INTRODUCTION - Hydrology: An Introduction - page 2

With this definition it is now also possible to delineate the practical scope of hydrologic
analysis in engineering and in other applied disciplines. It consists of the determination
of the amount and/or flow rate of water that will be found at a given location and at
a given time under natural conditions, without direct human control or intervention.
The latter specification, that no human control be involved, is necessary to distinguish
hydrology from the related discipline of hydraulics. Hydraulics is concerned with the
study of controlled fluid motion in well-defined and often in human-made environments.
For instance, problems involving pipe flow, irrigation water distribution or pumping of
groundwater are not hydrologic in nature, but are more properly assigned to the realm
of hydraulics.

1.2 THE HYDROLOGIC CYCLE

The water cycle, also called the hydrologic cycle, refers to the pathway of water in nature,
as it moves in its different phases through the atmosphere, down over and through the land,
to the ocean and back up to the atmosphere. When atmospheric water vapor condenses
and precipitates over land, initially it moistens the surface and some amount of it is
stored as interception , which later evaporates. As precipitation (and in a similar way
snowmelt ) continues, part of it may flow over the surface in the form of overland flow
or surface runoff , and part of it may enter into the soil as infiltration . This surface runoff
soon tends to collect locally, either in puddles or small ponds as depression storage ,or
in gullies or larger channels where it continues as streamflow , which ultimately ends up
in a larger water body, such as a lake or the ocean. Streamflow is normally described
by a hydrograph , that is the rate of flow at a gaging station as a function of time. The
infiltrated water may flow rapidly through the near-surface soil layers to exit into springs
or adjacent streams, or it may percolate more slowly through the profile to join the
groundwater , which sooner or later seeps out into the natural river system, lakes and
other open water bodies; part of the infiltrated water is retained in the soil profile by
capillarity and other factors, where it is available for uptake by the roots of vegetation.
Soil layers and other geologic formations, whose pores and interstices can transmit
water, are called aquifers . When an aquifer is in direct contact with the land surface, it is
referred to as unconfined . The locus of points in an unconfined aquifer, where the water
pressure is atmospheric, is called the water table . Although the water table is not a true
free surface separating a saturated zone from a dry zone, it is sometimes assumed to be the
upper boundary of the groundwater in an unconfined aquifer. The partly saturated zone
in an unconfined aquifer, between the water table and the ground surface, is sometimes
referred to as the vadose zone . In an unconfined aquifer, the term groundwater refers
usually to the water found below the water table; soil water or soil moisture refers to the
water above the water table. A water bearing geologic formation, that is separated from
the surface by an impermeable layer, is referred to as a confined aquifer . Streamflow is
fed both by surface runoff and by subsurface flow from riparian (i.e. located along the
banks) aquifers. The streamflow, resulting from groundwater outflow is often called base