A form of precipitation in which rainfall water reaches the earth from the atmosphere is measured by an instrument called rain gauge. Rain gauge is also variously known as hyetometer, Ombrometer, or Pluviometer.
Selection of site for rain gauge:
While selecting the site for the installation of rain gauge the following points should be take kept in mind.
- The site should be on level ground and other types of the ground like hilltops, hill slopes, etc undulation type of slope is not suitable.
- The site should be an open space.
- The nearest object from the rain gauge should be kept at a distance of twice the height of the object.
- Sad should be away from continuous wind forces.
- Other metrological instruments and the fencing of the site should maintain step 3 above.
- The site should be easily accessible.
- The gauge should be truly vertical.
- 10% of the total number of rain gauge stations in any basin should be self-recording.
- The observer must visit the site regularly to ensure its proper reading readiness for measurements.
Types of rain gauges
The precipitation is measured by a hydrological instrument called a rain gauge. A rain gauge is also variously known as a hydrometer, Ombrometer, or Pluviometer. Rain gauges are mainly of two types.
- Non-recording or ordinary rain gauges.
- Recording type or automatic rain gauges.
1. Nonrecording or original rain gages:
The non-recording gauge (original gauge) extensively used in the world is the Symon’s gauge. Non-recording gauges don’t record the rain but only collect the rain. Once the rain is collected, then it is measured by using a graduated cylinder.
The volume of rain measured in the measuring cylinder directly represents the rainfall volume in cm of water depth i.e. the volume of water collected in cm/3 divided by the area of the aperture (hole i.e. opening that admits light) of the gauge in cm2 is equal to the depth of rainfall in cm.
The receiver (connecting bottle) has a capacity of 175 mm of rain. In a region of heavy rainfall, rain gauge with receivers of 375 mm or 1000 mm capacity to measure rainfall depths may be used.
Nonrecording rain gauges give the amount of rainfall only. This type of rain gauge cannot provide information regarding when exactly the rain commenced, the rain ended, what is the intensity of rainfall, and how much intensity of rainfall varies within the duration of the storm.
In order to record the beginning and end of the rain and to measure the intensity of rainfall, a continuous recording of rainfall with time is required, for this purpose, we have to use the recording rain gauges.
In Symon’s rain gauge there are concrete block sizes for the foundation are 600 mm x 600 mm x 600 mm and other dimensions are in mm as shown in the figure above.
2. Recording type automatic rain gauges:
In the recording gauge, it plots the rainfall against time. From that plot of data, we can easily extract information about the intensity and duration of rainfall, on the basis of which we may make a hydrological analysis of storms.
- Recording-type rain gauges are those rain gauges, which can give for permanent automatic rainfall records without any bottle reading.
- In this type of rain gauge, no man is required to measure or read the amount of rainfall from the rain gauge.
- The record was started and gets recording was recorded automatically on graph paper.
- The gauge, focuses on a record of humidity range versus (VS) time, in the form of a graph, which is known as the mass curve of rainfall.
The figure below shows the mass curve of rainfall.
The following are some of the commonly used recording rain gauges:
- Tipping (or tilting) bucket type
- Siphon float type
- Weighing bucket type
1. Tipping or tilting bucket-type rain gauges:
The tipping or tilting bucket-type rain gauge design principle is very simple. A container is taken and divided into two vertical compartments and is balanced in an unstable equilibrium about a horizontal axis. In its normal position.
The instrument arrangement is in the given ways:
- It does not produce the mass curve of rainfall.
- It consists of a 30 cm diameter sharp-edge receiver.
- Sad the end of the receiver, a funnel is provided.
- A pair of buckets are pivoted on the funnel in such a way that when one bucket receives 0.2 mm of precipitation it tips, discharging its contents into a tank and bringing the other bucket under the funnel.
- The teaching of the work completes an electric circuit coming to the movement of a pen to mark on a clock-driven revolving drum that carries a record sheet.
Disadvantages of the Tipping or tilting bucket type rain gauges:
- When tipping of pockets takes place, rainfall at that instant is not recorded.
- Very high-intensity of rainfall gives close signals, which can make it difficult to record the number of tips and.
- Calibration of tips may change due to rusting and dirt accommodation.
Syphon ( Float) type automatic rainfall recorder
The working principle of float-type rain gauge is similar to the waiting bucket-type rain gauge. The reason water enters into the container of the gauge through the funnel. A float is provided at the bottom of the container as shown in the figure below. This float is lifted off by the rainwater which is collected in the container.
- This type of recording rain gauge is also known as a float type gauge.
- Here, rainfall collected by funnel-shaped collection is led into a float chamber causing the upload to rise.
- When the float rises, a pen (pointer) attached to the float through a level system records the elevation of the float on a rotating drum driven by a clock mechanism.
- When the float has reached a preset maximum level, the siphon arrangement empties the float chamber.
All types of information about the storm can be obtained from the accumulator plot of graph paper. The beginning and end of the storm, its intensity, duration, distribution of rain, and the depths of total storm precipitation can easily e be obtained from the plot of the graph.
Disadvantages of syphon type automatic rainfall recorder:
- Cost instruments than other recording-type instruments.
- Mechanical defects sometimes give enormous results.
Waiting bucket type rain gauges:
This type of rain gauge can be used for recording rainfall as well as snowfall.
Rain is collected in a receiver vacate supported on a spring balance. A mechanical lever arm of the balance is connected without pain which touches a clock-mounted drum with the graph paper. As it rains, the weight of the bucket gradually increases, which changes the position of the pen of the balance. With the time the pen marks a line Cu continuously moves graph paper. The recorded shows the accumulation of precipitation our time. The recording can be chicken after 24 hours or 7 days depending on the clock and drum size.
These gases are generally used in the USA and are becoming increasingly popular but not used in India and South Asia.
Disadvantages of waiting bucket type rain gauges:
- When very heavy precipitation occurs, there is a good chance that the bucket will overflow and
- Expensive instruments are used
Errors in measurement of rainfall:
The error in measurement of rainfall are given below:
- The main likely serious of errors in measuring rainfall are,
- In general rain gauge underestimates the rainfall that would have fallen on the ground in the absence of a gauge.
- 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.
- Improper reading of measuring cylinder, the spilling of some of the water when transferring it.
- To the measuring jar and the inability to transfer all the water from the receiver to the measuring jar. New For You: Field Density Test of Soil by Core cutter method for soil compaction
Double mass curve method of adjustment:
The recorded data from rain gauge inconsistency of record is measured by the double mass curve technique.
In the mass curve method of adjustment, the recorded data come from the same parent population are consistent.
The double mass curve technique is used to test the consistency of rainfall data records at any rain gauge station which if suspended contains certain discrepancies. The inconsistencies in the rainfall data of a station may be due to various reasons. The rain gauge station might have been installed at different sites in the past that is there is a long and continuous rainfall record. The data obtained are not homogeneous with respect to the present location of the rain gauge.
The consistency and accuracy of the data are tested by double mass curve analysis and the double mass curve also gives a correction factor to assure that the data is homogenous throughout its period and is related to an unknown site.
A double mass curve enables the data to be estimated for the missing periods or to be extrapolated beyond the existing length of record on the basis of the neighborhood data.
Procedure of Mass curve:
The main procedures of the double mass curve are as follows:
- A Minimum five number of stations (5 as usually minimum and 5 to 10 base stations) with reliable data of approximation the same length and in the same climate region is selected as a base station.
- The best stations should not have any e inconsistencies.
- The average of the annual rainfalls at the base stations is computed for every year and cumulatively added beginning with the last year.
- A double mass curve graph is placed 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.
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