## 2 Materials and Methods
### Research Methodology
#### 2.2.2 Gradient Division at the Spatial Level  
The corridors formed by the land-water interface zones are an integral part of the water body landscape and can better reflect the disturbances to the water bodies. The corridor formed by the land-water interface zone is an integral part of the water bodies landscape, which can better reflect the disturbances suffered by the water bodies. In order to show the gradient variation of the ESV transverse profiles in the aquatic-land interface zone, after considering the ecological spatial characteristics and the scope of the study area, the gradient along the river was divided into four distances, namely, 500 m, 1000 m, 1500 m, and 2000 m, as the unit of gradient division according to the geographic characteristics of the Xijiang River Basin. After a comprehensive comparison, the smaller the spacing, the more favorable this is for the expression of spatial differentiation, but the less amount of data contained in the gradient. The larger the spacing, the more unfavorable this is for the expression of spatial differentiation, but the more data contained in the gradient, the greater the analysis that can be completed. When combined with the purpose of this study, which needs to achieve a certain accuracy of spatial differentiation as well as a sufficient amount of data, 1000 m was finally chosen as the increment of the gradient. The along-river gradients are uniformly named in the text: along-river gradient 1, along-river gradient 2, and so on, dividing the study area (starting from the centerline of the Xijiang River mainstem) into 10 gradient classes to determine the ESV gradient.  
In order to investigate the ESV gradient upstream and downstream of the study area, a buffer zone of 10 km on both sides of the mainstem of the Guangdong section of the Xijiang River was divided into 10 zones based on the centerline of the mainstem of the river, taking into account the ecological and spatial characteristics of the study area. The specific division operation was as follows: the centerline of the mainstem of the Guangdong section of the Xijiang River Basin (approximately 342 km) was divided into 10 equal parts, and a dividing line was made perpendicular to the centerline of the river at the point of segmentation, and the buffer zones in the range of 10 km on each side from the centerline of the river (to a total of 20 km) were cut vertically perpendicular to the centerline of the river so that we obtained 20 km land-water intersection zones in the Guangdong section of the Xijiang River mainstem in 10 equal parts, corresponding to the ten gradients, respectively. These were named the first level of the river section, the second level of the river section, the third level of the river section, and so on; each gradient basically represents the area from the upper reaches to the lower reaches of the geospatial characteristics and different types of land-use modes, and at the same time, it is also convenient for analyzing comparisons with the gradient along the river. Each gradient was calculated and analyzed as a separate ESV research unit (Figures 2 and 3).  
Figure 3: The change trends of ecosystem service value per square kilometer and area of land use types with lateral gradient.  
Figure 2: Land use type map of a 1–10 km stretch along the river of a land and water interface zone of the Guangdong section of the Xijiang River Basin, as well as the profiles of the study subarea lines.