Let us revisit some school geography. How does the earth get rain? The sun’s heat on the water bodies like oceans, seas, lakes, rivers, streams results in movement of water to the atmosphere which is described as Evaporation. This is evident by the fact that the water levels in a swimming pool will keep depleting if it is not refilled on a continuous basis. What is not apparent is the process of transpiration, in which plants absorb water from the ground and send it back to the atmosphere through its leaves.
These two processes account for the movement of water from the earth’s surface to the atmosphere. Now this gets condensed and results in precipitation or rain. The rain that falls on the earth’s surface results in runoff, infiltration and evaporation. Runoff is the water that flows off after hitting a surface – for instance the water that runs down from your roof when it rains. This, if not tapped could eventually find its way to a water body like a stream or river, and end up in the ocean and again get evaporated. Infiltration is the movement of water deeper into the soil surface. This can either go deeper resulting in recharge of the ground water reserves or it could flow below the surface as ground water flow and get discharged above soil somewhere downstream through streams, springs or just ground water seepage. This in effect completes the water cycle which can be very simply summarized by the following equation
Rain precipitation = Evaporation + Transpiration + Contribution to Ground water reserves + Runoff
A similar cycle exists at a much granular level in our cities. We get water either from the public utility or, for areas unconnected, from ground water sources. This water is consumed for different end uses and close to 90% of the water consumed (not including the gardening requirement) goes back to the utility sewer systems. The following graphic indicates a water balance at a home-city level.
We can observe that a significant amount of water that we use is sent back as waste water to the sewerage utility for treatment from where it is discharged to land/ water bodies.
The water balance is thus an effective tool to quantify our supply and demand. If required, one can drill down to finer detail to analyze the supply across different source points (rainwater, ground water, city utility), demand across categories (residential, nonresidential, high and low grade end use), waste water (generated, available for reuse, discharged to water bodies). This kind of analysis is greatly useful for charting the water management strategies at a much bigger scale.