a) Availability of funds If funds are not available, one has to keep a smaller design period.

b) Interest rates If interest rate is small, a higher value of the design period may be economically possible.

**c) Rate of population growth If the population of the city is getting doubled with in a period of ten years & have a fast growth of industries, using huge amount of water there is no point to keep this design period too long keeps it short.

d) Quality of material Design period should not be longer than the life time period of the material used in water supply scheme.