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Precipitation And Evapotranspiration
Precipitation is any form of solid or liquid water that falls from the atmosphere to the earth‟s surface. Rain, d precipitation. In India, rain is the most common form of precipitation.
Evapotranspiration is the process which returns water to the atmosphere and thus completes the hydrologic cycle. Evapotranspiration consists of two parts, Evaporation and Transpiration. Evaporation is the loss of water molecules from soil masses and water bodies. Transpiration is the loss of water from plants in the form of vapour. We proceed on to discuss precipitation, and its most important component in India context, the rainfall.
Causes of precipitation
For the formation of clouds and subsequent precipitation, it is for necessary that the moist air masses to cool in order to condense. This is generally accomplished by adiabatic cooling of moist air through a process of being lifted to higher altitudes. The precipitation types can be categorized as.
This is the precipitation that is caused by the expansion of air on ascent along or near a frontal surface.
Precipitation caused by the upward movement of air which is warmer than its surroundings. This precipitation is generally showery nature with rapid changes of intensities.
Precipitation caused by the air masses which strike the mountain barriers and rise up, causing condensation and precipitation. The greatest amount of precipitation will fall on the windward side of the barrier and little amount of precipitation will fall on leave ward side.
For the Indian climate, the south-west monsoon is the principal rainy season when over 75% of the annual rainfall is received over a major portion of the country. Excepting the south-eastern part of the Indian peninsula and Jammu and Kashmir, for the rest of the country the south-west monsoon is the principal source of rain.
From the point of view of water resources engineering, it is essential to quantify rainfall over space and time and extract necessary analytical information.
Measurement of rainfall
ü One can measure the rain falling at a place by placing a measuring cylinder graduated in a length scale, commonly in mm.
ü In this way, we are not measuring the volume of water that is stored in the cylinder, but the „depth‟ of rainfall.
ü The cylinder can be of any diameter, and we would expect the same
„depth‟ even for large diameter g iscylinde uniformly distributed in space. Now think of a cylinder with a diameter as
large as a town, or a district or a catchment of a river.
ü Naturally, the rain falling on the entire area at any time would not be the
same and what one wouldaverageget depth‟would.be an
ü Hence, to record the spatial variation of rain falling over an area, it is better to record the rain at a point using a standard sized measuring cylinder.
ü In practice, rain is mostly measured with the standard non-recording rain gauge the details of which are given in Bureau of Indian Standards code IS 4989: 2002.
ü The rainfall variation at a point with time is measured with a recording rain-gauge, the details of which may be found in IS 8389: 2003.
ü Modern technology has helped to develop Radars, which measures rainfall over an entire region. However, this method is rather costly compared to the
ü conventional recording and non-recording rain gauges which can be monitored easily with cheap labour.
Variation of rainfall
ü Rainfall measurement is commonly used to estimate the amount of water falling over the land surface, part of which infiltrates into the soil and part of which flows down to a stream or river.
ü For a scientific study of the hydrologic cycle, a correlation is sought, between the amount of water falling within a catchment, the portion of which that adds to the ground water and the part that appears as streamflow.
ü Some of the water that has fallen would evaporate or be extracted from the ground by plants.
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