| In order to better understand the characteristics of our environment, we study the movement of water. First, it is important to grasp the fundamental concepts and terminology. The hydrologic cycle is the movement of water between mediums, and the cycle has no beginning and no end. The distribution of the water supply in the world is 97.2% saline water in the oceans, 2.8% in land areas, 2.14% in ice caps and glaciers, 0.61% in ground water to a depth of 13,000 feet, 0.005% in soil moisture, 0.009% in fresh-water lakes, 0.0001% in rivers, 0.008% in saline lakes, and 0.001% in the atmosphere (Fetter, 1994). Within and between these mediums, water is always in constant movement.
In this website we focus on the local hydrologic cycle for a watershed. The important factors are precipitation, evapotranspiration, overland flow, infiltration, groundwater flow and stream flow, along with special manifestations of each referred to, respectively, as baseflow (groundwater flow into streams), runoff (different authors use different definitions) and subsea flow (deep groundwater flow directly into the adjacent ocean or estuaries). The following notation will be used to represent the associated watershed components:
(Image by J.F. Hermance; 12/25/00)
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Components of Local Water Cycle
P = Precipitation
Qswi = Surface water in (e.g. stream flow into the watershed, and overland flow that enters directly)
Gin = Groundwater flow in
Qswo = Surface water out (e.g. stream flow, and overland flow that exits independently of streams)
ET = Evapotranspiration
Gout = Groundwater flow out
DS/Dt = Change in the volume of water in storage (S) per unit time
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Accordingly, the flux into a system can (usually) be represented by
Flux in = P + Qswi + Gin
The flux out of a system can be represented by
Flux out = Qswo + ET + Gout
So that the hydrologic cycle can be represented by the following equation that balances the various fluxes of water through a system:
P + Qswi + Gin - (Qswo + ET + Gout) = DS/Dt
where the last term (DS/Dt) allows for the possibility that there may be an instantaneous difference between the water entering and leaving a system, with a consequent increase or decrease in the total volume of water contained therein.
Definitions
(For convenience, the following definitions represent closed-systems, for which there is no surface water or goundwater input flux.)
Precipitation: Water that falls to the earth's surface as rain, snow or condensed fog (reference 1).
Infiltration: The process by which water passes through the interface between the atmosphere and the earth's subsurface (1).
Evapotranspiration: Evaporation is the process as water changes from liquid to vapor, and transpiration is the process when plants give off water vapor through their leaves.
Groundwater flow: The movement of water through the earth's subsurface. Usually refers to the flow of water in the saturated zone. However, in some situations, any water that infiltrates the earth's surface is considered groundwater. In such a case, the total input flux might be represented by the precipitation minus the overland flow (1).
Overland flow: The flow of water over a land surface due to direct precipitation.
Baseflow: The groundwater that seeps into and supplies water to a stream through its base (1).
Runoff: Used by some authors to refer to the total amount of water flowing in a stream, and thus delivered to the outflow zone of a watershed. In this context, runoff is assumed to consist of both overland flow and baseflow. Other authors use the term to denote the total flux of water leaving (or "running off") a watershed or a system (such as a state, country, continent, etc.). In the latter context, runoff might consist of a significant groundwater component. (1,2)
Subsea outflow: The deep groundwater flux that directly discharges to a distant water body such as the ocean, without first recharging a stream or river.
Animated Descriptions
Below is an animated visual description of each of the components defined above for a local hydrologic cycle. Please click on a term, and an animated image will appear below.
* Footnote: With editorial changes by J. F. Hermance.
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