2.4.1. Trend Analytical Method

The linear trend analysis of EVI in the growing season from 2000 to 2015 was carried out with the trend analytical method. The formula is as follows:

$$\text{Slope} = \frac{\mathbf{n} \times \sum\_{i=1}^{n} \left( \mathbf{i} \times \text{EVI}\_{i} \right) - \sum\_{i=1}^{n} \mathbf{i} \sum\_{i=1}^{n} \text{EVI}\_{i}}{\mathbf{n} \times \sum\_{i=1}^{n} \mathbf{i}^{2} - (\sum\_{i=1}^{n} \mathbf{i})^{2}},\tag{1}$$

where n stands for the time series (2000–2015), i.e., n = 16; EVIi the mean EVI of the year i; Slope the inter-annual change slope of certain EVI pixel. When the value of Slope is positive, it indicates that EVI shows a trend of increase as time goes by, and vice versa. The formula is highly reliable and has been widely used [11,43,44].
