RAIN GAUGE

Standard non recording rain gauge prescribed by the IMID is the Symon’ s gauge the details of which are shown below.

NON RECORDING RAIN GAUGE

The gauge consists of a funnel with a sharp edged rim of 127 mm diameter, a cylindrical body, a receiver with a narrow neck and. handle and a splayed base which is fixed in the ground.

The receiver should have a narrow neck and should be sufficiently protected from radiation to minimize the loss of water from the receiver by evaporation.

To prevent rain from splashing in and out, the vertical wall of the sharp edged rim is made deep enough and the slope of the funnel steep enough (at least 45° ).

The rain falling into the funnel is collected in the receiver kept inside the body and is measured by means of  special measure glass (supplied along with the gauge) which is graduated Trim.

The receiver has a capacity of 175 mm of rain. In regions of heavy rainfall,  rain gauge with receivers of 375 mm or 1000 mm capacity may be used.

The measure glass has a capacity of 25 mm and can be read to nearest to drum.

The gauge is fixed on a masonry or concrete foundation of size the base 60 cm 6 cm which is sunk into the ground.

Into this foundation the base of  the gauge is connected as shown in Fig 5.2.

So that the rim of is exactly 30 cm above the ground level. The top of the gauge  is perfectly horizontal.

Recently, the IMD has changed over to the use of fibre glass reinforced polyester rain gauge which are an improved version of the Symon’ s gauge.

Indian standard rain gauge

These gauges are available  in  different  combinations  of collector  areas  (100  cm and  200  cm  and receiver bottle capacities (2 to 10 litres).

They have the capacity to measure rainfall depths of 100mm to l0 mm.

They conform the Indian standards IS 5225-1969. Figure 5.3 gives the details of the collector of 100 cm2 area and the receiver for I.S. gauge.

The details of l.S. gauges can be obtained from IS : 5225.

At the routine time of observation the funnel is removed, the receiver is taken out and the rain water collected in the receiver is carefully poured into the measure glass and read without any parallax error.

When the rainfall exceeds 25 mm the measure glass will be used as many times, as required.

The measured rainfall in the 24 hours ending with 8.30.A.M. is recorded as the rainfall of the day on which 8.30 A.M. observation is taken.

In regions of heavy rainfall, if it is suspected that the receiver (though of larger capacity) may not hold the entire rainfall of the day the measurements must be done more frequently with the last measurement being taken at 8.30 A.M.

The sum of all the readings taken in the last 24 hours is recorded as the rainfall of that day.

RECORDING RAIN GAUGE

Non rain gauge give the amount of rainfall only.

They cannot provide the information regarding when exactly the rain commenced, when rain ended, what is the intensity of rainfall and how the 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 continous record rainfall of with time is required.

For this purpose we have to use recording rain gauge.  Recording rain gauge usually work by having clock-driven drum carrying a graph on which a pen records the cumulative depth of rainfall continuously.

Types of Rain gauge:

Although there are different types of recording rain gauge only three have gained widespread use.

They are

• Tipping (or tilting) bucket type
• Weighing bucket type
• Float type (with siphon arrangement)

Tipping Bucket Rain gauge:

The principle involved in this this type of gauge is very simple.

A container is divided vertically into two compartments and is balanced in an unstable equilibrium about a horizontal axis.

In its normal position it rests against one of the two stoppers which prevent it from tipping over completely as shown in Fig. 5.4.

The   rain   filled   from  a   conventional   collecting   funnel   into   the  uppermost compartment  and  after  a  predetermined  rain  (usually  0.25  mm)  has  fallen  but becomes unstable in its present position and tips over to its other position of rest.

The compartments of the container are so shaped that water can now flow out of the lower one and leave it empty; meanwhile the rain falls into the upper compartment again.

The movement of buckets as it tips over can be used to operate an electric circuit and produce a record.

The record thus consists of discontinuous steps, the distance between each step representing the time taken for small amount of rain to fall.

The disadvantages of this type of gauge are as follows. If the bucket are designed to tip at a convenient frequency for a particular intensity rainfall, they will tip either too soon or too late for other intensities.

As a result both the intensity and amount of rainfall recorded will be except during a storm which has the same intensity for which the buckets are designed.

The record obtained from this gauge is not in a convenient form. For higher intensities the bucket tips so rapidly that the jogs in the record during the period. They tend to overlap and blend into one broad solid line making it difficult.

The bucket takes a small but finite time to tip over and during the first half of its motion the rain is being led into the compartment already containing the calculated  amount  of  rainfall.

