## 3 Results and Analysis  
### Analysis of the Average RSEI in the Study Area  
The changes in the average RSEI value for the study area from 1990 to 2020 are shown in Figure 4. Over the past 30 years, the average RSEI in the study area generally exhibited an increasing trend, with an average of 0.669. The lowest value was 0.618 in 2005, and the highest was 0.745 in 2020. Combining the characteristics of the RSEI histogram distributions from different periods, the study area was divided into five ecological environment quality grades (Figure 5): excellent (0.8-1), good (0.6-0.8), moderate (0.4-0.6), fair (0.2-0.4), and poor (0-0.2) [8]. The areas and their respective proportions for each ecological grade were calculated across seven time periods. The results indicated that from 1990 to 2020, the ecological environment quality grade of the study area was predominantly classified as good, with an average proportion of 70.94%. The next most prevalent grade was moderate, with an average proportion of 19.28%. The average proportions for poor, fair, and excellent were relatively low, at 0.11%, 2.41%, and 6.92%, respectively (Table 2). Figure 4: Interannual changes in RSEI average values in the study area from 1990 to 2020. (Different colored lines represent the fitting of the change trends of RESI over different time periods). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} &
\begin{tabular}{c} **RSEI** \\ **Grades** \\ \end{tabular} & **Poor** & **Fair** & **Moderate** & **Good** & **Excellent** \\ \hline \multirow{3}{*}{1990} & Area & 115.13 & 1436.65 & 16,086.99 & 39,391.83 & 1169.4 \\  & Proportion & 0.2 & 2.47 & 27.64 & 67.68 & 2.01 \\  & Area & 166.69 & 2135.91 & 17,050.44 & 37,719.16 & 1127.79 \\  & Proportion & 0.29 & 3.67 & 29.3 & 64.81 & 1.94 \\  & Area & 59.36 & 2249.41 & 15,717.13 & 39,092.31 & 1081.79 \\  & Proportion & 0.1 & 3.86 & 27.01 & 67.17 & 1.86 \\  & Area & 69.19 & 2407.39 & 15,844.12 & 39,294.97 & 584.33 \\  & Proportion & 0.12 & 4.14 & 27.22 & 67.52 & 1 \\  & Area & 3.13 & 436.29 & 4395.63 & 47,009.76 & 6297.63 \\  & Proportion & 0.01 & 0.75 & 7.55 & 80.77 & 10.82 \\  & Area & 3.13 & 436.72 & 4399.98 & 47,056.3 & 6303.86 \\  & Proportion & 0.01 & 0.75 & 7.56 & 80.85 & 10.83 \\  & Area & 2.65 & 394.98 & 3063.51 & 40,288.86 & 14450 \\  & Proportion & 0 & 0.68 & 5.26 & 69.22 & 24.83 \\ \hline \multicolumn{6}{l}{Unit: Area (km\({}^{2}\)), Proportion (\%).} \\ \end{tabular}
\end{table} 
Table 2: Area and proportion statistics of ecological environment quality grades in the study area. Figure 5: Distribution of ecological grades in the study area from 1990 to 2020. Spatiotemporal Characteristics Analysis of Ecological Environment Quality in the Study Area 3.2.1. Temporal Characteristics Analysis of Ecological Environment Quality in the Study Area  
Over the past 30 years, the ecological environment quality of the study area exhibited a two-stage evolution, characterized by a gentle decline followed by a substantial improvement (Figure 4). From 1990 to 2005, the RSEI decreased from 0.639 to 0.618, indicating a downward trend. During this period, the ecological environment quality was predominantly classified as good and moderate, with their combined area proportion exceeding 94%. From 2005 to 2020, the RSEI increased from 0.618 to 0.745, demonstrating a significant upward trend. The primary ecological environment quality grades were good and excellent, with their combined proportion surpassing 68.52%. Since 2010, the combined proportion classified as good and excellent consistently exceeded those classified as good and moderate. Specifically, the excellent proportion has significantly increased, rising from 10.82% in 2010 to 24.83% in 2020. The ecological environment quality grades of the study area from 1990 to 2020 are presented in Table 3. On average, over the 30 years, the area with an unchanged ecological environment quality grade accounted for 50.06% of the study area.