water cycling and, furthermore, ecophysiological processes. Many researches [14, 15, 16] suggested that the overlook of topographical-controlled lateral water fluxes would cause significant biases in local estimation of water (evapotranspiration, runoff and soil moisture) and carbon fluxes (GPP: Gross primary productivity, NPP: Net primary productivity and NEP: Net ecosystem productivity). For the traditional hydrological models, although they can completely consider the watershed hydrological processes, they always neglect or simplify the impact of vegetation on soil water and their ecophysiological processes. Evapotranspiration (ET) and photosynthesis (as GPP) are two critical ecophysiological processes for the connection between land surface water, energy and carbon cycling, and both of them are highly governed by local soil hydrological processes. For GPP, the surrounding water content of the rhizome controls photosynthesis efficiency through the effect of vegetation stomatal and enzyme activity [17], ET has a stronger interaction with the local water and energy balance. About 70% of the precipitation that falls on the land surface gets back to the atmosphere by vegetation ET, and in arid areas ET loss can be up to 90% [18]. Simultaneously, because vaporization and sublimation would take the absorption of heat, it is also important for local thermal balance. The coupling of water and carbon cycling can better explain these ecohydrological processes and, therefore, help make more efficient decisions for local environmental management. The Heihe River is the second largest inland river in northwest China and originates from the Qilian Mountains which is in the northern margin of the Tibetan Plateau. It is located in the center of the Eurasian continent, playing an important role in the ancient Silk Road [19]. The upper stream of the watershed takes up around 7% of the Heihe basin area, and is the water source of the whole basin with a wide distribution of grassland which plays a significant role for local carbon assimilation and water cycling [20]. Influenced by the westerly belt circulation and the cold continental polar air mass, the Upper Heihe watershed has an arid to semiarid climate with scarce and concentrated precipitation [21,22].