4.1. Annual and inter-annual average variation of ecological sustainability
The coefficient matrix of weight for each indicator was obtained by the principal component analysis (Table 4).
The tendencies of annual average variation of press, state, response, and sustainability of Yilong Lake from 1952 to 2006 are shown in Figure 5. During this period, the annual average press values fluctuated and decreased (Figure 5a).
The annual average state value decreased sharply from 0.99 to 0.37, with the minimum of 0.23 observed in 1993 as shown in Figure 5b. As the decrease of the annual average state value, the annual average response increased sharply from 1978 to 2006 (Figure 5c). From 1952 to 1978, the annual average sustainability value decreased from 0.73 to 0.33. In recent years, it increased slowly due to the sharply increase of the human response (Figure 5d).