MEASUREMENT OF RAINFALL

Illustration of various types of rain-gauges.

Advantaged and disadvantages of various types of gauges.

Types of Rain-Gauges

Following are the main types of rain-gauges used for measurement of rainfall.

Non-automatic / Non-recording Rain-gauge Symon’s Rain-gauge

Automatic / Recording Rain-gauge Weighing Bucket Rain-gaugeTipping Bucket Rain-gaugeFloat type Rain-gaugePressure Transducer Rain-guage

Non-automatic (Non-recording) Rain-gauge

These are called non-recording rain gauges 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, i.e.

Depth of rain water in cm =

Note: The amount of precipitation/rainfall is expressed as the

depth in cm or inches.

3

2

Volume of water collected in cm

Area of aperatureof the guage in cm

Symon’s Rain-gaugeMost common type of non-automatic rain-gauge.

Consists of cylindrical vessel 127 mm (5”) dia with a base enlarged to 210 mm (8”) dia.

The top section is a funnel provided with circular brass rim exactly 127 mm (5”) internal dia.

The funnel shank is inserted in the neck of receiving bottle which is 75 to 100 mm (3 to 4”) dia.

Capacity of bottle is 75 to 100 mm of rainfall.

The rain-gauge is placed in concrete block 60cm×60cm×60cm (2’×2’×2’).

The rim should be 305 mm (12”) above the ground surface.

Water contained in the bottle is measured by suitably graduated measuring glass, with an accuracy up to 0.1 mm.

During a heavy rainfall, the rain should be measured 3 or 4 times in a day.

Automatic (Recording) Rain-gauge

Gauges which can give permanent and automatic rainfall record without any bottle reading.

Man has not to go to the gauge to measure the amount of rain fallen.

There is mechanical arrangement by which the total amount of rain fallen, since the record was started, gets recorded automatically.

The gauge thus produces a record of cumulative rain vs. time in the form of graph known as Mass Curve of rain fallen.

Measurement of rainfall by an Automatic (Recording) Rain-gauge

In general, automatic rain-gauge consists of rotating drum with a graph paper fixed around it.

There is pen in contact with graph paper, which moves up with the collected rain, and thus recording cumulative rain, with the passage of time.

These are of four types

1. Weighing Bucket Rain-gauge2. Tipping Bucket Rain-gauge3. Float Type Rain-gauge4. Pressure Transducer Rain-guage

1.Weighing Bucket Rain-gauge

Consists of a receiver bucket supported by a spring or lever balance or any other weighing mechanism.

The movement of bucket due to its increasing weight is transmitted to a pen which traces the record on a clock driven chart.

2.Tipping Bucket Rain-gauge

Consists of 30 cm dia sharp edge receiver.

At the end of receiver funnel is provided.

Under the funnel a pair of buckets are pivoted (the central point which balances) in such away that when one bucket receives 0.25 mm (0.01”) of rainfall it tips (to fall or turn over), discharging its contents into reservoir bringing other bucket under funnel.

Tipping of bucket completes an electric circuit causing the movement of pen to mark on clock driven revolving drum which carries a record sheet.

3.Float Type Rain-gauge Working is similar to weighing

gauge bucket.

Funnel receives the rain water which is collected in rectangular container.

Float is provided at the bottom of container.

Float is raised as the water level rises in the container.

Movement of float is being recorded by a pen moving on recording drum actuated by clock work.

When the water level in the container rises so that float touches the top, the siphon comes into operation, and release the water; thus all the water in the box is drained out.

4. Pressure Transducer Rain-guage

Advantages of Recording Gauge over the Non-recording Gauge. In recording gauge rainfall is recorded

automatically & therefore, there is no necessity of any attendant.

Recording rain-gauge gives the intensity of rainfall @ any time while the non-recording gauge gives the total rainfall in any particular interval of time.

As no attendant is required such rain-gauge can be installed in far–off places also.

Possibility of human error is obviated.

Disadvantages of Recording Gauge over the Non-recording Gauge.

It is costly in comparison with non-recording gauge.

Fault may be developed in electrical or mechanical mechanism or recording the rainfall.