Rain is an essential component of the water cycle and is an inevitable source of fresh water on the Earth. In simple terms, rain can be defined as the precipitation in the form of water droplets resulting from the condensation of water vapour and are heavy enough to fall under gravity. Rainfall measurement is a vital requirement for many aspects of agriculture, forestry, hydrology etc. The rainfall measurement can be accomplished by two methods namely the manual method of rainfall measurement and remote (Radar) method of rainfall measurement.
Methods of Rainfall Measurement
A. Manual Method of Rainfall Measurement
The manual method of rainfall measurement is the simplest method of rainfall measurement. This method basically includes the measurement of rainfall by means of an instrument which is known as a rain gauge. A rain gauge can be thus defined as the instrument that is used for the measurement of rainfall that works by sampling the incidence of rainfall at a specific point basically through an orifice of a known area. A rain gauge can be classified into the following:
1. Non-Recording Rain Gauge
Non-recording type rain gauges are the simple forms of rain gauges that essentially collect the rainfall but do not record the quantity of the collected rainfall. The non-recording rain gauge generally consists of a circular collecting area of about 12.7cm in diameter used for collecting the rainfall. The circular collecting area is connected to a funnel. The objective of providing the funnel is to discharge the collected rainwater to the receiving vessel. The rim of the circular collector must be set at a horizontal plane at a height of about 30.5cm above the ground level.
The arrangement of the circular collector and the discharging funnel is kept inside a metallic container. The rainfall is allowed to accumulate in the vessel and is duly measured by a graduated measuring jar. The measuring jar has an accuracy of about 0.1mm. The rainfall is measured in mm or cm as the depth of water. The most commonly used non-recording rain gauge is the Symon’s rain gauge.
Figure: Non-recording Rain Gauge (Symon’s rain gauge)
2. Recording Rain Gauge
Recording Rain Gauge is defined as the type of rain gauge that delivers a permanent automatic record of the rainfall. This rain gauge is also commonly referred to as the Integrating rain gauges as it records the cumulative rainfall. The recording rain gauge is arranged in such a way that the total amount of rainfall recorded is directly recorded in a graph paper. Such rain gauge gives the mass curve of rainfall i.e. plot of cumulative rainfall vs time. The recording rain gauge also gives the duration of the rainfall as well as the intensity of the rainfall at any given time. The recording rain gauges can be further classified into the following types:
a. Tipping Bucket Type Rain Gauge:
It is the type of rain gauge that consists of a pair of bucket arrangement that is arranged in such a way that when 0.25mm of rain falls in one bucket, it tips automatically bringing the other bucket in position. The rain collected from the tipped bucket is then collected in a can. The tipping mechanism of the bucket actuates an electrically driven pen so as to trace the plot on the graph paper. The graph is mounted on a clock-driven drum. The collected water is then measured to determine the quantity of rainfall.
Figure: Tipping Bucket Rain Gauge
b. Weighing Bucket Type Rain Gauge:
The weighing bucket-type rain gauge consists of an arrangement of a bucket mounted on a weighing scale. The weight of the bucket and its counterparts are duly recorded on a clock-driven chart. Such rain gauge delivers the mass curve of rainfall i.e plot of the cumulative rainfall vs time.
Figure: Weighing Bucket Rain Gauge
c. Floating Type Rain Gauge:
It is also commonly referred to as the natural siphon type rain gauge. This type of rain gauge consists of a funnel-shaped collector that leads to the floating chamber. As the rainfall gets accumulated on the collector the float subsequently rises and the pen attached to the float records the rainfall on a rotating drum through a clockwise mechanism. The pen works through a lever system. When the float reaches the maximum level, the siphon arrangement is used to empty the float chamber by the siphonic action.
Figure: Floating Type Rain Gauge
Factors To Be Considered During Selection of Rain Gauge Stations
Some of the factors that must be considered for the selection of the rain gauge stations are as follows:
1. The station must lie on a level and open ground so that the gauge can represent a horizontal catch surface. The rain gauge must not be surrounded by any object at least up to 30m.
2. The instrument must be sufficiently high so as to prevent splashing, flooding etc.
3. The instrument must be placed near to the ground as far as possible. This is done to minimize the wind effect.
B. Rainfall Measurement by Radar
Rainfall measurement by radar is basically an integrated system for the rainfall measurement that makes use of the microwave radar. In this method, the intensity and the amount of rainfall are measured by a rain gauge while the extent, location and the movement of rainfall are measured by the microwave radar. This method is suitable for the measurement of rainfalls in large areas where high accuracy is required. The range of hydrological range of the radar is about 200km.
Read More: Soak Pit
Read More: Bill of Quantities