How Water Moves on Earth

The water cycle, also called the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of Earth. Powered by energy from the Sun and the force of gravity, water on Earth is constantly recycled and redistributed. About 97 percent of all water on Earth is salt water stored in oceans, while less than 3 percent is fresh water — and most of that fresh water is locked in glaciers and ice caps.

The cycle begins when solar radiation heats water in oceans, lakes, rivers, and puddles. This added energy causes liquid water to change into water vapor through evaporation. Plants also return water to the atmosphere through transpiration, in which water absorbed by roots is released through tiny openings in the leaves. Together, evaporation and transpiration are sometimes called evapotranspiration.

As warm, moist air rises, it expands and cools. When the air cools to its dew point, water vapor changes back into liquid water droplets through condensation. These droplets gather around tiny dust particles called condensation nuclei, forming clouds. Once cloud droplets become heavy enough, they fall to Earth as precipitation — rain, snow, sleet, or hail.

Precipitation that reaches the ground follows several paths. Water that flows over the surface into streams, rivers, and eventually the ocean is called runoff. Water that soaks into the soil through cracks and pore spaces is undergoing infiltration. Water that seeps deep into the ground becomes groundwater, filling spaces between soil and rock particles in an underground reservoir called an aquifer. The upper boundary of the saturated zone is called the water table.

On Long Island, more than three million people depend on groundwater stored in the Upper Glacial, Magothy, and Lloyd aquifers for their drinking water. Because Long Island has no major rivers or surface reservoirs, every drop of water used by Long Island residents comes from precipitation that infiltrated into the ground. Protecting these aquifers from contamination — and recharging them with clean precipitation — is critical to the region's water supply.

Earth's oceans cover about 71% of the planet's surface and supply the vast majority of water vapor that enters the atmosphere. The rate at which liquid water evaporates from the ocean depends mainly on temperature, wind speed, and humidity. Warmer water and warmer air can hold more energy, so evaporation rates are highest near the equator, where solar radiation is most intense, and lowest near the poles, where temperatures stay cold year-round.

Once water vapor enters the atmosphere, it begins to rise. As air rises, it expands and cools. When the air cools to its dew point temperature, water vapor begins to condense onto microscopic dust particles called condensation nuclei, forming the tiny droplets that make up clouds. Different cloud types form at different altitudes — cumulus clouds form low in the atmosphere as warm, moist air rises rapidly, while cirrus clouds form high in the atmosphere where temperatures are extremely cold.

When precipitation reaches Earth's surface, it can either flow over the ground as runoff or soak into the ground through infiltration. Several factors determine the balance between the two: the steepness of the slope, the permeability and saturation of the soil, the amount of vegetation, the intensity of the rainfall, and whether the ground is frozen. Adjust the variables below to investigate how each factor affects the percentage of water that runs off versus infiltrates.

Long Island’s Hidden Freshwater System

Long Island sits on a thick stack of sand, gravel, and clay that was deposited about 20,000 years ago when continental glaciers melted at the end of the last Ice Age. These glacial deposits are highly permeable, which means precipitation can soak quickly into the ground rather than running off into rivers — a fortunate property, since Long Island has no major freshwater rivers and no large surface reservoirs. Instead, the more than three million people living on Long Island rely entirely on groundwater stored beneath their feet for drinking, agriculture, and industry.

Below the surface, fresh water is held in three stacked layers called aquifers. The Upper Glacial aquifer lies closest to the surface and supplies many shallow private wells. Below it is the much larger Magothy aquifer, which provides the majority of public drinking water on Long Island. The deepest layer is the Lloyd aquifer, which is separated from the Magothy by a thick layer of nearly impermeable clay called the Raritan Clay. Because the Raritan Clay blocks the downward flow of water, the Lloyd aquifer recharges extremely slowly — once water is removed from the Lloyd, it can take centuries to be replaced.

Recharge of all three aquifers depends entirely on precipitation infiltrating through the surface soils. About half of all rain that falls on Long Island infiltrates into the ground rather than evaporating or running off. Once infiltrated, water moves slowly downward and laterally through the saturated zone of each aquifer. Pumping wells faster than precipitation can recharge them causes the water table to drop and creates two serious problems: along the coast, ocean water is drawn into the aquifers in a process called saltwater intrusion, and inland, contaminants from the surface (lawn chemicals, road runoff, leaking septic systems) are pulled deeper into the freshwater supply.