The Hydrologic (Water) Cycle

The cycle runs on two forces: energy from the sun, which evaporates water and lifts it into the air, and gravity, which pulls it back down as precipitation and pulls it downhill or underground afterward. Follow one molecule: sunlight warms the ocean surface and it evaporates into water vapor, rises, cools, and condenses into clouds. Eventually it falls as rain. From there it can take several paths. It might soak into soil as infiltration and keep percolating down into an aquifer, where it sits in storage, sometimes for a very long time. It might get pulled up through plant roots and released back to the atmosphere as transpiration. Or it might run along the surface as runoff, feeding streams that carry it back to the ocean, closing the loop.

Rain falling on a forest gets split three ways at once: some evaporates straight back off leaves and puddles, some gets pulled through plants and transpired, and some infiltrates or runs off toward streams. How that split happens depends on soil, vegetation, and slope, and it matters because it determines how much water reaches rivers and aquifers versus returning quickly to the air.

This is why the ocean's level stays steady despite constant river input: evaporation off the ocean roughly matches what rivers and rain deliver, so water cycles rather than accumulating. Groundwater, though, moves through this system far slower than surface water. Infiltration into deep aquifers can take centuries, so even though it's technically part of the cycle, pumping it out faster than it recharges makes it nonrenewable on any human timescale. This sets up the next question: what happens when we pave over the land that used to let water infiltrate at all.