The Water Cycle

The sun heats the ocean surface and water evaporates, turning liquid into vapor that rises into the atmosphere. As that vapor climbs and cools, it condenses into tiny droplets that form clouds. Once droplets grow heavy enough, they fall as precipitation: rain, snow, sleet. Where that water lands decides its next move. Some runs off along the surface into streams and rivers, eventually reaching the ocean again. Some soaks into the ground as infiltration, joining soil moisture or deeper groundwater. Plants pull groundwater back up and release it through transpiration, which joins evaporation in returning water to the air. Every drop is accounted for: nothing is created or destroyed, just relocated between ocean, air, land, and living things.

A question on this topic usually hands you a rainfall total split between runoff and infiltration and asks where the rest went. Since the whole system conserves water, whatever isn't accounted for by runoff and infiltration over a year went back to the atmosphere by evaporation and transpiration. That same conservation logic lets you reason about reservoirs: the atmosphere holds little water and cycles it through in days, while the deep ocean holds enormous volumes and only cycles over thousands of years. Paving over land breaks the natural balance between runoff and infiltration, which connects the water cycle directly to groundwater topics and to how cities manage flooding, since less infiltration means less recharge for wells and aquifers downstream.