Measurement of Precipitation | Methods of Measurement of Precipitation | How Do We Measure Rainfall

 Measurement of Precipitation

One of the most crucial and least known components of the global hydrologic cycle is the precipitation that is the basic data required to estimate any hydrologic quantity (such as runoff, flood discharge etc.). Therefore, measurement of precipitation is an important component of all hydrologic studies. Weather and water-balance studies too require information on precipitation.

1. Precipitation Gauges

Precipitation (of all kinds) is measured in terms of depth of water (in millimeters) that would accumulate on a level surface if the precipitation remained where it fell. A variety of instruments have been developed for measuring precipitation (or precipitation rate) and are known as precipitation gauges or, simply, rain gauges which are classified as either recording or non recording rain gauges.

Non-recording rain gauges only collect rain water which, when measured suitably, gives the total amount of rainfall at the rain gauge station during the measuring interval. TheIndian Meteorological Department has adopted Symon’s rain gauge (Fig. 1.). A glass bottle and funnel with brass rim are put in a metallic cylinder such that the top of the cylinder is 305 mm above the ground level. Rain water falls into the glass bottle through the funnel. The water collected in the bottle is measured with the help of a standard measuring glass jar which is supplied with the rain gauge. The jar measures rainfall in millimeters. At each station, rainfall observations are taken twice daily at 8.30 a.m. and 5.30 p.m

Fig.1. Symon's rain gauge

Recording rain gauges automatically record the intensity of rainfall and the time of its occurrence in the form of a trace (or graph) marked on a graph paper wrapped round a revolving drum. Following three types are the most widely used recording rain gauges :

(i) Tipping bucket rain gauge,

(ii) Weighing bucket rain gauge, and

(iii) Siphon rain gauge.

(i) Tipping bucket rain gauge : A 300 mm diameter funnel collects rain water and conducts it to one of the two small buckets (Fig. 2.) which are so designed that when 0.25 mm of rainfall is collected in a bucket, it tilts and empties its water into a bigger storage tank and, simultaneously, moves the other bucket below the funnel. When any of the two buckets tilts, it actuates an electric circuit causing a pen to make a mark on a revolving drum. The recording equipment can be remotely located in a building away from the rain gauge. At a scheduled time, the rain water collected in the storage tank can be measured to yield total rainfall in the measuring duration. The rainfall intensity (and also the total rainfall) can be estimated by studying the record sheet on which each mark indicates 0.25 mm of rain in the duration elapsed between the two adjacent marks.

Fig.2. Tipping bucket rain gauge

(ii) Weighing bucket rain gauge : This gauge (Fig. 3.) has a system by which the rain that falls into a bucket set on a platform is weighed by a weighing device suitably attached to the platform. The increasing weight of rain water in the bucket moves the platform. This movement is suitably transmitted to a pen which makes a trace of accumulated amount of rainfall on a suitably graduated chart wrapped round a clock driven revolving drum. The rainfall record of this gauge is in the form of a mass curve of rainfall (Fig. 4.). The slope of this curve at any given time gives the intensity of rainfall at that time.

Fig. 3. Weighing bucket rain gauge

Fig. 4. Rainfall record of bucket rain gauge (mass curve of rainfall)

(iii) Siphon rain gauge : This gauge (Fig. 5.) is also called float type rain gauge as this gauge has a chamber which contains a light and hollow float. The vertical movement of float on account of rise in the water level in the chamber (due to rain water falling in it) is transmitted by a suitable mechanism to move a pen on a clock-driven revolving chart. The record of rainfall is in the form of a mass curve of rainfall and, hence, the slope of the curve gives the intensity of rainfall.

Fig. 5. Siphon rain gauge

 Bureau of Indian Standards has laid down the following guidelines for selecting the site for rain gauges (IS : 4897-1968):

1. The rain gauge shall be placed on a level ground, not upon a slope or a terrace and never upon a wall or roof.

2. On no account the rain gauge shall be placed on a slope such that the ground falls away steeply in the direction of the prevailing wind.

3. The distance of the rain gauge from any object shall not be less than twice the height of the object above the rim of the gauge.

4. Great care shall be taken at mountain and coast stations so that the gauges are not unduly exposed to the sweep of the wind. A belt of trees or a wall on the side of the prevailing wind at a distance exceeding twice its height shall form an efficient shelter.

5. In hills where it is difficult to find a level space, the site for the rain gauge shall be chosen where it is best shielded from high winds and where the wind does not cause eddies.

6. The location of the gauge should not be changed without taking suitable precautions. Description of the site and surroundings should be made a matter of record.

2. Radar Measurement of Precipitation

In regions of difficult and inaccessible terrains, precipitation can be measured (within about 10% accuracy of the rain gauge measurements) with the help of a radar (radio detecting and ranging). A radar transmits a pulse of electromagnetic waves as a beam in a direction depending upon the position of the movable antenna. The wave travelling at a speed of light is partially reflected by cloud or precipitation particles and returns to the radar where it is received by the same antenna. The display of the magnitude of the energy of the returned wave on the radarscope (i.e., radar screen) is called an echo and its brightness is termed echo intensity. The duration between the transmission of the pulse and appearance of the echo on the radarscope is a measure of the distance (i.e., range) of the target from the radar. Direction of the target with respect to the radar is decided by the orientation of the antenna at the time the target signal is received. The echo is seen in polar coordinates. If there is no target (i.e., cloud or precipitation particles), the screen is dimly illuminated. A small target would appear as a bright point whereas an extended target (such as a rain shower) would appear as a bright patch. The radarscope being divided as per the coordinate system, the position of the target can be estimated. By having a proper calibration between the echo intensity and rainfall (or its intensity), one can estimate the rainfall (or rainfall intensity). The Indian Meteorological Department has a well-established radar network for the purpose of detecting thunderstorms besides a few cyclone-warning radars along the eastern coast of the country.

The wavelength of the electromagnetic waves transmitted by the meterological radars is in the range of 3 to 10 cm; the usual operating range being 5 cm (for light rains) to 10 cm (for heavy rains). The relationship among the characteristics of the waves and the rainfall intensity is represented by  

Pr = CZ/r²

where, Pr is the average echo power, r is the distance from radar to target and C is a suitable constant. The radar echo factor Z is related to the intensity of rainfall I (in mm/hr) as

Z = aI^b

in which, a and b are numerical constants that are determined by calibrating the radar. One may, thus, obtain

I = [r² P_r /(aC)]^1/b

 

Present day developments in radar measurements of precipitation include on-line processing of the radar data and Doppler type radars for measuring the velocity and distribution of raindrops.

3. Satellite Measurement of Precipitation

It is a common experience that gauge network for measuring precipitation in a large and inaccessible area (such as in desert and hilly regions) is generally inadequate, and non-existent in oceans. The satellite observation is the only effective way for continuous monitoring of precipitation events over a large or inaccessible area. Use of the meterological satellites for weather and water balance studies is, therefore, continuously increasing.

In satellite measurements, the precipitation is estimated by correlating the satellite derived data and observed rainfall data. These relationships can be developed for a part of electromagnetic spectrum using cloud life history or cloud indexing approach. The first approach uses data from geo-stationary satellites that produce data at every half an hour interval. The second approach, based on cloud classification, does not require a series of consecutive observations of the same cloud system (2).

Microwave remote sensing techniques that can directly monitor the rainfall characteristics have great potential in rainfall measurement.