Study on Calculation Method of Target Ecological Water Amount in Shortage Areas of Mintuo River Basin

Ecological water replenishment is the part of ecological flow deducting the remaining local existing water that needs to be recharged by other water sources. It has become the basic basis for water resources development and utilization and water affairs coordination. Ecological flow is a complex concept with numerous calculation methods and obvious regional differences. In view of the characteristics of the Mintuo River Basin, especially the problem that the extensive economic development exceeds the environmental carrying capacity of the river, considering the lack of natural rainfall, this paper recommends a one-dimensional water quality simulation method that maintains environmental capacity. Taking Jingyan County as a representative, a case study was carried out. The river was divided into several sections according to the water function zoning, and the water quality of each section was simulated to determine the section and the total river water replenishment, and through compared with the traditional calculation method of water conservancy planning, the rationality and benefit of the period are analyzed.


Introduction
The focus of the calculation of ecological water replenishment in rivers is to calculate the ecological flow. Ecological flow is an important indicator to ensure the ecological functions of rivers and lakes, and is an important basis for coordinating life, production and ecological water use, and optimizing the allocation of water resources. [1] It is related to regional water security and the healthy development of economic society. Defining a reasonable ecological flow is not only a requirement for current water resources management and control, but also a basis for supporting the rational development of the economic society of the area. Ecological flow is a complicated concept. At present, there is no clear and unified definition at home and abroad. Due to different research focuses and goals, there are still a lot of different appellations. For example, the European Union's "Water Framework Directive" is called Ecological Flow, the International Conference on Environmental Flows "Bruceban Declaration", and The Nature Conservancy is called Environmental flow is called river ecological water consumption, ecological base flow, basic ecological environmental water demand, and water ecological base flow in different fields such as environmental protection, water conservancy, and energy in China. In recent years, after a lot of discussion, integration and practice, the domestic concept of ecological flow has gradually reached a consensus. It is generally believed that the connotation of ecological flow considers the maintenance of the ecological function of the river and the protection of the healthy economic and social development. It can be divided into basic ecological flow and target ecological flow. Basic ecological flow refers to the minimum amount of water and process that needs to be retained in rivers, lakes and marshes to maintain the structure and function of rivers, lakes, swamps, and ecosystems; the target ecological flow is the normal use of rivers, lakes, and marshes Eco-environmental goals need to retain the flow in the river, which basically forms a consensus.
However, the calculation of ecological flow is still controversial. At present, there are more than 200 kinds of calculation methods for ecological flow at home and abroad [3], mainly hydrology method, hydraulic method, habitat simulation method and overall analysis method. The more typical hydrological methods are Tannant method, Q90 method, flow duration curve method, hydraulic method is typically R2-Cross method, wet cycle method, etc., habitat simulation method is typically water quality simulation method, IFIM method, etc., analysis methods include BBM method, DRIFT method, etc.
However, there are significant differences between regions in China. The level of economic and social development, hydrogeographical types, water resources endowments, and ecological function positioning are different. How to choose a calculation method is an extremely difficult problem, and statistical calculation methods are not suitable. If a unified ecological flow calculation method is adopted, it will inevitably cause the water ecological environment in some more sensitive areas to continue to deteriorate. For this reason, some scholars have put forward suggestions on ecological flow by zoning and classification. For example, Hai River 4 and Huai River 5 have proposed calculation suggestions and safeguard measures for ecological flow in their respective basins.
The Mintuo River Basin studied in this article is located in the central part of Sichuan Province. It has a special location, a large population, a developed industry, a concentrated population of cultivated land, and a prominent regional and seasonal water shortage. According to the statistics of the five major water systems in Sichuan, the water quality compliance rate of the main stream of the Yangtze River (Sichuan section), the main stream of the Jinsha River, and the main stream of the Jialing River are all 100%, while the water quality compliance rate of the main streams of the Min River and Tuo River are only 46.2% and 13.3%, respectively. The focus of water-saving environmental improvement, but the improvement of its water quality is extremely difficult. Practice has shown that it is difficult to achieve the desired effect through some general measures such as local pollution interception and drainage of ecological base flow from reservoirs. Aiming at the above problems, this paper adopts an idea of improving the water body's pollution holding capacity, using a one-dimensional hydrodynamic model to simulate the water quality process and calculate the ecological water demand. Taking Jingyan County in the transition zone of the Mintuo River as an example, a typical case study was carried out.

