The hydrologic cycle, also known as the water cycle is a way of describing the material flow of water throughout the Earth. This series of steps describes how water moves across the Earth and changes form. These specific steps result in the circulation of water between oceans, the atmosphere, and the land. The water cycle involves natural phenomena that include precipitation such as rain and snow, drainage from rivers, and the return of water to the atmosphere through evaporation and transpiration. This natural cycling of water through a variety of different locations on the Earth means that through the history of the Earth, the amount of water has been relatively constant. Little has been added or lost over the years, and this water has been continuously in motion.
The cycle has two primary components: storage and movement. Storage is where water in the system resides or "rests" as it moves from one water reservoir to another. For example, water enters a lake through some movement such as precipitation. After it enters the lake, it stays there for some period of time in its storage phase. Eventually, the water moves back out through some movement, such as evaporation into the atmosphere, discharge into a river, or migration into the subsurface groundwater system. This continuous movement of water among the various storage reservoirs is termed the hydrologic cycle.
It is important to note that water requires energy to change states - from solid to liquid is called the enthalpy of fusion, and from liquid to gas is called the enthalpy of vaporization. The evaporation of water from the oceans and evapotranspiration from continents is a particularly important component of the hydrologic cycle that requires a change of state and input of energy. Since nature follows the law of conservation of energy, the energy to fuel this cycle must come from somewhere. In the hydrologic cycle, this energy comes from the Sun.
The hydrologic cycle largely depends on the solar energy to the Earth, because hotter temperatures result in more evaporation, precipitation and higher humidity. With the concerning emissions of greenhouse gases, global warming will likely play a large effect on this important cycle. Since a warming Earth means a moister atmosphere, the hydrologic cycle will become more intense; precipitation and river flows are expected to increase by 10% to 40% in certain tropical locations, while decreasing up to 30% in dry locations. This means that floods will become more likely in areas with more precipitation, while droughts will affect the drier places. Other extreme weather events are also very likely to occur, such as hurricanes and typhoons.
The storage of water in the hydrologic cycle is vastly important on the Earth. Water can be stored in three main places: the atmosphere, on the surface of the Earth, and underground. Specifically these water storage areas are known as reservoirs and include oceans, glacier ice, groundwater, lakes, soil moisture, living organisms, the atmosphere, and rivers. Collectively, all water storage areas make up the hydrosphere. Most water on earth is found in oceans and seas, then in glaciers and groundwater. ~97% of the world's water is stored in the oceans as saltwater. Because the overwhelming majority of water is stored here, this can be seen as the start and end point of the cycle. Water that starts here evaporates up into the atmosphere, where the majority of it falls back into the ocean as rain, while a much smaller amount falls onto land.
Water moves from reservoir to reservoir through a variety of different transportation mechanisms, but this water can stay in storage for varying lengths of time. The movement of water from place to place can be fairly erratic, but there are trends for how long certain types of reservoirs maintain their water for. The average length of time that water stays in any reservoir in the hydrologic cycle is known as the residence time. Water the water in the atmosphere is renewed every 15 days, while soil moisture lasts a couple of months. Lakes replenish their water every 50 to 100 years, while groundwater can last anywhere from 100 to 10 000 years. Finally, ice caps hold water the longest with residence times of up to 200 000 years.
The type of storage that occurs on the land surface and under the ground largely depend on the geologic features related to the types of soil and the types of rocks present at the storage locations.
There are numerous different ways that water moves across the planet in the water cycle. All of these different types of movement are important in maintaining water levels in certain locations around the globe.
Here, water is heated by solar radiation from the Sun. Water molecules on the surface of oceans, rivers, lakes, ponds, and other bodies of water become energized by this. When sufficiently energized, the water molecules are able to break free from the forces binding them together and they evaporate, rising into the atmosphere as water vapour. 97.5% of the earth's water is contained in the ocean, so a large amount of water enters the atmosphere through evaporation at the oceans surface.
This is the process of water vapour being emitted by plant leaves.
In practical terms, when water vapour leaves a vegetated surface it is often difficult to distinguish between evaporation from the soil surface and transpiration from plants. Therefore, these combined processes are commonly referred to as evapotranspiration.
When water moves directly from a solid to a gaseous state without ever entering the liquid state. This process allows water in snow or glaciers to enter the atmosphere directly.
When water vapour rises, it cools slightly and condenses. Generally, the water condenses on dust particles in the air and becomes liquid. Sometimes the water skips the liquid phase and turns directly into a solid - in the form of ice, hail, or snow. In the liquid form the particles collect and form clouds.
Water falls from the sky in numerous different forms of precipitation including rain, snow, and hail. This precipitation comes from clouds, which are free to move around the world and are pushed by air currents. This allows water to move around the globe easily.
Rain or snowmelt can move water over land and form small creeks or collect in ditches. Runoff is the visible flow of water in rivers or creeks as water, previously stored in a basin, drains.
When precipitation falls on the ground, some of it moves downwards into cracks, joints, and pores in the soil. The entry of water into the subsurface is termed infiltration. The process of percolation refers to the subsequent movement of water through subsurface soil pores until it reaches the water table. At this point it becomes groundwater. This is a slow process, which is why more water flows back to the ocean through surface runoff than groundwater discharge.
Groundwater is water that is held in cracks and pore spaces below ground. This water can be tapped by water supply wells or continue moving below the ground until it eventually returns to the surface. The process by which groundwater exits the ground is known as groundwater discharge. This groundwater can either discharge directly into oceans, or more commonly, it discharges to surface water (lakes and rivers) and then travels to the ocean as surface runoff.
For more information please see Groundwater as a part of the hydrologic cycle.