Wednesday, March 21, 2007

The Definition of Urban Hydrology - Part.1

Hydrology may be defined as the physical science which treats the waters of the Earth, their occurrence, circulation and distribution, their chemical and physical properties, and their reaction with the environment, including their relation to living things (UNESCO, 1979).

These words serve to emphasize two particular aspects of the subject: its interdisciplinary nature, which embraces physical, chemical and biological as well as applied sciences; and its concern with the spatial and temporal distribution and movement of water in all its forms.

The latter is implicit in the concept of the hydrological cycle, which illustrates the multifarious paths by which the water precipitated on to the land surface finds its way to the oceans, where evaporation provides the supply of moisture for the renewal of the process.

The hydrological cycle is commonly presented in pictorial form, of which
FIG. 1.1, adapted from Todd (1959), provides a typical example.

FIG. 1. 1. The hydrological cycle in pictorial form (source Todd, 1959).

The Hydrological Cycle

From the time the earth was formed, water has been endlessly circulating. This circulation is known as the hydrologic cycle. Groundwater is part of this continuous cycle as water evaporates, forms clouds, and returns to earth as precipitation.

Surface water is evaporated from the earth by the energy of the sun. The water vapor forms clouds in the sky. Depending on the temperature and weather conditions, the water vapor condenses and falls to the earth as different types of precipitation. Some precipitation runs from high areas to low areas on the earth's surface. This is known as surface runoff. Other precipitation seeps into the ground and is stored as groundwater.

Think of groundwater as water that fills the spaces between rocks and soil particles underground, in much the same way as water fills a sponge. Groundwater begins as precipitation and soaks into the ground where it is stored in underground geological water systems called aquifers. Sometimes groundwater feeds springs, lakes, and other surface waters or is drawn out of the ground by humans. The water then can evaporate, form clouds, and return to the earth to begin the cycle over again. be continued --> Part. 2

Posted by: JRP