Spatial-temporal variability analysis of the coupling coordination relationship between rainfall erosivity and NDVI in the Yangtze River Economic Belt

Abstract: Rainfall erosivity and vegetation cover are two of the most important factors in regional soil erosion. The greater the rainfall erosivity is, the more favorable the soil erosion is, whereas, the better the vegetation coverage is, the less favorable the soil erosion is. Moreover, the Yangtze River Economic Belt has been a prominent ecological and national strategic area in China. It is of great practical significance to determine the relationship between rainfall erosivity and vegetation cover, in order to protect the ecological environment. In this study, the daily-scale rainfall erosivity model and the Mann-Kendall trend test were applied to analyze the temporal and spatial evolution of monthly-scale rainfall erosivity and NDVI using daily precipitation and monthly NDVI data. The Pettitt test and coupling coordination degree theory were then used to identify the temporal and spatial variation of the coupling coordination relationship between rainfall erosivity and NDVI. Finally, the reasons were proposed for the change of the coupling coordination relationship between them. The results showed that: 1) Multi-year average rainfall erosivity from July to October was evenly distributed in the whole region, while there was a decrease from the southeast to the northwest in the rest months from 1965 to 2019. A significant increasing trend of rainfall erosivity was also found in the eastern region in July and September (P<0.05). The rainfall erosivity in the central region showed a significant decrease in January, whereas, there was a significant increasing in the central and southern regions in March. The rainfall erosivity in the southwest showed a significant decrease trend in May. 2) On a multi-year scale, the NDVI of the whole gradually increased from June to September, and it reached the highest level in August. The ratio of the area where the NDVI increased significantly in different months to the total area of the study area all exceeded 30% (P<0.05) from 1999 to 2019. The regions with the significant NDVI increase were distributed mainly in the southwest, central and southern regions. But, the NDVI in the eastern region decreased significantly, where the soil erosion risk was also high. 3) There was a high multi-year average in the coupling coordination degree of rainfall erosivity and NDVI in the east, but a low in the west. A significant decrease was detected in the coupling coordination degree in the central and southwestern regions, while a significant increase was in the eastern regions (P<0.05). The significant change-points were found in the coupling coordination degree of rainfall erosivity and NDVI, which were distributed mainly in the southwest, central and eastern regions in the occurrence period from 2003 to 2014. 4) The reduction of forest area (such as evergreen coniferous forest, evergreen broad-leaved forest, mixed forest, and closed shrub forest) was the important reason for the significant increase of coupling coordination degree in the east, indicating a high percentage of vegetation cover but a very small percentage of the area. Consequently, it is a high demand to rationally increase the forest land in space pattern for the ecological environment. The finding can greatly contribute to further understanding the risk of soil erosion in the Yangtze River Economic Belt.