Hydrologic Cycle

The movement of water between these reservoirs, primarily driven by solar energy influx at the Earth's surface, is known as the hydrologic cycle.

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Figure 6. Diagram showing the main components of the hydrologic cycle, including evaporation, transpiration, precipitation, runoff, infiltration, and groundwater runout.
Source: Michael Arthur and Demian Saffer

The hydrologic cycle is a conceptual model that describes the fluxes of water between the oceans, surface water bodies (lakes, rivers, and streams), groundwater in subsurface aquifers, the atmosphere, and the biosphere. One important aspect of the cycle is that no water is gained or lost: water moves between reservoirs but the total mass remains the same. Another way to say this is that the water that currently exists on Earth is the same water that has been here since the time the Earth formed. (Technically, there are small fluxes of water from the Earth's interior to the surface and atmosphere through volcanism and venting, and small influxes of water from comets and debris, but these are negligible in comparison to the mass of water in the primary reservoirs shown above.)

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1. There are five processes that control the movement of water between reservoirs in the hydrologic cycle. Looking at Figure 6 above, what do you think they are? Name as many as you can.

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The movement of water between reservoirs, or the "limbs" of the hydrologic cycle includes five primary processes:

• Evapo-transpiration: the movement of water from oceans or land to the atmosphere, through the combined processes of evaporation and transpiration. Evaporation and transpiration both involve a change in state, from liquid to vapor, which requires an input of energy. Evaporation is simply the change from liquid to vapor as a result of molecular motion, and is affected by temperature and ambient humidity. Transpiration is the movement of water to the atmosphere by plant respiration. In most terrestrial basins, transpiration is the dominant process by which water moves from the Earth's surface to the atmosphere, whereas over lakes and the oceans, evaporation dominates.
• Precipitation: the movement of water from the atmosphere to the land surface or oceans, in the form of rain, snow, sleet, ice pellets, etc... Precipitation involves a change in state from vapor to liquid, known as condensation. This change in state releases heat energy. After precipitation falls on the land surface, it may flow into surface water bodies (lakes or streams), or percolate through soils and rock into the groundwater system.
• Runoff: the movement of water from the land surface to the oceans in streams or rivers.
• Infiltration: the percolation of water from the land surface or from surface water bodies through soils and into the subsurface. Water that infiltrates becomes part of the groundwater system, and is also known as groundwater recharge.
• Groundwater outflow, also known as subsea outflow: the seepage of water from the groundwater system directly into the oceans. The flux of groundwater outflow is the least constrained component of the hydrologic cycle, and is often estimated by balancing the other fluxes in the cycle.

Because the changes in state that accompany evaporation and precipitation also take in and release energy, the movement of water through the hydrologic cycle is paralleled by redistribution of heat and energy.

• Formative Assessment 1: The Water Cycle