Sources of water

A water supply scheme starts with a reliable source that can provide enough water of acceptable quality through dry years and for the design population decades ahead. Sources are surface water, from rivers, lakes and reservoirs, and groundwater, from wells, tube wells and infiltration galleries. Rainwater harvesting, desalination and treated wastewater reuse are increasingly important supplements.

Rivers provide large quantities but their flow and quality vary with season, and they carry sediment, pathogens and pollution from upstream. Intakes must work at low flow and survive floods. Storage reservoirs even out seasonal variation; their yield is assessed from long flow records using mass curves or sequent-peak analysis. Lakes have more stable quality but can suffer algal blooms. Groundwater is usually clearer and cooler, free of most pathogens, and available close to demand, but its yield depends on aquifer properties and recharge. Over-pumping lowers water tables, raises pumping costs and can draw in saline or fluoride- and arsenic-bearing water.

Source selection weighs quantity, quality, reliability, distance and elevation (which decide pumping cost), treatment needs, environmental flows for downstream users and long-term sustainability. Demand is estimated from population forecasts (arithmetic, geometric, incremental increase or logistic methods) multiplied by per capita demand, with allowances for institutions, industry, fire fighting and losses.