Table of Contents
The manual method of rainfall measurement is the simplest method and includes the measurement of rainfall by means of an instrument which is known as a rain gauge.
The rain gauge works by sampling the incidence of rainfall at a specific point basically through an orifice of a known area.
1. Types of Rain Gauge
2 types of rain gauges are explained below:
a. Non-Recording Rain Gauge
Non-recording type rain gauge is a simple form of rain gauge that essentially collects the rainfall but does 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.
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 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 Symon’s rain gauge.
Figure: Non-recording Rain Gauge (Symon’s rain gauge)
b. Recording Rain Gauge
Recording Rain Gauge delivers a permanent automatic record of the rainfall.
This types of rain gauge is also commonly referred to as the Integrating rain gauge 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 on 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:
i. Tipping Bucket Type Rain Gauge:
It is the type of rain gauge that consists of a pair of bucket arrangements that is arranged in such a way that when 0.25 mm 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 to trace the plot on graph paper.
The graph is mounted on a clock-driven drum. The collected water is then measured to determine the quantity of rainfall.
The diameter of the tunnel which collects rainwater in the tipping bucket rain gauge is 300mm (30cm).
Figure: Tipping Bucket Rain Gauge
ii. 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
iii. Floating Type Rain Gauge:
It is also commonly referred to as the natural siphon-type rain gauge.
This types 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 Types of Rain Gauge
2. 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 to prevent splashing, flooding, etc.
3. The instrument must be placed near the ground as far as possible. This is done to minimize the wind effect.
3. Errors in the Measurement of Rainfall
The major error in the measurements of rainfall is mentioned below:
a. In general rain gauge misjudged the rainfall that would have dropped on the field in the non-presence of a gauge.
b. The measuring value difference of the type of gauge, the height of the rim of the gauge above the ground surface, the intensity of rainfall, the wind speed, etc.
c. Inappropriate reading of measuring cylinder, the dropping of a few of the water when moving it.
d. To the measuring jar and the incapacity to move all the water from the gauge to the measuring jar.
4. Double Mass Curve Method of Adjustment
The noted data from the rain gauge inconsistency of record is calculated by the double mass curve method.
In the mass curve method of adjustment, the noted data arrive from the same parent population and are even.
The double mass curve method is employed to check the consistency of rainfall data records at any rain gauge station which is stopped if it contains specific discrepancies. The inconsistencies in the rainfall data of a station can be arrived because of different reasons.
The rain gauge station might have been established at various sites in the past that is there is a long and continuous rainfall record. The data received are not homogeneous concerning the current location of the rain gauge.
The consistency and precision of the data are inspected by double mass curve analysis and the double mass curve also provides a correction factor to ensure that the data is homogenous throughout its period and is affiliated to an unknown site.
A double mass curve allows the data to be evaluated for the absent periods or to be extrapolated beyond the existing length of record on the reference of the neighborhood rain gauge data.
5. Procedure of Mass Curve
The primary procedures of the double mass curve are mentioned below:
a. A Lowest five number of stations (5 as usual minimum and 5 to 10 base stations) with dedicated data of approximation the same length and in the same climate region is set as a base station.
b. The best stations should not contain any inconsistencies.
c. The average of the annual rainfalls at the base stations is calculated for every year and cumulatively counted beginning with the last year.
d. A double mass curve graph is positioned between the accumulator with rainfall of the base station abscissa and the cumulative rainfall of station X (i.e. Neighborhood problem station) as the ordinate.
This was for the types of rain gauge.
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