The Water Cycle



What is Hydrology?

Hydrology is the scientific study of the waters of the earth. Hydrology examines the properties of water as well as its planetary occurrence, distribution, and movement.


The Hydrological Cycle

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Components of the Water Cycle (Image: pmm.nasa.gov)

The hydrological cycle is often called the water cycle. It is the vertical and horizontal movement of water as either vapor, liquid, or solid between the earth’s surface, subsurface, atmosphere, and oceans.

When rain falls on the ground, some of it travels slowly down through the ground to the aquifer. As water makes its slow journey, it is filtered by soil, sand and gravel. This water is called groundwater.

Some of the rainwater runs over the land into lakes, rivers, and streams. This water is called surface water.


Water Budget Accounting

The amount of water in a particular location can be calculated in a general sense by using an accounting budget approach. The volume of water at any point in a hydrologic system can be viewed simply as the difference between the inflow and outflow of the system and the resulting change of storage. In other words, inflow minus outflow equals change in storage.

The Accounting Budget for Water
Inflow (I) – Outflow (O) = ± Change in storage (S)

Hydrologists also call this the continuity equation or the conservation of mass. Water is neither created nor destroyed, but water can move from place to place and take different forms (i.e. solid, liquid or vapor).


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