31-03-2012, 01:21 PM
HYDROLOGY
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The word hydrology means science of water which deals with the spatial and temporal
characteristics of the earth’s water in all its aspects such as occurrence, circulation, distribution,
physical and chemical properties, and impact on environment and living things. Engineering
hydrology deals with all these aspects which are pertinent to planning, design, and operation
of hydraulic engineering projects for the control and use of the available water.
. HYDROLOGIC CYCLE
The total water of earth, excluding deep ground water, is in constant circulation from the
earth (including oceans) to atmosphere and back to the earth and oceans. This cycle of water
amongst earth, oceans, and atmospheric systems is known as hydrologic cycle.
Figure 2.1 is an enormously simplified sketch of the hydrologic cycle for which sun is the
source of energy. The hydrologic cycle (Fig. 2.1) can be visualized to begin with the evaporation
(due to solar heat) of water from the oceans, streams and lakes of the earth into the earth’s
atmosphere.
Hydrologic cycle
The description of the hydrologic cycle should not lead one to conclude that there is a
continuous mechanism through which water moves steadily at a constant rate. The movement
of water through the cycle is evidently variable, both in time and space although the total
water resources of the earth remains invariant since the formation of the earth system. Further,
the hydrologic cycle is a very complex phenomenon that has been taking place since the earth
formed. It should also be noted that the hydrologic cycle is a continuous recirculating cycle
with neither a beginning nor an end.
PRECIPITATION
The atmospheric air always contains moisture. Evaporation from the oceans is the major source
(about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes
only about 10% of the atmospheric moisture for precipitation. The atmosphere contains the
moisture even on days of bright sun-shine. However, for the occurrence of precipitation, some
mechanism is required to cool the atmospheric air sufficiently to bring it to (or near) saturation.