The Hydrologic Cycle

The amount of water on Earth today is the same as it was billions of years ago: no more and no less. Water is continually recycled through the water cycle (or hydrologic cycle), a system that moves rainfall from the atmosphere to land, through surface and groundwater systems, to the ocean, and back into the atmosphere. Water changes its form throughout this cycle between solid, liquid, and gas—and it moves over the Earth’s surface, underground, or through the atmosphere.

The water cycle is constant, with no beginning or end, but it is made up of basic components: evaporation (and transpiration), condensation, precipitation, infiltration, groundwater recharge, and runoff.

Click to View the Animated Diagram of a Watercycle

Heat from the sun causes water to evaporate from the surface of creeks, streams, rivers, lakes, and oceans and to transpire from trees, plants, and the ground. Water vapor rises into the atmosphere, where cooler temperatures causes it to condense into clouds. Air currents move clouds around the planet; when conditions are right, cloud particles collide, grow, and fall back to Earth as precipitation. Most of the precipitation falls into the ocean, which covers most of the planet’s surface. Some of the precipitation that hits the ground is quickly absorbed by the soil, some is used by plants, some filters into the ground and recharges groundwater supplies. The rest flows over land surfaces as runoff, and eventually into other parts of the system or surface waters (the creeks, rivers, lakes, and oceans)… When precipitation falls as snow, it may remain in solid form in ice caps or glaciers, but most eventually melts and flows over land.

The hydrologic cycle is a dynamic system of interdependent parts in constant movement. Altering one part of the cycle impacts other parts, because the overall water balance must be maintained. Removing trees and paving land surfaces, for example, reduces the amount of infiltration and evapotranspiration, and increases the amount of runoff.