Overview of the study area
Jingyan County is a county under the jurisdiction of Leshan City, Sichuan Province. It is located in the southwest of Sichuan Basin, with Renshou in the north, Rong County in the east, Qianwei and Wutong Bridge in the south, the central district of Leshan City in the west, and Qingshen in the northwest. It covers an area of 840km 2 , which occupies 6.6% of the Leshan city. Jingyan County is located in the central area of Leshan Town and the core economic zone of Leshan. It is an important channel for Leshan City to merge into Chengdu. It will become a first-level sub-central city of Leshan City, with good location advantages and development prospects.
Mangxi River is a first-level tributary on the left bank of the lower reaches of the Minjiang River. It runs through the entire Jingyan area from north to south and is the most important river in Jingyan County. The average precipitation depth in Jingyan County for many years is 963.8mm, and the county's per capita water resources are 1352m 3 , which is only 38% of the per capita water resources of Leshan City. The current water resources development utilization rate reaches 32%, which is close to the sustainable development utilization rate of water resources. The upper limit is a severe resource shortage area; at the same time, the water volume of the Mangxi River Basin is unevenly distributed during the year, and the runoff during the dry season (November to the next 4 years) accounts for only 13% of the entire year. The precipitation time is uneven Sex has further exacerbated the contradiction between water supply and demand. In addition, the long-term use of production and domestic water has squeezed the ecological water of the river, resulting in a serious shortage of water in the dry season of the river, deterioration of water quality, and a water shortage problem. At the same time, Jingyan County's poor water resources endowment conditions have become increasingly prominent in contradiction with the healthy development of the society and economy. The bearing pressure on resources and the environment is increasing. The local water resources conditions cannot support the healthy development of the local economy and society, and water must be transferred from outside the region to solve the problem of water shortage in Jingyan County.

Calculation unit
The Jingyan section of the Mangxi River involves 3 water function zones. The Mangxi River is divided into three sections, namely, Jingyan Reserve, Jingyan Development and Utilization Zone, and Jingyan Wutongqiao Reserve with the water function zone as the boundary; The nodes (involving Dafo Town, Jinfeng Town, Yancheng Town, Qianfo Town, Sanjiang Town, Mata Town and Wangcun Town) are divided into 7 river sections; finally, considering the main tributaries and important sewage outlets as nodes, the Mangxi River Divided into 19 river sections, as shown in Table 1.

Mangxi River Hydrological Data
Mangxi River is a primary tributary on the left bank of the lower reaches of the Minjiang River. It is the main river in Jingyan County and the upper reaches is called Yanxi River. Originated in Laoyashan, Dafo Town, Jingyan County, the mainstream spans, Jingyan County, Wutongqiao District, and merges into Minjiang Chahe Yongsijiang at Siwangguan, Zhugen Town, Wutongqiao District, Leshan City, with an elevation of 352.0m. The drainage area is 1238km 2 , the main stream is 95.0km in length, and the annual average runoff is 582 million m 3 ; in Jingyan County, the area is 663km 2 , the river length is 75.0km, and the annual average runoff is 300 million m 3 .
In the Mangxi River Basin, Jingyan Hydrological Station and Qianfo Hydrological Station have been set up in Yancheng Town and Qianfo Town of Jingyan County, and there are hydrological data of a short period of time. Xinglong (II) Hydrological Station is located on the Weiyuan River in the nearby basin. However, due to the short period of hydrological observation data at Jingyan Station, there is no other station to interpolate and extend, the quality is poor, the water level change is small, and its representativeness is also insufficient, and it is not suitable for calculation basis and can only be used for reference; Qianfo Hydrological Station has the flow data in 1963～ 1964 and the measured data have a short life span, which is difficult to use in engineering. The Xinglong (II) Hydrological Station is located on the Weiyuan River in the adjacent basin, and its climate and underlying surface conditions are similar to those of the Mangxi River Basin, which can be used as the basis for this hydrological analysis and calculation.
The hydrological analogy method was used to calculate the runoff of various stations along the Mangxi River from 1988 to 2017.

Point source pollution source
According to the survey results of the "One River  Table 3.

