## 2 Materials and Methods  
### Description of the Study Area  
This study is conducted at five sites in Northern China with different vegetation types, viz., typical arid, semi-arid, and semi-humid ecosystems, and located around 40\({}^{\circ}\)N with annual precipitation ranging from 62.0 to 541.0 mm and annual mean temperature varying between \(-\)5.0 and 12.5 \({}^{\circ}\)C. Alpine meadow, arid shrub, deciduous forest, and crops are the four vegetation types chosen for the present study (Table 1). Arou is located in a river valley highland on the south of Babao River, upstream of Heihe River (Figure 1) [44]. The climate of this site is a typical highland continental climate characterized by temperature variation. The average annual temperature, precipitation, and evapotranspiration are \(-5.0\)\({}^{\circ}\)C, 445.0 mm, and 1300.0 mm, respectively. The site mainly consists of grassland, comprising alpine and mountain meadow. The height of the meadow varies during the year, with 20\(-\)30 cm in summer and several centimeters in other periods. The surface and the deeper soil are composed of clay and sandy clay loam, respectively; however, permafrost and seasonally frozen soil also exist in this area. Daman is a typical irrigation district and lies in an artificial oasis of the middle Heihe River watershed. The average annual temperature and precipitation over the past five decades (1961-2010) are around 5.4 \({}^{\circ}\)C and 204.0 mm, respectively, with the annual potential evaporation of 2365.0 mm [45]. Spring wheat and maize (C4 plant) are the major crops in this site, of which spring wheat is generally sown in late March and harvested in the middle 10 days of July, while maize is sown in late April and harvested in the middle 10 days of September [46]. The maximum canopy height and the maximum LAI are 2.1 m and about 5.6 m\({}^{2}\)m\({}^{-2}\), respectively. Flood irrigation is conducted several times during the growing season. The soil is characterized by silty day loam and silt loam for the surface and deeper layer, respectively. Sidaoqiao is located in the Ejin Banner desert-oasis downstream of Heihe River watershed. This site represents an extremely arid region with scarce precipitation (116.8 mm per year) and high potential evapotranspiration. The potential evaporation is approximately 3755.0 mm per year and the average temperature is about 9.6 \({}^{\circ}\)C per year, with a frost-free period of 165 days [46]. Vegetation covers ranging from 15% to 20% and consists of desert shrubs comprising _Populuseuphratica_, _Tamarix_, and other arid region species. The soil is mainly composed of unconsolidated sand and grains and is highly susceptible to wind erosion [47]. Xinier lies on the alluvial plain of the Kaidu-Kongqi River under the southern foot of Tianshan Mountain and northeast edge of Taklimakan Desert. It is characterized by a typical continental desert climate with average temperature of 12.5 \({}^{\circ}\)C and scarce precipitation of 62.0 mm. Annual mean potential evaporation measured with evaporation pan is 2788.0 mm [48]. The major crop of the study area is cotton, which is the main economic crop in Xinjiang Province, China. Cotton is planted in April and harvested from September to November 2012 [49]. Mulched drip irrigation is applied in