The water cycle

The water cycle, or hydrological cycle, is the continuous movement of water between oceans, atmosphere and land. The sun evaporates water from seas, lakes and soil, and plants release it by transpiration. Vapour rises, cools and condenses into clouds, then falls as precipitation: rain, snow or hail. On land some water is intercepted by vegetation, some soaks into the ground (infiltration), some is stored in depressions, and the rest runs off over the surface to streams. Infiltrated water recharges groundwater, which slowly feeds rivers as base flow, and the cycle continues back to the sea.

Hydrology quantifies these flows for engineering. Over a catchment and a chosen period, the water budget says that precipitation equals runoff plus evapotranspiration plus the change in storage (soil moisture, groundwater, reservoirs, snow). This simple balance underlies reservoir planning, irrigation scheduling, groundwater management and flood studies.

India's water cycle is dominated by the monsoon: most rain falls in about four months, unevenly in space and between years. That is why storage reservoirs, groundwater recharge and careful flood management are so central to Indian water engineering. Urbanisation changes the cycle locally: paved surfaces cut infiltration and raise runoff peaks, causing urban flooding and falling groundwater. Climate change is altering rainfall intensity and timing, which engineers now consider in design.

For engineers the cycle is also a reminder of scale. A city drawing water from a reservoir, discharging treated sewage downstream and pumping groundwater is part of the same loop, so upstream decisions on land use, abstraction and pollution show up downstream in quantity and quality.