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Hydrology is the science of water which deals with the spatial and temporal characteristics of the earth’s water. In other words, hydrology is the study of water in all its phases, including the occurrence, movement, distribution, circulation, storage, exploration, development and management.
The atmospheric air always contains moisture. Under favourable conditions, this moisture gets condensed and water drops are formed. These water drops ultimately fall on the earth’s surface as precipitation in the form of rain, snow, fog, drizzle, dew, hail (frozen rain drops) and sleet (snow mixed with rain).
Rainfall is the water that reaches the surface of the earth from the atmosphere due to the condensation of the atmospheric vapour. Part of the natural precipitation (i.e., rainfall) is transported on the earth’s surface through natural surface channels which are known as streams or rivers.
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This is known as runoff which includes:
(i) Surface runoff or overland flow received in a stream immediately after rain,
(ii) Interflow, i.e. the portion of soil moisture which flows through the upper soil layers and joins the stream, and
(iii) Delayed runoff which includes part of rain water which enters the stream after flowing through deeper portions of the ground, and that part of the runoff which has been temporarily stored in natural lakes or as snow cover.
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Overland flow and interflow together are termed direct runoff, while the ground water flow that enters a stream is termed base flow (or base runoff). Thus, runoff is the total quantity of water received by a stream from its drainage basin (or watershed or catchment). A drainage basin of a stream is defined as that part of the earth’s surface which diverts all its runoff to the same stream (i.e., drainage outlet).
Characteristics of a drainage basin such as size, shape, surface characteristics, topography and geology and those of rain (like its intensity and duration) considerably affect the runoff. In general, a larger catchment would yield higher runoff. A fan-shaped catchment (Fig. 2.1) produces larger runoff compared to that produced by fern-shaped catchment.
Similarly, a catchment with vegetal cover yields lesser runoff compared to that yielded by a catchment with paved surface or no vegetal cover. A steeply sloping catchment will result in higher runoff. Again, a catchment with pervious soil will yield lesser runoff due to larger infiltration into the ground. Also, the runoff increases with the increase in the intensity of rainfall and decrease in the duration of rainfall.
The runoff (or surface runoff) ultimately meets the sea. Part of the runoff may percolate deep into the ground and become part of ground water just as part of the natural precipitation that falls on the ground may percolate directly deep into the ground without contributing to the runoff. Part of this ground water may join runoff later as delayed runoff.
For the purpose of providing irrigation facilities, one has to plan, construct and operate different types of irrigation (or water resources) structures, such as clams, barrages, irrigation channels, canal structures, etc. Besides, bridges have to be constructed across rivers, and several types of measures have to be taken to minimise loss of life and property due to floods in rivers.
Likewise, hydropower projects are constructed for the benefit of mankind. All such projects require knowledge about the stream flow and its behaviour. It is this (i.e., the behaviour of a stream) and related aspects which make study of hydrology useful and important for irrigation engineers.
