1 The Hydrologic Cycle

Water moves through a connected cycle

The hydrologic cycle, also called the water cycle, describes how water is stored and moves among the ocean, atmosphere, land, living things, and underground. It has no fixed starting point. A water molecule may remain in the ocean for a long time, pass through the atmosphere in days, or be stored in ice or groundwater for much longer.

The cycle is driven mainly by energy from the Sun and gravity. Together, these drivers help move water between stores and across and beneath Earth's surface.

Stores and flows

A store, or pool, is a place where water is held. The ocean is Earth's largest water store. The atmosphere holds much less water at any one time, but that water is constantly moving. Other major stores include glaciers and snowpack, lakes and rivers, soil moisture, groundwater, and water in plants and animals.

A flow, or flux, transfers water between stores. The stores differ greatly in size and in how quickly water moves through them.

Processes that transfer water

Water moves between stores through several processes:

• Evaporation: Liquid water becomes water vapor, mainly from oceans and other wet surfaces.

• Transpiration: Plants release water vapor from their leaves. Evaporation and transpiration together are called evapotranspiration.

• Condensation: Water vapor cools and becomes liquid droplets or ice crystals, forming clouds.

• Precipitation: Water falls from clouds as rain, snow, sleet, or hail.

• Infiltration and percolation: Water enters the soil and may move deeper underground, replenishing groundwater.

• Runoff and streamflow: Water flows over land and through streams and rivers toward lakes, wetlands, or the ocean.

• Freezing, melting, and sublimation: Water changes among liquid, solid, and vapor, allowing it to move into or out of snow and ice stores.

Water pathways and storage changes

After precipitation reaches land, water may evaporate, be taken up by plants, soak into the ground, remain temporarily in snow or lakes, or flow overland into streams. These routes can connect: groundwater may discharge into a stream, and stream water may eventually reach the ocean and evaporate. The route and timing depend on local conditions.

Water is conserved as it moves between stores. The water balance describes how a store changes according to its total water inputs and outputs:

Here, $\Delta S$ is the change in storage, $I$ is the water entering the store, and $O$ is the water leaving it. A lake's storage increases when precipitation, streamflow, and groundwater inflow together exceed evaporation, seepage, and outflow.

Solar energy and gravity drive movement

Sunlight warms Earth's surface and supplies energy for evaporation and plant transpiration. Evaporation takes up energy from the surface, which is carried with water vapor. When vapor condenses, that energy is released to the surrounding atmosphere. These energy transfers help shape atmospheric circulation and weather.

Gravity pulls precipitation toward the surface, drives runoff into streams and rivers, and moves water through soil and rock toward lower elevations. Solar energy helps lift water into the atmosphere; gravity helps return it to the surface and carry it across and beneath the land.

Climate and the water cycle

Climate influences the water cycle by shaping temperature, precipitation, humidity, winds, and the timing of snow and ice melt. Warm, sunny, and often windy conditions can increase evaporation, while humid air can slow it. Winds transport water vapor, so precipitation may fall far from where the water evaporated.

Climate differences also affect which stores dominate. Cold regions can hold much of their water as snow and ice, while dry regions may have low soil moisture and intermittent streamflow.

The water cycle also influences climate. Evaporation and condensation transfer energy; clouds affect how much sunlight reaches the surface and how much heat escapes to space. Changes in temperature can alter evaporation, atmospheric moisture, precipitation patterns, and the timing of snowmelt. Some places may experience longer dry intervals, while others may receive heavier precipitation. These effects vary by region; they do not mean every location becomes wetter or drier in the same way.