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Precipitation is expressed in terms of the vertical depth to which water from it would stand on a level surface area if all the water from it were collected on this surface.
The depth is to which water would accumulate on the horizontal projection of the earth’s surface, if there were no losses by evaporation, infiltration and runoff and if any part falling as snow etc melted.
The amount of precipitation is measured using a rain gauge (also called pluviometer, ombrometer, hyetometer etc). A breif study on different rainguages and selection of raingauage stations are explained below.
Measurement of Rainfall Using Raingauges
A rain gauge consists of a cylindrical vessel assembly kept in the open to collect rain. Rainfall collected in the rain gauge is measured at regular intervals.
Rainfall catch is affected by the exposure conditions of the rain gauge. Rain gauges may be broadly classified into 2 categories:
- Non-recording raingauges and
- Recording raingauges
These rain gauges just collect the rainwater but do not record the quantity of rainfall. The most extensively used non-recording rain gauge is Symon’s gauge. Circular collecting area of 12.7 cm diameter connected to a funnel. The rim of the collector is set in a horizontal plane at a height of 30.5cm above the ground level.
The funnel discharges the rainfall catch into a receiving vessel. The funnel and collecting vessel (bottle) are housed in a metallic container. Water collected in the bottle is measured using a suitably graduated measuring jar with 0.1mm accuracy. Rainfall is measured in mm or cm of water depth
Recently IMD has started adopting fibreglass reinforced polyester raingauges. These are available in different combinations of collector and bottle. The collector is in two sizes – having 100 and 200 sq.m area. For details see IS:5225 and IS:4986.
Rainfall is measured every day at 8.30AM IST and is recorded as the rainfall of that day.
The receiving bottle cannot hold more than 10 cm of rain. Hence on days of heavy rainfall, measurements are taken more frequently and the last reading is taken at 8.30AM IST. The sum of the rainfall measurements over the past 24h is entered as the total rainfall of that day.
When snow is expected, the funnel and the receiving bottle are removed and snow is collected in the outer metal container. It is then melted and the depth of resulting water is measured. In areas of heavy snowfall, snow gauges with shields and storage pipes are used. Snow surveys are also conducted.
Recording Rain Gauges
Recording raingauges give a permanent automatic record of rainfall. It has a mechanical arrangement by which the total amount of rainfall since the start of record gets automatically recorded on a graph paper. It produces a plot of cumulative rainfall vs time (mass curve of rainfall). These rain gauges are also called integrating raingauges since they record cumulative rainfall.
In addition to the total amount of rainfall at a station, it gives the times of onset and cessation of rains (thereby gives the duration of rainfall events). The slope of the plot gives the intensity of rainfall for any given time period.
They can provide continuous record for a number of days. They are very useful in hilly and far off areas. In other areas, they are installed along with a non-recording raingauge.
The types of Recording Rain Gauges are:
1. Tipping Bucket Type
This is ideally suited for use as a telemetering rain gauge.The catch from the funnel falls onto one of a pair of small buckets. These buckets are so balanced that when 0.25mm of rain falls into one bucket, it tips bringing the other bucket in position. The water from the tipped bucket is collected in a can.
Tipping actuates an electrically driven pen to trace a record on the graph paper mounted on a clockwork driven drum. Water collected in the can is measured at regular intervals to check the total rainfall recorded. The record from this rain gauge gives the intensity of rainfall. These can be installed in stations located in hilly and inaccessible areas.
2. Weighing Bucket Type
The catch empties into a bucket mounted on a weighing scale. The weight of the bucket and its contents are recorded on a clock work driven chart. The instrument gives a plot of cumulative rainfall against time (mass curve of rainfall). In some of the instruments, the recording unit is so devised that the pen reverses its direction at a preset value.
3. Natural Syphon Type (Float Type)
The rainfall collected in the funnel shaped collector is led into a float chamber, causing the float to rise. As the float rises, a pen attached to the float through a lever system records the rainfall on a rotating drum driven by a clockwork mechanism.
A syphon arrangement empties the float chamber when the float has reached a preset maximum level. The vertical lines in the chart correspond to the sudden emptying of the float chamber by syphonic action, which resets the pen to zero level.If there is no rainfall, the pen traces a horizontal line
Selection of Rain Gauge Stations
In order that the rainfall catch of a raingauge accurately represents the rainfall in the area surrounding the raingauge, certain standards are followed while establishing raingauge stations.The rain gauge station must fulfill following requirements:
- The ground must be level and in the open and the instrument must represent a horizontal catch surface
- The raingauge must be set as near the ground as possible to reduce wind effects
- The raingauge must be set sufficiently high to prevent splashing, flooding etc
- The instrument must be surrounded by an open fenced area of at least 5.5m x 5.5m. No object must be closer to the raingauge than 30m or twice the height of the obstruction
Rainfall Measurements by Radar
In an integrated system for measurement of rainfall, rain gauges are used for measuring the total amount as well as intensity of a rainstorm and a microwave radar (wavelength ~ 3 to 10 cm) is used for determining the areal extent, location, and movement of rainstorms. Also the amount of rainfall over large areas can be determined using radars with a good degree of accuracy.
- The hydrological range of the radar is about 200km.
- Heavy rains – 10 cm radar
- Light rains and snow – 5cm radar
- Doppler type radars are used for measuring the velocity and distribution of raindrops.
Also Read: Analysis and Presentation of Rainfall Data