# Study of the Dynamic Adaptive Calculation Method for River Water Environmental Capacity
## 2 Study Area  
The study area is the Wei River Basin in Shaanxi Province (Figure 1), located between 104-110 \({}^{\circ}\)E and 34-37 \({}^{\circ}\)N, with an area of 134,300 km\({}^{2}\). The Wei River, a perennial river in northern China, with an average annual runoff of 7.57 billion m\({}^{3}\)[32], flows through the provinces of Gansu, Ningxia, and Shaanxi, with a total length of 818 km. The Shaanxi section of the main stream of the Wei River flows through Baoji, Yangling, Xianyang, Xi'an, and Weinan [33]. It mainly supplies production and living water, as well as drainage, to cities along the river in this area. Most of the water flows through the Loess Plateau from northwest to southeast, with a high sediment content and a gentle slope. The younger geological units in the Shaanxi section of the Wei River mainly include Cenozoic strata, which are composed of clay, sand, and gravel mixed layers, forming shallow pore water aquifers and confined water aquifers in the alluvial plain. These aquifers are well developed near the riparian zone of the Wei River, with thicknesses ranging from 10 to 90 meters, indicative of a significant water resource in the area. The hydraulic conductivity of the riverbed varies greatly, ranging from 316 to 1465 \(\upmu\)s/cm, with an average of 860 \(\upmu\)s/cm. There are many tributaries of the Wei River, mainly including nine large-flow tributaries, such as the Qian River, the Hei River, the Ba River, and the Jing River (Figure 1), as well as five large water intakes and sixty-nine sewage outlets.  
The Wei River supports the people's life, social progress, and economic development in the administrative regions along the Wei River in Shaanxi Province, and serves as the basis for the economic, social, and human development of the entire province. However, since the 1980s, the Wei River has not only become the main source of urban water use in Shaanxi Province but has also transformed into a natural sewage discharge site within the city. As the pollutant discharge exceeds the pollution carrying capacity of the river itself, the water pollution problem is deteriorating, and the natural functional state of the Wei River has been greatly changed, making a water use and water shortage crisis for the city. Moreover, with the expansion and transformation of urban construction, the Wei River has changed its original landform and hydrological characteristics, resulting in the continuous shrinking of the river system. Therefore, in order to scientifically implement the total amount control of water pollution and the prevention and control of water environmental pollution, it is a key issue to determine a reasonable and feasible WEC.  
Water function zoning is the basis for the implementation of total water pollution control in river basins. It is a scientific basis for determining the dominant functions of different river reaches, formulating water environment quality standards, and carrying out water resources protection and management. According to the \"Shaanxi Province Water Environmental Function Zoning\", the Wei River can be divided into 14 functional areas, as shown in Figure 1 and Table 1. The main pollutants in the Wei River are chemical oxygen demand (COD), NH\({}_{3}\)-N, total phosphorus (TP), and dissolved oxygen (DO) [34; 35; 36], among which COD and NH\({}_{3}\)-N are the most important pollutants in the Shaanxi section of Wei River. Therefore, COD and NH\({}_{3}\)-N are the main research objects in this paper.