In order to estimate the effects of precipitation, it is necessary to measure rainfall and snowfall (or precipitation) and to find out its distribution at various places on the earth.
All forms of precipitation are measured as the vertical depth of water that would accumulate on a level surface if the entire precipitation remained where it fell. The total amount of precipitation falling on earth in a given period will hence be expressed as the depth to which it would accumulate on the horizontal projection of the Earth’s surface if there were no losses by evaporation or run-off, and if any part of precipitation falling as snow or ice were melted. The two types of precipitation (ie. rain and snow) are measured separately by measuring devices called rain gauges and snow gauges respectively.
Measurement of Rainfall
Since the amount of precipitation varies from place to place, it is necessary to install measuring devices at key points.
The simplest method of measuring rainfall is by setting up a rain gauge with a horizontal circular aperture of known area, and collecting and measuring at regular intervals, the rainfall collected in that gauge. It is assumed that the amount of rainfall collected in the gauge is representative of a certain area around the point where the measurement is made.
Types of Rain Gauges.
Any open receptacle with vertical sides can be used as a gauge for measuring rainfall. These refined receptacles which are used for measuring rainfall are called rain gauges. Two kinds of rain gauges are generally used. First is the ‘non-recording type’ rain gauges, and the second is the ‘recording type’ rain gauges. These two types of rain gauges are summarised below:
Non-recording rain gauges
Non-recording rain gauges are most widely used in India. They are known as the ‘non-recording type’, because they do not record the rain, but only collect the rain. The collected rain is then measured by means of graduated cylinders, so as to directly represent the rainfall volume in cm of water depth:
Various models have been designed for such gauges. Out of them, the Symon’s type (Fig-2) was mostly used in India till the year 1969 or so. However, since then, the Indian Meteorological Department (IMD) has adapted another model called Standard gauges (Fig-3) for their use at all rainfall stations in this country. Symon’s and other types of rain gauges have, therefore, been replaced in India, and have become obsolete.
The complete installation of the new standard type of a non-recording rain gauge, which is now being used in India, is shown in Fig-3. Such a rain gauge consists of a collector with a gun-metal rim, a base, and a polythene bottle. Both the collector and the base are made of fibreglass reinforced polyester. The collector has a deep-set funnel, and the complete rain gauge has a slight taper with the narrower portion at the top, as shown. The collector and the base are locked to each other by means of two complementary locking rings, fixed inside the collector at its lower end and the base at its top end.
The collectors can have apertures of either 100 sq-cm or 200 sq-cm area. The polythene bottles are available in three sizes having capacities of 2, 4 and 10 litres of water. The rain gauge with different combinations of aperture size and bottle size, may have capacities of 100, 200, 400 and 1000 mm of rainfall. The 200 mm capacity rain gauge with 200 sq-cm collector and 4 litre bottle, however, is most widely used. When the rain falls, it is collected in the bottle. A man comes daily at 8.30 AM (in India) and records the amount of water collected in the bottle, and thus the rainfall is recorded. The process is common throughout this country and rain is recorded at 8.30 AM every day. It the rainfall is too much and is likely to exceed the capacity of the bottle, then two or three intermediate readings are taken, and their sum will have to be recorded as the rainfall of the past 24 hours of the day on which the final reading is taken.
Recording type rain gauges.
Recording type rain gauges are those rain gauges that can give us a permanent automatic rainfall record without any bottle reading. In this type of gauge, a man has not to go to the gauge to measure or to read the amount of rain falling. There is a mechanical arrangement by which the total amount of rain, fallen since the record was started, gets recorded automatically on graph paper. The gauge thus produces a record of cumulative rain vs time in the form of a graph, which is known as the mass curve of rainfall and is shown in Fig- 3.
Besides giving the total amount of rain fallen at a station, such a curve will also help in indicating the times of onset and cessation of rain, and thereby indicating its duration. Moreover, the slope of the curve gives us the intensity of rainfall for any given period, as indicated in Fig- 3.
Since such gauges represent the cumulative rain, they are sometimes called the Integrating rain gauges. Moreover, such gauges can provide us with continuous recorded measurements for a number of days. They are of great utility in hilly and far off areas, where it is not practically feasible to daily Visit the gauge stations. Such gauges are, therefore, also sometimes known, as Continuous rain gauges.
A recording rain gauge is generally installed in conjunction with all ordinary non-recording gauge exposed closeby. The nearby installed ordinary gauge will serve as an aid and a standard for checking and adjusting the readings of the recording gauge.
Types of recording rain gauges
Various models have been designed for recording gauges; on the basis of which, various gauges, such as “Tipping bucket type”, “Weighing type” and “Floating type” etc. are available in the market. Out of these three important types, the floating type is most widely used in India at almost all its meteorological stations. The tipping bucket type is, however, used at remote hilly stations, because it can transmit its records, through a telecommunication system, directly to the control room at the main meteorological station (Pune in India).
A float type gauge (Fig- 4), in general, consists of a rotating drum with a graph paper fixed around it. There is a Pen point in contact With the graph paper, which moves, up with the rise of the float. The float, in turn, rises up with the rainwater collected in the gauge chamber and entering through a funnel. The moving pen thus goes on recording the cumulative rain with the passage of time.
When the chamber gets filled with water to such an extent that the float touches the top, a natural siphon starts working, and the rain water collected in the chamber is drained out.
In a tipping-bucket gauge, the rain Water is first caught in a collector and then passed through a funnel. The funnel discharges the water into a two-chamber bucket. When 0.1 mm of rainwater gets filled up in one compartment, the bucket tips, emptying into a reservoir, and moving the second compartment into place beneath the funnel, as shown in Fig- 5.
The tipping of the bucket completes an electric circuit, causing a pen to mark on a revolving drum. Since the movement of the tipping of the bucket can be transmitted electronically over distances, such gauges are generally installed in hilly and inaccessible areas, from where they can supply measurements directly in the control room, as pointed out earlier.
Measurement of Snow
Snow is the second important part of precipitation. Although it does not yield immediate run-off, but is generally responsible for delayed supplies of water. Thus, most of our Himalayan rivers are snow fed and are, therefore, in a position to provide continuous perennial supply of water. Snowfall is generally measured by a non-recording type of rain-gauge. The snow is melted and measured. To measure the snow, snow surveys are also conducted. In India, snow surveys are generally conducted in March or April, i.e., at the start of summer season, When snow starts melting. These surveys are useful in estimating the amount of available water expected due to the melting of snow in summer.
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