Non-point source pollution source
Non ② The emission quotas of various pollution sources for rural life, farmland runoff, aquaculture, and livestock and poultry breeding shall be calculated in accordance with the values given in the National Water Environment Technology Verification Guide. The coefficient of entering the river adopts related research results such as "Accounting and Source Analysis of Pollution Load at Cuntan Section of the Main Stream of the Yangtze River", using the following formula: Among them, r is the river entrance coefficient, p is the annual rainfall, and ̅ is the multi-year average rainfall in mm. Rainfall uses data from Jingyan Rainfall Station. After calculation, the non-point source inflow coefficient is 0.15. How to measure the non-point source is shown in Table 4.

Governing equation
In this paper, the water balance method based on the onedimensional river water quality model is adopted. In order to simplify the calculation, two control equations are used in each calculation area in this paper. The complete mixed model is used at the starting point of each calculation partition: In the formula, Ci is the concentration of pollutants in the initial section of the calculation zone, in mg/l, and Cm and Qm are the concentration and flow of pollutants flowing into the unit from the upstream computing unit, and the units are mg/l and m 3 /s respectively, the same below, Cp and Qp are the concentration and flow of pollutants flowing into the unit by the tributaries, Cb and Qb are the concentration and flow of pollutants flowing into the unit from point sources, and W is the flow into the river from non-point sources, in g/s. Cin and Qin are the pollutant concentration and flow of the external diversion water into this unit. In this paper, it is assumed that the Mangxi River draws water from the neighboring Minjiang River. Cin is the water quality of the Minjiang River in 2030, COD is 10mg/L, ammonia nitrogen is 0.5mg/L, and Qin is To meet the trial calculation value of Ci not exceeding the standard, in our calculation, the Ci is limited, COD is 20mg/L, and ammonia nitrogen is 1mg/L.
After the pollutants are fully mixed in the initial section, the pollutants are degraded when flowing along the river. A one-dimensional degradation model is adopted: In the formula, C i+1 is the final section concentration of the calculation unit, k is the degradation coefficient, unit is d, refer to the relevant calculation results of Jingyan County, and take 0.1; t is the flow time of the pollutant in the calculation unit, unit is s, taking t= L/V in calculation, L is the length of the calculation unit, and V is the average flow rate.

Calculation steps
Two indicators of COD and ammonia nitrogen were selected, in accordance with the water quality requirements of the water function zone, so that the water quality of the Mangxi River after replenishment reached the surface water level III requirements, and the threshold values of COD and ammonia nitrogen were 20mg/L and 1.0mg/L, respectively. The tributaries are generalized as point source sewage outlets, and the water quality is considered to meet the standard class III. At the same time, it is considered that the river sediment has been treated and no new pollutants are released into the water body.
The calculation steps are: starting from the most upstream sewage outlet of the first river section, using a one-dimensional water quality simulation model to calculate each sewage outlet in turn until the end of the river section; determine whether the water quality of the downstream end of the river section meets the surface water quality requirements of Class III. If it meets the requirements, calculating the next river section; if it does not meet the requirements, trying to calculate the water supply of the Minjiang River until the water quality of all sections meets the surface water quality requirements of Class III.

Simulation results of typical years
In this paper, the monthly ecological flow from 1988 to 2017 was calculated, and the ecological water replenishment after deducting the background flow of the river was calculated. Table 5 lists the calculation results of 4 typical years, and selected 25%, 50%, 75%, and 95% of the incoming water years to represent the four types of years of high, flat, dry, and extremely dry. There is a big difference between the annual and monthly ecological water requirements. The amount of ecological water replenishment in extremely low water years can reach 2.4 times that of wet years, and the amount of ecological water replenishment in dry months can reach more than 6 times that of wet months. Figure 6 lists the changes in the ecological water demand along the river under the average conditions for many years. The ecological water supply has increased sharply in the outlet section of the research town, which is closely related to the pollution caused by urban development. Table 5 he average annual water supply for each section purification capacity of the river, and establishes a set of calculation methods to be applied to the Mangxi River in Jingyan County to calculate the multi-year average ecological demand of the Mangxi River. The water volume is 26.64 million m 3 , which is about 9% of the natural runoff of the Mangxi River, which is higher than the calculation result of the traditional tenant method and reflects the pressure on the ecological environment of the river channel from water outside the river.
Through theoretical calculations, the ecological water demand proposed in this paper can ensure that the Mangxi River reaches Class III water quality in 2030, while supporting regional economic development. The calculation method is more suitable for the characteristics of the Mintuo River Basin. However, there are no practical cases of this method at present, but it has been suggested to the local government that with the help of the Minmang connection project to be implemented in Jingyan County, the effect of this method can be further verified in the future and provide reference cases for similar areas.