## 4 Results
### Spatial Variation Characteristics of Annual TN and TP Discharge Loads  
Under the simulated results of the SWAT model, the annual TN and TP outputs were 618 and 140 kg km\({}^{-2}\), respectively. The output per unit area of TN and TP in the river section was divided into five categories. The annual TN discharge load was highest in the middle parts of the basin, in subbasins 5, 11, 15, 23, and 25. The second grade was in subbasins 3, 9, and 22. The lowest TN output was mainly located in subbasins 2, 16, 17, 18, 19, and 24 (Figure 3). The annual TP discharge load generally exhibited similar spatial distribution characteristics as the annual TN discharge load. The annual TP discharge load  
\begin{table}
\begin{tabular}{l l l} \hline \hline \multicolumn{1}{c}{**Parameter**} & \multicolumn{2}{c}{**Range**} \\ \cline{2-3}  & **Minimum** & **Maximum** \\ \hline A\_GWQMN.gw & Threshold depth of water in the shallow aquifer & \multirow{2}{*}{0} & \multirow{2}{*}{1000} \\  & required for return flow to occur (mm). & & \\  & Groundwater “revap” coefficient has effects on & & \\  & the amount of water that recharges the capillary & \multirow{2}{*}{0.02} & \multirow{2}{*}{0.2} \\ V-GW-REVAP.gw & fringe after evaporation during dry periods. & & \\  & The capillary fringe is recharged by shallow & & \\  & aquifera during dry periods. & & \\ A\_RCHRG\_DPgw & Deep aquifer percolation fraction. & \(-\)0.05 & 0.05 \\ A\_GW\_DELAY.gw & Groundwater delay (days). & 100 & 300 \\ R\_CN2.mgt & SCS runoff curve number. & \(-\)0.1 & 0.1 \\ R\_CH\_W2.rte & Average width of the main channel at the top of & \(-\)0.15 & 0.15 \\  & the bank (m). & & \\ R\_CH\_S2.rte & Average slope of the main channel along the & \multirow{2}{*}{0.15} & \multirow{2}{*}{0.15} \\ V\_CH\_N2.rte & Manning’s N value for the main channel. & 0.01 & 0.2 \\ V\_CH\_K2.rte & Effective hydraulic conductivity (mm/h). & 0 & 10 \\ R\_CH\_L2.rte & Length of the main channel (km). & \(-\)0.09 & 0.012 \\ V\_ESCO.hru & Soil evaporation compensation factor. & 0.70 & 0.95 \\ R\_OV\_N.hru & [PERSON]’s N value for overdland flow. & \(-\)0.29 & \(-\)0.078 \\ V\_SURLAG.bsn & Surface runoff lag coefficient. & 5.54 & 14 \\  & The calibration coefficient is used to control the & & \\  & impact of the storage time constant (km) for & \multirow{2}{*}{0.004} & \multirow{2}{*}{0.46} \\ R\_MSK\_CO1.bsn & normal flow (where normal flow is when the & \\  & river is at bankfull depth) upon the \(K_{m}\) value & & \\  & calculated for the reach. & & \\  & The calibration coefficient is used to control the & & \\  & impact of the storage time constant (km) for low & \multirow{2}{*}{0.63} & \multirow{2}{*}{\(-\)0.01} \\ R\_MSK\_CO2.bsn & flow (where low flow is when the river is at 0.1 & \(-\)0.63 & \(-\)0.01 \\  & bankfull depth) upon the \(K_{m}\) value calculated & & \\  & for the reach. & & \\ R\_SOL\_BD\_sol & Moist bulk density (g/cm\({}^{3}\)). & \(-\)0.23 & \(-\)0.07 \\ r\_ALPHA\_BFgw & Baseflow alpha factor (days). & 0.05 & 0.13 \\ r\_SOL\_K & Saturated hydraulic conductivity (mm/h). & \(-\)0.9 & 0.9 \\ v\_CANMX & Maximum canopy storage (mm H\({}_{2}\)O). & 0 & 10 \\ r\_SOL\_AWC.sol & Available water capacity of the soil & \(-\)0.3 & 0.3 \\ r\_SOL\_Z.sol & layer (mm H\({}_{2}\)O/mm soil). & \(-\)0.3 & 0.3 \\ r\_SOL\_Z.sol & Depth from the soil surface to the bottom of the & & \\  & layer (mm). & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters and their ranges used for the Soil and Water Assessment Tool (SWAT) model.  
was highest in the middle parts of the basin, in subbasins 3, 5, 15, 23, and 25. The second grade was in subbasins 9, 10, 11, 12, 20, and 22. The lowest annual TP discharge load was mainly located in subbasins 2, 17, 18, 19, and 24 (Figure 4). The areas with large annual TN and TP discharge loads were situated in the middle parts of the basin, where the cultivated and construction land are concentrated and the landscape pattern is relatively complex.