1.5.3 Measurement of Precipitation

Measurement of precipitation

As most of the precipitation that occurs in our country is through rainfall, these two terms will be treated as synonymous. Precipitation is measured in terms of vertical depth to which the water from the rainfall would stand on a level surface if all rainwater were collected on it, assuming no loss by evaporation or runoff. Since the amount of precipitation varies from place to place it is necessary to install measuring devices at key points. A rain gauge is a device that collects and measures the precipitation in mm occurring over that area.

Selection of site for rain gauge station

• The site should be on level ground.
• The site should be on open space.
• Site should be away from continuous wind forces.
• Horizontal distance between rain gauge and the nearest object should be 30m or twice the height of that object.
• Site should be easily accessible.
• The gauge should be truly vertical.
• Ten percent of total number of rain gauge stations should be self recording.
• The observer must visit the site regularly to ensure its proper readiness for measurement.

Optimum number of rain gauge stations (As per IS (4987 of 1968)

Rain gauges

There are two types of rain gauges.

Non recording type (non automatic type) – they do not record the rain but only collect the rain. The collected rain is then measured by measured by means of graduated cylinders so as to directly represent the rainfall volume is cm of water depth. These types are normally used in India.

Till recently ‘Symons’s type’ gauge was used mostly. Since 1969, Indian meteorological department adopted model called ‘Standard Gauge’ which replaced all previous models.

Standard Non recording rain gauge

This 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 fiber glass 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 collectors have either apertures of 100 or 200 sq cm area and are so made that they are interchangeable. The polythene bottles are of three sizes having capacity 2, 4 and 10 liters of water. Thus standard rain gauges may have capacities of 100,200,400 and 1000mm of rainfall. Capacities of rain gauge can be altered by interchanging collectors and polythene bottles. 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.

All the standard rain gauges can be installed at a standard height of 30cm above the ground level which was not possible in Symon’s type.

Rainfall collected in the gauge in past 24 hours is poured in standard measuring graduated measuring jar. This way rainfall is recorded at 08.00 am manually every day. However if rainfall is heavy and likely to exceed capacity of bottle, then two or three intermediate readings may be taken.

Recording (Automatic) type of rain gauges

Recording type gauges can give total rainfall between given period only. These gauges cannot determine variation in the intensity of rainfall. Automatic type is used for continuous recording of intensity of rainfall. In this type manual recording of rainfall is not necessary but there is mechanical arrangement by which the total amount of rain fallen since when record has started gets recorded automatically on a graph paper.

Tipping Bucket type –This consists of circular collector, funnel, tipping bucket, measuring cylinder as shown in figure. Rain water is first caught in collector which is then passed to funnel. Funnel discharges water to tipping bucket supported between two compartments. When one compartment is filled, the bucket tips and empties in measuring cylinder below, while the other compartment takes place beneath funnel & collects rain water. The tipping of bucket completes an electric circuit, causing pen to mark on a revolving drum. Since movement of the tipping bucket can be transmitted electronically over distances, such gauges are installed in hilly and inaccessible areas from where they can supply measurements directly in control room. As rainfall measurements are directly recorded at control room the gauge can be left without watch and ward except periodical repairs and visits.

Weighing type – This consists of a pan, supported on scale. Rain water falls in collector/bucket through funnel. The bucket is placed on the platform of a spring or a lever balance or any other weighing mechanism. The increasing weight of bucket is automatically recorded on a graph paper fixed on a rotating drum by means of pen. Pen traces the rainfall on a clockwise operated chart. This type is not widely used in India.

Float type – The rain water entering the gauge at the top of the cover is led via funnel to the receiver, consisting of a float chamber and a siphon chamber. A pen is mounted on the stem of the float and as the water level rises in the receiver the float arises and the pen records on a chart wrapped round a clockwise rotating drum, the amount of water in the receiver at any instant. The rotating drum completes one revolution in 24 hours, or sometimes in 7 days, depending upon the gauge, and the graph chart will have to be replaced after this much of time. Siphoning occurs, automatically when the pen reaches the top of the chart, and as the rain continues, the pen rises again from zero to top of the chart. If there is no rain, the pen traces horizontal line from where it leaves off rising.

Computation of Average rainfall over a basin

To compute average rainfall over basin the rainfall is measured at number of rainguage stations suitably located in the area. Network of rain gauge stations should be so planned as to give representative picture of catchment. Following table may be referred to decide no of rain gauge stations

If basin contains more than one rain gauge stations average precipitation may be computed by flowing methods

Arithmetic mean method – If rainfall is uniformly distributed on its areal pattern this is the simplest method of estimating average rainfall. Thus if P₁, P₂, P₃,………,P_n are precipitation or rainfall values measured at n gauge stations then we have,

This is most approximate method since rainfall varies in intensity and duration from place to place.

Thissen Polygon Method - In this method rainfall recorded by each rain gauge station should be weighed according to area it is assumed to represent. This is also called weighted mean method

Procedure

· Join adjacent rain gauge stations A,B,C,D etc by straight lines.

· Construct perpendicular bisectors of each of these lines.

· A Thiessen network is thus constructed. The polygon formed by the perpendicular bisectors around a station encloses an area which is closer to that station than any other station.

· Multiply the area of each Thiessen polygon by the rain gauge value of enclosed station.

· Find total area A of the basin.

Compute average precipitation as

Isohytal Method

An Isohyet is a line on a rainfall map of a basin joining places of equal rainfall readings.

Procedure

· Prepare Isohyetal map from the rainfall values recorded at various rain gauge stations.

· Measure the areas enclosed between successive isohytes with the help of planimeter.

· Multiply each of these areas by the average rainfall between isohyets.

Average rainfall is then computed as