Explain why population forecasting cannot provide an exact estimate of population sizes for designing and operating water supply and waste treatment.

Population is a dynamic thing and is affected by many internal and external factors. When we do population estimation, we mainly do it based on statistics. We note the past trends of population growth and try to predict the future size of the population at the same rate of growth. We can not take into account all the factors that affect the growth rates at different manner. When the population is growing, its characters are changing constantly and thus it follows different rates of growth at every moment. Final size of a population is the total summation of these different rates of growth acting upon different sizes of population.