For water supply scheme design of a town or a city, the suitable method of estimating future population by the end of the design period is  

Concept:

Various methods for population forecasting as suitable for that city, considering the growth pattern, are as follows:

1. Arithmetical increase method:

• In this method assumed that the population is increasing at a constant rate.
• This method is suitable for a large and old city with considerable development.

2. Geometrical increase method (or geometrical progression method):

• In this method, the percentage increase in population from decade to decade is assumed to remain constant. 
• This method gives higher values and hence should be applied for a young and rapidly increasing city, but only for a few decades.

3. Incremental increase method:

• This method is a modification of the arithmetical increase method and it is suitable for an average size town under the normal condition where the growth rate is found to be in increasing order.

4. Exponential curve method

• It is a suitable method of estimating the future population by the end of the design period of the water supply scheme for a town or a city.