## 3 Results and Analysis
### Spatial Variation Characteristics of Soil Salinity  
#### 3.3.1 Isotropic Analysis  
The traditional statistical analysis of soil salinity can only summarize the whole picture of soil salinity, but it is difficult to express the randomness and structure of soil salinity. Therefore, it is necessary to analyze and discuss its spatial variation structure using geostatistics theory and methods [4]. The selection of the optimal semi variance function model generally follows the principle that the larger the determination coefficient (R\({}^{2}\)), the smaller the residual error (RSS) [16]. See Table 3 and Figure 6 for the results. It can be seen from Table 3 that the fitting models of the salinity variation function of the three layers of soil were the spherical model, spherical model, and exponential model, respectively, and the determination coefficients (R\({}^{2}\)) of the theoretical model to the variogram were 0.756, 0.830, and 0.761, respectively, indicating that the theoretical model had a high degree of fitting with the actual variation function. In the semi-variance function model, the nugget (C\({}_{0}\)) represents the variation caused by random factors, including human factors, such as irrigation, cultivation, and management. Partial Sill(C) represents the variation caused by structural factors, including natural factors such as soil parent material, topography, and groundwater level during soil formation [27; 28]. Nugget coefficient [C\({}_{0}\)/ (C\({}_{0}\) + C)] can reflect the degree of correlation of variables. Wherein, when C\({}_{0}\)/(C\({}_{0}\) + C) \(<\) 25%, soil salinity has a strong spatial correlation; when 25% \(<\) C\({}_{0}\)/(C\({}_{0}\) + C) \(<\) 75%, soil salinity has a moderate spatial correlation; and when C\({}_{0}\)/(C\({}_{0}\) + C) \(>\) 75%, soil salinity has a weak spatial correlation [29; 30]. The nugget coefficient of soil salinity in the layer between 0 m and 0.25 m was 69.5%, showing moderate spatial correlation, which indicates that the spatial heterogeneity of surface soil salinity is affected by both random factors and structural factors. However, on the scale of the basin as a whole, the surface soil is affected by random factors more than structural factors. The nugget coefficients of soil salinity in the layers between 1 m and 2 m and between 2 m and 3 m were 0.8% and 10.2%, respectively, which is far less than 25%, showing a strong spatial correlation. This ratio indicates that the salinity of deep soils is almost completely spatially autocorrelated under the action of structural factors. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Depth/m**} & \multirow{2}{*}{**Model**} & **Nugget** & **Sill** & **Range** & **Nugget Coefficients** & \multirow{2}{*}{**R\({}^{2}\)**} & \multirow{2}{*}{**RSS**} \\  & & **C\({}_{0}\)** & **C\({}_{0}\) + C** & _a/km_ & **C\({}_{0}\)/(C\({}_{0}\) + C)** & & \\ \hline
0–0.25 & Spherical & 1.459 & 2.086 & 56.0 & 69.5 & 0.756 & 0.108 \\
1–2 & Spherical & 0.010 & 1.216 & 18.2 & 0.8 & 0.830 & 0.042 \\
2–3 & Exponential & 0.139 & 1.309 & 24.9 & 10.2 & 0.761 & 0.129 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Variogram model and parameters. Figure 6: Semi variance function of soil salt content: variogram of soil salinity in the layers between 0 m and 0.25 m (**a**); variogram of soil salinity in the layers between 1 m and 2 m (**b**); variogram of soil salinity in the layers between 2 m and 3 m (**c**). Range(_a_) reflects the autocorrelation range of the variable [16], and range(_a_) of the soil salinity in the three layers was 56 km, 18.2 km, and 24.9 km, respectively. The layer between 0 m and 0.25 m had the largest range(_a_) of soil salinity (56 km), indicating that the soil salt content is consistent in a large range. This is because the study area mainly implements a highly intensive management mode of agricultural technology, so the agricultural management measures in different areas are similar.