Energy input and the four stages
The water cycle is driven by solar energy and gravity working in concert. Evaporation removes water from oceans, lakes, and soil using heat from the sun. Transpiration releases moisture from plants as part of their metabolism. Combined into evapotranspiration, these processes lift water vapor into the atmosphere where it is transported by winds across entire continents.
Condensation occurs when air cools enough that vapor becomes liquid, forming clouds and visible moisture. Precipitation returns water to the surface as rain, snow, or other forms depending on temperature. Runoff carries surface water toward rivers and oceans, while infiltration allows it to percolate into soil and groundwater.
Time scales and regional variations
Water molecules complete a cycle at vastly different rates depending on where they sit. A molecule in the deep ocean may cycle only once every thousand years, while water in the atmosphere recycles in days. This means tropical regions with rapid evaporation and rainfall can complete ten cycles annually, while polar ice stores water in the same form for millennia.
Human activities now alter cycling rates through dam construction, groundwater extraction, and land-use changes. Deforestation reduces transpiration, urbanization increases runoff, and irrigation accelerates cycling in agricultural regions, creating local effects that ripple through regional climate.