2.3.1. Trend Analysis Method

Trend analysis is a linear regression analysis of the changes of variables over timescales. It can not only track and analyze the change trend of variables, but also predict the change trend of variables. In the analysis of the change trend of inter-annual NDVI, the slope is the minimum power of the raster value of the time series, and the change value of spatial pixel on the time scale can be calculated by traversing pixel by pixel, and the change trend can be obtained [42]. The calculation method is as follows:

$$\mathcal{Z}\_{Slope} = \frac{n \times \sum\_{i=1}^{n} (i \times NDVI\_i) - \sum\_{i=1}^{n} i \sum\_{i=1}^{n} NDVI\_i}{n \times \sum\_{i=1}^{n} i^2 - \left(\sum\_{i=1}^{n} i\right)^2} \tag{1}$$

where, ∅*Slope* is pixel regression Slope, *NDVIi* is NDV value in the n year, and n is time length. When ∅*Slope* > 0, it indicates an increasing *NDVIi* trend, and when ∅*Slope* < 0, it indicates a decreasing NDVI trend.
