## 2 Materials and Methods  
### Site and Dataset Descriptions  
A comprehensive and continuous set of in situ measurements of flux is crucial for research on land-atmosphere interactions. The SACOL (shown in Figure 1) is among the few stations for the long-term and continuous measurements of flux in land-atmosphere interactions in the transitional zone of China's summer monsoon. The reliability of data provided by the SACOL has proven to be instrumental for research on the surface energy and water and carbon cycles over transition zones [22; 23; 24]. The SACOL station is located in the western part of China's transitional climate zone (35\({}^{\circ}\)57\({}^{\prime}\) N, 104\({}^{\circ}\)08\({}^{\prime}\) E), with an elevation of 1965.8 m. The mean annual volume of rainfall and Ta are 386.5 mm and 7.4 \({}^{\circ}\)C, respectively [25]. More than 80% of the annual precipitation occurs in the growing season [13], while desert grasslands constitute the dominant vegetation on the land surface. The growing season is the period of the year when crops and other plants grow successfully. The growing season dates in a region are closely related to the climate. In China's transition climate zones, the growing season typically spans from early April to the end of September [13]. Figure 1: Map and photographs of the SACOL station. The star in the middle represents the location of the SACOL station, and the shaded area represents the transitional climate zone in China. The two figures on the left show the observation field and the gradient tower, while the three figures on the right show observations of the surface radiation, parameters of the soil, and surface fluxes. An eddy covariance (EC) system was installed at SACOL that consisted of a three-dimensional (3D) sonic anemometer (CSAT3, Campbell Scientific, Salt Lake, UT, USA) and an open-path infrared gas analyzer (LI7500, LI-COR, Lincoln, RI, USA). The sonic anemometer was used to measure the 3D wind speed and the virtual temperature. The open-path infrared gas analyzer provided simultaneous measurements of the concentrations of water vapor and carbon dioxide. Boundary layer meteorological measurements were used to record the wind speed (014A-L, Met One, Salt Lake, UT, USA), the Ta of air, and the relative humidity (HMP45C-L, Vaisala, Helsinki, Finland) at heights of 1, 2, 4, 8, 16, and 32 m. The upward and downward shortwave radiations were measured using CM21 radiometers (Kipp & Zonen, Delft, The Netherlands), while the upward and downward longwave radiations were measured using CG4 radiometers (Kipp & Zonen, Delft, The Netherlands). The volumetric water content of the soil was collected by using CS616 (Campbell Scientific) at depths of 5, 10, 20, 50, and 80 cm. In addition, reliable methods were used to control the quality of the data, as well as the annual calibration of the instruments. Linear interpolation and Falge's method [26] were used to enhance the reliability of the dataset [24]. Each time series was screened for spikes, and the discarded values were interpolated. Spikes are defined as points exceeding 4.0 times the standard deviation in a 60-s window. If spikes and unrealistic values surpassed 1% of the total data points, the entire run was discarded. Incomplete records (<5 min) were removed. The results indicate that spikes insignificantly impact turbulent flux measurements. Variances and covariances were computed through 30 min block-averaging. Addressing potential time delays, cross-correlation analysis corrected for sensor separation, and lag-time bounds were set at 1.0 s for each interval. Then, a series of tests were conducted to check the reliability of the data [24]. A specific QA/QC system, including data post-processing and quality control, was developed for SACOL. The procedure of post-processing included time delay correction, coordinate rotation, frequency response corrections, sonic temperature correction, and WPL correction.