The hydrologic cycle, commonly known as the water cycle, describes the continuous movement of water on, above, and below the Earth’s surface. This solar-powered system exchanges moisture between the oceans, the atmosphere, and the land, explaining how water constantly changes state—from liquid to gas to solid. While the total mass of water on Earth remains constant, its distribution among different reservoirs is always changing.
Key Processes Driving Water Movement
The mechanics of the hydrologic cycle involve several interconnected processes that move water between its various storage locations. The cycle begins with energy from the sun driving the process of evaporation, which transforms liquid water from the Earth’s surface into an invisible gas called water vapor. A vast majority of this atmospheric moisture, about 86% of global evaporation, originates from the world’s oceans.
Water also enters the atmosphere through transpiration, where plants take up liquid water through their roots and release it as vapor through small pores on their leaves. The combined effect of evaporation from the ground and water bodies and transpiration from plants is often referred to as evapotranspiration. As water vapor rises into the cooler upper atmosphere, it undergoes condensation, changing back into a liquid state to form tiny droplets or ice crystals that accumulate as clouds.
When these microscopic cloud droplets grow large enough through collision and coalescence, gravity causes them to fall back to the Earth’s surface as precipitation. This release of water can take the form of rain, snow, sleet, or hail. Once on the land, the water follows one of two primary paths: it can flow across the surface as runoff, eventually entering streams, rivers, and the ocean. Alternatively, precipitation can soak into the ground through a process called infiltration or percolation, becoming soil moisture or groundwater.
Water moving underground as subsurface flow can slowly travel to lower elevations, sometimes discharging into surface water bodies or being drawn up by plants.
Natural Water Storage Locations
Water resides temporarily in various natural storage locations, often called reservoirs. The oceans and seas represent the largest reservoir by far, holding roughly 97% of all water on the planet, though this water is saline. Due to their immense volume, water molecules have a long residence time in the ocean, averaging around 3,100 years before cycling out.
The planet’s ice caps, glaciers, and permanent snowpack constitute the second-largest store of water, representing the largest reservoir of fresh water. Water stored in the massive ice sheets of Antarctica and Greenland can remain frozen for hundreds of thousands of years. Below the surface, groundwater is stored in aquifers and is the largest reservoir of usable fresh water.
Groundwater residence times vary significantly, ranging from less than a year for shallow stores to tens of thousands of years for deep or “fossil” water. Surface water bodies like rivers and lakes hold a much smaller portion of the total water, with rivers having a rapid replacement rate, renewing their water on average every 16 days. The atmosphere also acts as a temporary reservoir, holding water vapor and clouds that are completely replaced very quickly, on average about every eight days.
The Cycle’s Role in Earth Systems
Beyond simply moving water, the hydrologic cycle is deeply integrated with and supports several of Earth’s major systems. The cycle plays a significant part in climate regulation by transferring heat energy around the globe. When water evaporates, it absorbs energy, known as latent heat, which cools the surface where the evaporation occurred.
This absorbed heat is then released into the atmosphere when the water vapor condenses to form clouds, driving much of the atmospheric circulation and transferring warmth from the tropics toward the poles. Water vapor itself is a potent greenhouse gas, meaning the volume of moisture in the atmosphere directly influences the Earth’s energy budget.
The cycle supports all terrestrial and aquatic ecosystems. It provides the fresh water required for the survival of nearly all land-based life, delivering moisture through precipitation and maintaining soil water content. The movement of surface runoff and groundwater sustains rivers, lakes, and wetlands, which are home to diverse biological communities.
The cycle profoundly shapes the Earth’s physical geography over geological time. Surface water flow, in the form of runoff and rivers, is the primary agent of erosion, weathering, and deposition. This action carves out valleys, transports sediments, and sculpts the continents, constantly altering the landscape.