# Study of the Dynamic Adaptive Calculation Method for River Water Environmental Capacity
## 4 Results and Discussions
### Water Environmental Capacity Calculation
#### 4.1.1 Yearly Scale  
In order to facilitate the comparative analysis, the WEC results of COD and NH\({}_{3}\)-N in the same water function area under different design hydrological conditions were calculated using the standard model, the section-beginning control model, the section-end the smooth summation model through the simulation system. At a 90% design frequency, the COD WEC of the Shaanxi section of the Wei River based on the four calculation models is 42,449.18 t/a, 35,981.36 t/a, 60,824.15 t/a, and 42,574.95 t/a, respectively. The NH\({}_{3}\)-N WEC is 2453.97 t/a, 2163.43 t/a, 3088.36 t/a, and 2699.58 t/a, respectively. At a 75% design frequency, the COD WEC of the Shaanxi section of the Wei River based on the four calculation models is 49,130.73 t/a, 42,826.27 t/a, 68,318.24 t/a, and 48,680.20 t/a, respectively. The NH\({}_{3}\)-N WEC is 3259.76 t/a, 2959.55 t/a, 3928.37 t/a and 3571.13 t/a, respectively. At a 50% design frequency, the COD WEC of the Shaanxi section of  
Figure 7: WEC dynamic adaptive simulation system calculation results display interface. the Wei River based on the four calculation models is 99,086.76 t/a, 89,302.05 t/a, 130,889.13 t/a, and 103,152.35 t/a, respectively. The NH\({}_{3}\)-N WEC is 4528.62 t/a, 4129.47 t/a, 5377.03 t/a, and 4857.39 t/a, respectively. Based on the four calculation models, the WEC values of both COD and NH\({}_{3}\)-N in the Shaanxi section of the Wei River increase as the design frequency decreases from 90% to 75% and then to 50%. It shows that the size of WEC is proportional to the design flow. In addition, the WEC of COD is larger than that of NH\({}_{3}\)-N, because the attenuation coefficient of NH\({}_{3}\)-N is small. By analyzing the results of the four models, it can be concluded that the calculation results of the section-end control model are the largest, followed by the subsection summation model, then the standard model, and finally the section-beginning control model, which is the smallest. The calculation results of the models verify the characteristics of each one. The section-end control model ensures that the water quality of the downstream control section meets the standard, as well as determining the maximum allowable discharge of each pollution source in the upstream. The subsection summation model considers the influence of tributaries and water intakes on WEC. It avoids the situation that the calculation requirements for WEC are too strict or too loose. The standard model is based on the generalized sewage outlet inflow section as the reference section. Its actual self-purification length is half of the river length. The section-beginning control model controls the water quality of the upstream section, and requires the water quality of the whole functional area to reach the standard. The change process of river WEC obtained by calculation of the four models is shown in Figures 8 and 9. The ordinates of Figures 8 and 9 are the size of WEC, and the abscissa is the ID of the water function areas (corresponding to the 14 water function areas in the Shaanxi section of Wei River in Table 1). Figure 8 illustrates the WEC of different models for COD at 90%, 75%, and 50% design frequencies, and Figure 9 illustrates the WEC of different models for NH\({}_{3}\)-N at 90%, 75%, and 50% design frequencies. Based on the comprehensive analysis of Figures 8 and 9, it can be seen that the WEC calculation results of the four calculation models of nearly half of the water function zoning follow the rule of the section-end control model \(>\) the subsection summation model \(>\) the standard model \(>\) the section-beginning control model. However, the WEC calculation results of the subsection summation model in water function areas 2, 10, and 12 are the smallest, and the WEC calculation results of the subsection summation model water function areas of 6, 9, 11, and 14 are the largest, which is due to the characteristics of the model. The subsection summation model takes into account the influence of tributaries and water intakes when calculating the WEC.