## 3 Model of Relationship between Water Pollution and Land Use
### Variable Calculation
#### 3.3.3 External Influence of Water Environment  
The calculation formula of domestic sewage discharge is  
\[\rm{Q_{s}}=KQ_{1}V_{1}/1000 \tag{8}\]  
where \(\rm{Q_{s}}\) represents the sewage discharge from the living area (t/day), \(\rm{Q_{1}}\) is the daily domestic sewage quantity quota per person, \(\rm{V_{1}}\) is the number of people living in the area, K is the sewage discharge  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Land Use Type** & **Area (m2)** & **Proportion (\%)** \\ \hline Residential land & 24,659,579 & 0.221 \\ Commercial and service facility land & 4,743,765 & 0.042 \\ Green and square land & 76,816,428 & 0.687 \\ Land for public administration and public service facilities & 5,371,766 & 0.048 \\ Industry & 156,060 & 0.001 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Proportion of different land use types.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Latent Dependent Variable** & **Variable** & **Unit** \\ \hline \multirow{3}{*}{Water environmental quality} & COD & mg/L \\  & TP & mg/L \\  & TN & mg/L \\ \hline \multirow{3}{*}{Water purification capacity} & Green area ratio & \% \\  & River meandering & - \\ \hline \multirow{3}{*}{Water environment} & COD environmental capacity & t \\  & TP environmental capacity & t \\  & TN environmental capacity & t \\ \hline \multirow{3}{*}{Water environment external influence} & COD emissions & t \\  & TP emissions & t \\  & TN emissions & t \\ \hline \multirow{3}{*}{Upstream incoming water quality} & COD concentration & mg/L \\  & TP concentration & mg/L \\  & TN concentration & mg/L \\ \hline \multirow{3}{*}{
\begin{tabular}{c} \end{tabular} } & COD point source emissions & t \\  & TP point source emissions & t \\  & TN point source emissions & t \\ \hline \multicolumn{3}{c}{Non-point source load} & Construction land area & \% \\ \hline \multicolumn{3}{c}{Point source load} & Green area & \% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Structural equation model (SEM) measurement variables and potential variables.  
coefficient, which is generally 0.6-0.9, and 0.9 was selected as the coefficient according to the location of the research area. Table 5 lists the pollutant concentration parameters.  
According to the pollutant concentration parameters of each land use type and the calculation formula for domestic sewage discharge, the pollutant discharge of each land use type was obtained. The discharge of pollutants from different types of land use (urban drainage and sewage treatment regulations) in the research area was calculated and weighted by a geographic information system (GIS) with water quality. Bootstrapping was used to combine the two kinds of data into a new dataset. According to the pollutant emission concentration distribution in the research area, the land use was divided into four categories: areas of general, severe, minor, and basically no impact.  
Figure 7 shows that the output of COD, TP, and TN in the upstream was small, with higher distribution in the downstream. The COD, TN, and TP emission concentrations were the highest in the residential land. Residential areas (including rural settlements) were the main factors affecting water quality in this region. The discharge of domestic sewage is the main factor causing the deterioration of water environment quality, followed by commercial land and public service land. The relationship between pollutants in rivers and land use is well understood. Residential areas and some rural settlements show a strong correlation with the concentration of pollutants in rivers. The relationship between pollutant concentration and different types of land gradually decreases as the distance from the river increases, and only some residential areas still impact pollutant concentration in the river.