| 2003 | 2010 | 2017 | |||
面积 | 占比 | 面积 | 占比 | 面积 | 占比 | |
I级 | 1.65 | 7.14% | 1.52 | 6.60% | 2.19 | 9.29% |
II级 | 1.47 | 7.37% | 1.24 | 5.40% | 3.01 | 12.73% |
III级 | 5.43 | 23.56% | 5.30 | 23.03% | 6.48 | 28.11% |
IV级 | 7.14 | 30.97% | 7.01 | 30.44% | 5.81 | 25.18% |
V级 | 7.37 | 31.96% | 7.95 | 34.55% | 5.57 | 23.93% |