## 3 Results
### Dynamic Characteristics of Seasonal Drought
The spatial distribution of drought index in the fall was similar to that in the summer, reflecting that drought was more serious in areas with less precipitation, the climate being drier in the northwest, where grassland is the main vegetation type, but more humid in the southeast and southwest, where forest is the main vegetation type (Table A1). It is worth mentioning that the SPEI value was the highest and the drought magnitude was the least in Qinghai and the central Loess Plateau (Figure 3(a3)). The magnitude of drought in fall was mainly mild (25.9%) or no drought (74.1%); the former was mainly distributed in Inner Mongolia, Ningxia, and some areas of Gansu, whereas other areas experienced no drought (Figure 3(b3), Table 2). The spatial distribution of SPEI values in the winter was similar to that in the spring, with SPEI values in the northwest being higher than those in the southeast and southwest, that is, the more precipitation there was, the more serious the winter drought was (Figure 3(a4)). Except for the most regions of Qinghai and the southern Loess Plateau, which are dominated by grassland and experienced mild drought (5.5%), most of the regions generally experienced no drought (94%) in winter (Figure 3(b4), Table 2). WUE changed from 0.1 to 24 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\); the average value was 3.1 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\), the low value occurred in the southwest study region with higher elevation, and the grassland had the highest WUE (2.17 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\)) (Table 3). The spring WUE decreased generally with the decrease in precipitation from the southeast to the northwest, with a decreasing trend from 2001 to 2019. In the summer (June-August), the spatial distribution of vegetation WUE was relatively uniform, in the range of 0.2-5.5 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\), with an average value of 3.6 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\). Of all seasons, the average WUE in summer was the highest, presenting the spatial distribution features of \"high in the southeast and low in the northwest\". The border zone between Gansu and Ningxia had the lowest WUE, whereas the highest WUE values were in the southeast forest area and the Qinghai grassland area. In fall (September-November), the spatial distribution characteristics of WUE were quite similar to those in the spring; the WUE range was 0.1-16 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\), with an average value of 3.4 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\)and the forests had the highest WUE (1.74 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\)) (Table 3). The average WUE in the fall was slightly higher than that in the spring. Compared to spring, summer, and fall, winter (December-February) WUE exhibited a lower value and an increasing trend. The WUE value ranged from 0 to 11 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\), with an average value of 1.4 mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Forests** & **Shrublands** & **Grasslands** \\ \hline Spring & 2.06 & 1.88 & 2.17 \\ Summer & 1.81 & 1.54 & 1.98 \\ Fall & 1.74 & 0.83 & 0.49 \\ Winter & 0.78 & 0.35 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Seasonal WUE of different vegetation types (mg C\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\)). Figure 4: _Cont._