The model shown by the graph is an exponential decay model.
Suppose that the y-axis represents the population of City A in thousands and the x-axis represents the time elapsed in months.
The graph gives that the population decreases every year by due to pollution, migration, or other factors.
The initial population of the City in the first month of the survey is 100,000 people. After 60 months, the population has decreased to approximately 20,000 people.
This model described above matches the graph.