## 4 Analysis of Results  
### Vegetation NDVI Trend Analysis  
The interannual variation trend of vegetation NDVI in Ordos City is shown in Figure 2, which is obtained by the trend analysis method. Vegetation NDVI shows an overall improvement trend (97.97% of the region shows a positive trend), but there are significant spatial differences. The average NDVI of Ordos City over 21 years is 0.324, with an average annual increase rate of 0.0075/a. The area of extremely significant improvement account for a large proportion, about 51.92% (Table 2), and it is distributed in all banners, among which the proportion of area with extremely significant improvement is larger in Ejin Horo Banner, Dongsheng District, Kangbashi District, and Jungar Banner in the east, especially in Ejin Horo Banner and Jungar Banner. The area with a significant increase account for about 16.74%, mainly distributed in the central and western parts of Erdo City. The area with no obvious change is 31.02%, mainly distributed in Hangjin Banner, Otok Banner, and Etoke Front Banner. The area of significant degradation and extremely significant degradation is relatively small, accounting for 0.13% and 0.19% of the total area, respectively, mainly distributed in the urban area of Ordos City and the northern Yellow River. In order to further analyze the vegetation change in Ordos City, the trend analysis of vegetation NDVI in spring, summer, and autumn for 21 years was carried out separately. In spring, the mean value of vegetation NDVI in Ordos City is 0.182, and the average annual increase rate is 0.0041/a, the slope of vegetation NDVI in 97.46% of Ordos City (Figure 2b) is positive, and 52.94% of the area is extremely significantly improved. It mainly appears in Jungar Banner, Dalat Banner, Dongsheng District, Kangbashi District, Ejin Horo Banner, and other areas; the area of significantly improved area is 16.56%, which is widely distributed and scattered throughout the region; the area of significant degradation and highly significant degradation are 0.1% and 0.06%, respectively, which are mainly found along the Yellow River and some areas in the north of Dalat Banner. In summer, the mean value of vegetation NDVI in Ordos City is 0.319, with a small difference from the interannual mean value, the average annual increase rate is 0.0063/a, and the slope of vegetation NDVI in 94.52% of Ordos City (Figure 2c) is positive, the area where the improving trend is greater than 0.02 is much larger than that in spring. A total of 39.16% of the area is extremely significantly improved, mainly concentrated in the east of Ordos City; the area of significantly improved area is 15.02%, which is more concentrated in Wushen Banner, Dalat Banner, and Etoke Front Banner; the percentage of significant degradation and highly significant degradation is 0.16% and 0.19%, respectively. In autumn, the mean value of vegetation NDVI in Ordos City is 0.254, which is greater than it in spring but less than the value of summer, and it is consistent with the characteristics that vegetation starts to grow in spring, grows in summer, and gradually dies in autumn. The average annual increase rate is 0.0067/a, the slope of vegetation NDVI in 98.47% of Ordos City (Figure 2d) is positive, the area where the improving trend \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Trends** & **Annual** & **Spring** & **Summer** & **Autumn** \\ \hline Extremely significant improvement & 51.92 & 52.94 & 39.16 & 58.94 \\ Significant improvement & 16.74 & 16.56 & 15.02 & 17.78 \\ No significant change & 31.02 & 30.34 & 45.47 & 23.21 \\ Significant degradation & 0.13 & 0.10 & 0.16 & 0.04 \\ Extremely significant degradation & 0.19 & 0.06 & 0.19 & 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Variation trend and significance of vegetation NDVI in Ordos City.