THE HYDROLOGICAL CYCLE

The hydrological cycle consists of the global processes affecting the distribution and movement of water.

	More water evaporates from the oceans than is returned via precipitation, whereas in land more water is received via precipitation than is lost by evaporation.
	The amount of water in the atmosphere is small, its retention time is low and its cycle on average 9 days.
	On land water is absorbed by soil, stored as groundwater and moves by gravity to stream channels and lake depressions. Retention times within groundwater depends on:

• soils and rocks composition

• slope gradients

• vegetation cover

• climate

• retention time in lakes is short (6-7years)

Changes in water storage and retention in lakes results from alteration between input rates from all sources and rates of water losses.

Water income results from :

• Precipitation directly on the lake surface

• Water from surface influents of the drainage basin

• Groundwater seepage below the surface of the lake

Losses of water from lakes:

• Flow from an outlet in the most common drainage lakes or by seepage through the basin walls into groundwater.

• Direct evaporation from lake surface.

• Evapo-transpiration from emergent and floating-leafed aquatic plants.

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Copyright Schmid & Schmid. Last revised:  April 2005

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