# Study of the Dynamic Adaptive Calculation Method for River Water Environmental Capacity
## 1 Introduction  
With the development of the human economy and society, the problem of river water environmental pollution has become increasingly prominent [1]. At present, many sustainable development goals related to water environments have been put forward internationally [2]. In order to manage rivers scientifically and effectively, the calculation of water environmental capacity (WEC) has become a key issue [3]. WEC plays a crucial role in evaluating and managing the resources and quality of river water [4]. The scientific and reasonable calculation of WEC determines the development of a river and economy. Water resource management agencies in most countries have stipulated restrictions on the discharge of sewage into rivers and other natural water bodies, that is, the maximum allowable concentration of specific pollutants that may be discharged into water bodies [5], and have reduced the impact on the environment by formulating pollution control strategies. In different countries and regions, different concepts and definitions have been derived from the restrictions on the discharge of sewage into rivers and other natural water bodies. Japanese scholars first put forward the concept of environmental capacity in1968 [6]. Concerning WEC, European and American scholars suggested similar concepts, such as assimilation capacity, maximum allowable pollutant capacity, the allowable pollutant discharge level of a water body, and dilution capacity [7; 8]. In 1998, Chinese scholars proposed the concept of water pollution carrying capacity, which is an extension of WEC, and the basic principles of the two are the same. The best definition is provided by the US Environmental Protection Agency, which developed a regulatory application called total maximum daily load (TMDL), which can quickly reflect the dynamic relationship between water quality and water environment quality [9]. WEC refers to the maximum amount of a certain pollutant that can be contained in the water in a certain period of time when the water quality target requirements are met under the designed hydrological conditions [10]. WEC reflects the balanced relationship, feedback mechanism, and self-regulating ability of water resource protection and social and economic development systems [11]. WEC is affected by various processes and factors, such as water environment characteristics, pollutant characteristics, pollutant emission pathways, and emission space. These include the types and concentrations of pollutants from different sources and their spatial distributions [12]; changes in hydrological processes [13]; factors controlling the degradation, transformation, mixing, and migration of pollutants [14; 15]; and the coupling of these processes and factors. Based on these physical and chemical processes, various models have been studied to calculate WEC. Based on a one-dimensional water quality model, [PERSON] et al. [16] proposed a river WEC calculation method combined with the matrix calculation algorithm, and they applied this method to analyze the temporal and spatial variation patterns of WEC. [PERSON] et al. [17] proposed the combination of a one-dimensional water quality model and a uniform mixing formula to calculate WEC, and they applied it to Yongkang city. For rivers with insufficient data and low calculation accuracy, scholars [18; 19; 20] have studied some simple WEC calculation models. For basins with abundant data and high calculation accuracy, scholars [21; 22; 23] have studied the WEC calculation model based on hydrological processes. [PERSON] and [PERSON] [24] proposed a system for calculating river WEC by simulating the changes in river hydrology, water quality, and hydraulic conditions combined with calculation models. The system integrates relevant calculation models, various influencing parameters, and databases, which comprehensively considers the influence of river hydrology, water quality, and hydraulic conditions on WEC. [PERSON] et al.