Content o Water demand o Determining quantity of water o Types of water demand o Factors affecting water demand o Population forecasting method o Types of population forecasting methidWater Demand » The rate of water required for a particular town or a city to successfully carry out its day to day activities is known as water demand. » While designing the water supply scheme for a town or city, it is necessary to determine the total quantity of water required. As a matter it is a first duty of an engineer to determine water demand and then to find a probable source from where the demand can be met.Determining quantity of water The quantity of water required for a city can be tackled by two factors: 1. Rate of demand: The requirements of water for various uses are properly analyzed and ultimately, the rate of consumption per capita per day is worked out. 2. Population: The population to be served by the water supply scheme is estimated and estimate of future population is worked out with help of population forecast method.Types of water demand The types of water demand of a city or a town: » Domestic water demand » Industrial water demand » Institutional and commercial water demand » Demand for public uses » Fire demand » Compensate losses demandFactors affecting water demand Following are the main factors that affect water demand Size of the city Living standard of the people Climatic conditions Quality of water Industrial and commercial activities Pressure in the distribution system System of sanitation Cost of water System of supply . Metering and method of charging Cer aveweo 5Population forecasting method © Two types of population estimates are needed for the operation and the design of the water supply i. Short term estimates in the range 1-10 years ii. Long term estimates in the range 10-50 years or more. e The population is increased by births, decreased by deaths, increased by migration and increased by annexation.Types of population forecasting methods Arithmetical increase method - Geometrical increase method Incremental increase method Decreasing rate of growth method Simple graphical method Comparative graphical method Zoning method or master plan method a oe aerithmetical increase method This is the most simple method of population forecast and is based on the that the pepulanoe is increasing from decade to decade at a constant rate. © dp/dt=C Where, dp/dt is the rate of change of population. © The population at the end on ‘n’ decades is given b P,= P+ni Where, P,, = population after ‘n’ decades. P= Present population ee average increase per decade n= numberof decades Geometrical increase method e In this method it is assumed that the percentage increase in population from. decade to decade is constant. From the population data of previous three to four decades, the percentage increase on poouleton is found and its average is found. ° The population at the end on ‘n’ decades is given by: P,= P [1+(r/100)}" Where, P, = population after ‘n’ decades. P_ = Present population r= average incremental increase n = numberof decadescremental Increase method e The average increase in population is determined by the arithmetical _increase method and to this as added the average of the net incremental increase once for each future decade. © The population at the end on ‘n’ decades is given by: P,= P+ni+ [{n(n+1)r}/2] Where, P,, = population after ‘n’ decades. = Present population = average increase per decade = average incremental increase P i r n number of decades Decreasing rate of growth method e In this method, the average decrease in the percentage increase is worked out , and is then subtracted from the least percentage increase for each successive decades. Simple graphical method e In this method, a curve is plotted between the population pand time T, with the help of of census data of previous few decades. The curve is smoothly extended to forecast the future population.Graphical comparison method e In this method, the cities having conditions and characteristics similar to the city whose future population is to be determined are first of all selected. It is then assumed that the city under consideration will develop as the selected similar cities have developed in the past. Zoning method or master plan method e The city and town are divided into various zones such as commercial, industrial, residential etc. The future expansion of cities is strictly regulated by various bye-laws of corporation and other local bodies. © The population of a particular zone is fixed and according to that the water supply schemes are designed.