Add environmental capacity calculation in ICM, user can easily find how much pollution the river can receive at each time step and each river’s total environment capacity during the whole simulatiton.
Water Environmental Capacity Calculation
Background
Water environmental capacity is an essential component of water environmental assessment and must be monitored and managed for economic, engineering, and human health reasons. Many efforts have been made to study methods for the calculation of water environmental capacity. This content reviews available literature on water environmental capacity. The evolutionary history and application scenarios of major water quality models, and water environmental capacity calculation formulas are summarized.

Water environmental capacity calculation formulas
General Equations Up to Standard
The basic equation of water environmental capacity can be represented as follows :
Wij= Q·(Cs-Co)+K·Cs·V
Wij is the water environmental capacity of minimum computing unit in both time and space,
Q is the flow discharge,
Cs is the water quality standard,
C0 is the natural background concentration of the pollutants in the water,
K is the degradation coefficient of the pollutants.

The equation of water environmental capacity is applied to river network region as follows:
W=∑(j=1)^n▒∑(i=1)^m▒a(ij ) × Wij

Wij is the water environmental capacity of minimum computing unit, especially in calculating the minimum space unit of river reach (that is , the space between two breaks); the default minimum time unit is day.
aij is non-uniformity coefficient, aij∈(0,1] ; If wider the river reach is , aij will be smaller.

Equations of Reciprocating Flow Area
According to the boundary events of hydrology, we can calculate the minimum time and space unit value of water environmental capacity using mathematical model of river network towards the research area. Summarizing the values of each control unit according to the following formulas.
Using the two direction flow(double-flow model) formulas to calculate as below:
W=A/(A+B)W+ + A/(A+B)W-
A: Positive flow calculating relative days
B:Negative flow calculating relative days
W+ : Positive flow value of water environmental capacity
W- : Negative flow value of water environmental capacity

Positive flow formula:
W+ = Q01 (CS-C01 )+K1 V1 Cs
Negative flow formula:
W- = Q02 (CS-C02 )+K1 V1 Cs

In order to ensure the water environmental capacity calculation result that is consistent with the actual uneven phenomenon, we can use the non-uniformity coefficient of correction in the river network area during the process of the water environmental capacity calculation, which will be multiplied by the mixing of the water environmental capacity of water. It is concluded that water can meet certain control of water environment capacity under the condition of non-uniform coefficient values between 0 and 1. The wider the river reach , the more difficult it is for pollutants to be discharged into the water body to achieve uniform mixing, and the smaller the non-uniformity coefficient.
The relationship between the width of river reach and aij ( non-uniformity coefficient ) as follows:
river width(m)） aij
0-50 0.8-1.0
50-100 0.6-0.8
100-150 0.4-0.6
150-200 0.1-0.4

References:
1. J.Yuan, B. Wei, J. Zhu, H. Zhang & G.Yu., SKLOE, Jiao Tong University, E3S Web of Conferences.,. 81. 01001. 10.1051/e3sconf/20198101001 (2019)
2. Zhang, Doctoral dissertation, Hefei Un iv. Technol., (2017)
3. Y. D u a n, G. Liu, J. Geosci. and Environ. Prote., 04, 100 (2016)