population forecasting
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This document discusses various methods for population forecasting to aid in designing water supply and sanitation systems. The key methods discussed are: 1) Arithmetical Increase Method - Assumes population increases at a constant rate over time. 2) Geometrical Increase Method - Assumes the percentage increase in population remains constant from decade to decade. 3) Master Plan Method - Uses zoning plans from a city's master plan to determine population densities and forecast water/wastewater needs. 4) Logistic Curve Method - Models population growth that follows a typical S-shaped curve under normal birth/death/migration conditions without disruptions. The appropriate forecasting method depends on the city size, growth stage,
population forecasting
- 2. Population forecasting Design of water supply and sanitation scheme is based on the projected population of a particular city, estimated for the design period.Any underestimated value will make system inadequate for the purpose intended; similarly overestimated value will make it costly. Changes in the population of the city over the years occur, and the system should be designed taking into account of the population at the end of the design period. Factors affecting changes in population are: increase due to births decrease due to deaths increase/ decrease due to migration increase due to annexation
- 3. Population forecasting Methods ARITHMETICAL INCREASE METHOD GEOMETRICAL INCREASE METHOD (OR GEOMETRICAL PROGRESSION METHOD INCREMENTAL INCREASE METHOD GRAPHICAL METHOD COMPARATIVE GRAPHICAL METHOD MASTER PLAN METHOD LOGISTIC CURVE METHOD
- 4. ARITHMETICAL INCREASE METHOD • This method is suitable for large and old city with considerable development. • If it is used for small, average or comparatively new cities, it will give lower population estimate than actual value. • This method is based on assumption that the population increases at constant rate. Or rate of change of population is constant.
- 5. • Hence, dP/dt = x i.e., rate of change of population with respect to time is constant. • Therefore, Population after nth decade will be Pn= Po + n.x • Where, Pn is the population after ‘n’ decades and ‘P’ is present population
- 6. GEOMETRICAL INCREASE METHOD (OR GEOMETRICAL PROGRESSION METHOD n for ig= no of % • In this method the percentage increase in population from decade to decade is assumed to remain constant. • Geometric mean increase is used to find out the future increment in population. • Since this method gives higher values and hence should be applied for a new industrial town at the beginning of development for only few decades.The population at the end of nth decade ‘Pn’ can be estimated as: • Pn= Po (1+ IG/100) 𝑛 • Where, IG = geometric mean (%) P = Present population N = no. of decades.
- 7. INCREMENTAL INCREASE METHOD • This method is modification of arithmetical increase method and it is suitable for an average size town under normal condition where the growth rate is found to be in increasing order. • In this method per decade growth rate is not assumed to be constant as in arithmetical increase method, but progressively increasing and decreasing depends the average incremental increase in past is positive or negative. • Hence, population after nth decade is Pn = Po+ n.X + {n (n+1)/2}.Y • Where, Pn = Population after nth decade X =Average increase Y = average Incremental increase
- 8. GRAPHICAL METHOD • In this method, the populations of last few decades are correctly plotted to a suitable scale on graph . • The population curve is smoothly extended for getting future population. This extension should be done carefully and it requires proper experience and judgment. • The best way of applying this method is to extend the curve by comparing with population curve of some other similar cities having the similar growth condition.
- 9. MASTER PLAN METHOD • The big and metropolitan cities are generally not developed in haphazard manner, but are planned and regulated by local bodies according to master plan. • The master plan is prepared for next 25 to 30 years for the city. • According to the master plan the city is divided into various zones such as residence, commerce and industry. • The population densities are fixed for various zones in the master plan. From this population density total water demand and wastewater generation for that zone can be worked out. By this method it is very easy to access precisely the design population.
- 10. LOGISTIC CURVE METHOD • This method is used when the growth rate of population due to births, deaths and migrations takes place under normal situation and it is not subjected to any extraordinary changes like epidemic, war, earth quake or any natural disaster, etc., • and the population follows the growth curve characteristics of living things within limited space and economic opportunity. • If the population of a city is plotted with respect to time, the curve so obtained under normal condition looks like S-shaped curve and is known as logistic curve