## 2 Study Area and Materials
### Datasets and Preprocessing
#### 2.2.3 Validation Data  
(1) Aboveground fresh weight and NPP  
The aboveground fresh weight was derived from the Qinghai Forestry and Grassland Administration (QFGA) ([[https://lcj.qinghai.gov.cn/](https://lcj.qinghai.gov.cn/)]([https://lcj.qinghai.gov.cn/](https://lcj.qinghai.gov.cn/)), Xining, China, accessed on 7 November 2022). The monitoring sites were determined by the grid sampling method. The monitoring was performed during the peak growth period in the summer. The area of each monitoring site was not less than 1 km\({}^{2}\), and 3 to 6 sample plots were selected at equal intervals for each monitoring site. The distance between the sample plots was about 250 m. The sample area of each plot was set to 1 m \(\times\) 1 m and 2 m \(\times\) 2 m, respectively, for grassland and shrubland. In accordance with this principle, about 225 monitoring sites were arranged for each year from 2012 to 2019. The aboveground plant samples were cut and averaged as field aboveground fresh weight. The aboveground samples of each plot were cut, and their average was taken as the aboveground fresh weight of this monitoring site. Then, the aboveground fresh weight was converted to aboveground biomass (AGB). The conversion coefficient between the aboveground fresh weight and the AGB was obtained from the field survey data of the research group from 2017 to 2018. First, the research group obtained the fresh weight of 11 sampling sites in 2017 and 12 sampling sites in 2018 using the same sampling method. Second, the aboveground fresh samples were dried at a thermostat of 65 \({}^{\circ}\)C until the weight remained stable; the weight was recorded as AGB. Then, the conversion coefficient between the aboveground fresh weight and the AGB was calculated, and the fresh weight of QFGA to AGB was converted using this coefficient. In this way, a total of 1793 AGB records were obtained from 2012 to 2019. Yearly and point-based outlier detection was first conducted for the NPP data using boxplots [66] as follows:  
\[\mathrm{Q_{1}-k(IQR)<x_{i}<Q_{3}+k(IQR)} \tag{8}\]  
where IQR is the difference between the upper quantile (Q\({}_{3}\)) and the lower quantile (Q\({}_{1}\)), \(k\) is 1.5, and \(x_{i}\) is the reasonable data range. When the measured data were out of the \(x_{i}\), they were considered outliers or extreme values and were excluded. After the outlier detection, a total of 1527 AGB records remained. For deciduous grasses, AGB is generally considered comparable to aboveground NPP (ANPP); therefore, the ANPP can be calculated using the following equation [67]:  
\[\mathrm{ANPP}=\mathrm{AGB}\times\mathrm{C_{a}} \tag{9}\]  
While the belowground biomass data are missing for the sample, the belowground NPP (BNPP) was converted using the following method [68]:  
\[\mathrm{BNPP}=(\mathrm{AGB}\times\mathrm{C_{a}})\times\left(\frac{R}{S} \times\frac{C_{b}}{C_{a}}\times t\right) \tag{10}\]  
\[t=0.0009\times\mathrm{AGB}+0.25 \tag{11}\]  
where AGB is the aboveground biomass, R/S is the root and shoot ratio, C\({}_{b}\) is 1.43, C\({}_{a}\) is 0.37 for the grassland, representing the carbon content below and above the ground, and t is the root turnover, indicating the ratio of root growth or death to the total biomass per unit time, which is correlated with the aboveground biomass. In total, the field sampling NPP was calculated using the following equation:  
\[\mathrm{NPP}=\mathrm{ANPP}+\mathrm{BNPP} \tag{12}\]  
(2) Soil water data  
This paper collected ground-measured soil moisture data from the Qinghai Meteorological Bureau ([[http://qh.cma.gov.cn/](http://qh.cma.gov.cn/)]([http://qh.cma.gov.cn/](http://qh.cma.gov.cn/)), Xining, China, accessed on 7 November 2022). The observation dataset consisted of the Guide station and the Tuotuo River station in the study area from 2014 to 2017. Each station recorded the gravimetric water 2 to 3 times every month, and the observation time was 2:00 pm. The observation data included the following three soil layers: 0-10 cm, 10-20 cm, and 20-30 cm.