## 3 Results
### Spatiotemporal Analysis of Water Quality  
According to the \"Surface water Environmental Quality Standards\" (GB3838-2002) [31] that establish index evaluation standards, as shown in Table 4, and according to the surface water environmental functions and protection objectives, Class I water is suitable for source water and nature reserves, Class II water is suitable for primary drinking water sources, class III water is suitable for secondary drinking water sources, and class IV water is suitable for industrial water. Class V water is suitable for agricultural water use. Turbidity is not included in water quality assessment because there is no corresponding standard limit value, but it can be used as a reference index to judge whether water quality is affected by sediment and salinity. High NH\({}_{3}\)-N levels indicate water pollution, which is mainly caused by agricultural fertilizers and domestic sewage. CODMn indicates organic content; high DO levels indicate the health of the water body and support the survival of aquatic  
Figure 4: Scatter diagram of prediction using the water quality inversion model. life. In Figure 5, the spatial variation characteristics of water quality inversion for TUB, CODMn, NH\({}_{3}\)-N, and DO are introduced. As shown in Figure 5, in the water quality inversion diagram of the Yellow River basin in Ningxia, it was found that the TUB content in the middle reaches of the Yellow River basin in Ningxia has increased significantly, which is primarily due to the increase in sediment content caused by soil erosion. As such, soil and water control in this region still need to be strengthened. In the entire basin, the CODMn concentration in the upper and lower reaches meets the Class II water quality standards, and that in the middle reaches meets the Class III and Class IV water quality standards, indicating that the impact of CODMn pollution is relatively serious. The average content of NH\({}_{3}\)-N reached the Class I water quality standard, and, in some areas, it reached the Class II water quality standard. In general, the influence of NH\({}_{3}\)-N on the Yellow River basin in Ningxia was minor. The average concentration of DO content in the entire basin meets the water quality standards of Classes I and II. According to the inversion maps of the four water quality parameters, the water quality in the upper and lower reaches of the Yellow River basin in Ningxia is \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Evaluation Factor**} & \multicolumn{6}{c}{**Evaluation Level**} \\ \cline{2-6}  & **I** & **II** & **III** & **IV** & **V** \\ \hline CODMn (mg/L)\(\leq\) & 2 & 4 & 6 & 10 & 15 \\ NH\({}_{3}\)-N (mg/L)\(\leq\) & 0.15 & 0.5 & 1 & 1.5 & 2 \\ DO (mg/L)\(\geq\) & 7.5 & 6 & 5 & 3 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Environmental quality standard limits for surface water. Figure 5: Spatial variation characteristics of water quality parameters. satisfactory; in comparison, the water quality in the middle reaches is poor, but the overall water quality is satisfactory, mainly constituting Class I and Class II water quality. These findings are consistent with the actual information on surface water environmental quality released by the local environment office. From 2004 to 2018, the pollution of the Yellow River basin in Ningxia was relatively serious, the main pollution indicators were NH\({}_{3}\)-N and CODMn, and the rainy season and dry season were dominant, but since 2010, the water quality in the Yellow River basin has gradually improved [32]. At present, the average erosion intensity of the land in the Yellow River basin still reaches 178 tons per square kilometer per year, and the turbidity content is relatively high. However, over the past 10 years, a series of measures have been taken, such as afforestation and grassland restoration, to increase vegetation coverage, reduce soil erosion by a considerable degree, and reduce water pollution.