This  error  is  appreciable  in  heavy  rainfalls.  For example, in a rainfall of intensity 15 cm/h the bucket tips every 6 seconds.

About 0 s is required to complete the tip. This makes the intensity of rainfall recorded by the

gauge less by 5%. Therefore the total rainfall of the day is always measured (as is done in the case of non gauge) and this data may be used to correct the error.

Owing to the discontinuous nature of the record, the instrument is not satisfactory for use in light drizzle or very light rain.

The time of beginning and ending of rainfall cannot be determined accurately. This gauge is no suitable for measuring snow without heating the collector.

Advantage:

The biggest advantage of the tipping bucket gauge is that it is the only  recording  rain gauge  which  can  be  used  in remote  places  by  installing  the recorder at a convenient and easily accessible location.

Weighing Bucket Rain gauge:

In this type of gauge the rain falling on the receiving area is collected by the funnel and, is led into a storage bucket which rests on a weighing platform.

The weight of the rainfall received since the recording began is recorded continuously by transmitting the movement of the platform through a system of links and levers to a pen which makes a trace on a suitably graduated chart secured around a drum as shown in Fig. 5.5.

The drum is driven mechanically by a spring clock. The drum may be made to revolve once a day, once a week or in any other desired period.

This type of gauge normally has no provision for emptying itself.

To overcome this difficulty the mechanism may be arranged to reverse the of the pen alter certain amount  of  precipitation  has  accumulated  and  reverse  again  after  another  equal amount so that the gauge may operate unattended  for a week at a time except in regions of very intense precipitation which may exceed the capacity of the gauge.

A typical chart of rainfall record of a day from the weighing-bucket type rain gauge with reverse mechanism in action is shown in Fig. 5.6.

In this chart the pen had reversed its path of travel (from upward movement to downward movement) at 3.40 h of the next day after recording 100 mm of rainfall.

Therefore, for instance, the cumulative rainfall recorded by the gauge at say 5 h of the next day is 100 + 30 =130 mm. The bucket is  set to zero whenever the chart is changed.

The main usefulness of this type of gauge is that it can record snow, hail and mixture of rain and snow. All forms of precipitation are weighed and recorded automatically.

Disadvantages:

The effects of temperature and friction on weighing mechanism may introduce errors in the record, ( shrinkage and expansion of the chart paper caused by changes in humidity may distort the time and the scale of rainfall Failure of reverse mechanism results in the loss of record.

The last difficulty may be eliminated by using only a single traverse of the pen but by reducing the scale of record graph.

Float Type Rain gauge:

This type of rain gauge is also known as the siphon rain gauge as it uses the siphon mechanism to empty the rain water colleted in the float chamber.

This is adopted by LM.D. The construction  of this type of rain gauge are shown in Fig. 5.7. Rain water entering the gauge at the top is led into the float chamber through a funnel and filter.

The purpose of the filter is to prevent dust and other Particles from entering the float chamber which may hinder the siphon mechanism.

The  float  chamber  consists,  of  a  float  with  a  vertical  stem  producing outside, to the top of which a pen is mounted.

This pen rests on a chart secured around a clock driven drum.

There is a small compartment by the side of the float chamber which is connected to the float chamber through a small opening at the bottom.

This is called the siphon chamber which houses a small vertical pipe with bottom end open and the top end almost touching the top of the chamber.

During the storm the rain water collected in the float chamber raises the water surface in it and along with the water surface the float also rises enabling the pen to make a trace cumulative depth of rain fall on the chart.

When the float chamber is completely filled with water, the pen reaches the top of the chart.

At this instant the siphoning occurs automatically through the pipe in the siphon chamber, he float chamber is emptied and the pen is brought to zero the chart again.

As the rainfall continues the pen rises again from the zero of the chart.

The complete siphoning should be over in less than 15 seconds of time.

This gauge cannot record precipitation in the form other than rain unless some sort of heating device is provided inside the gauge.

The float may be damaged if the rainfall catch freezes.

A chart from a float typical rain gauge with siphoning taking place during the storm is shown

Chart indicates that the gauge has siphoned once at 1:30 h of the next day.

Thus the cumulative depth of rainfall recorded by the gauge at 5 h of the next day, for example, is 10 + 4 = 14 mm.

If the rainfall is of large intensity, the siphoning may occur more than once during the period of the chart.

Other links:

HYDROLOGIC CYCLE
PRECIPITATION
EVAPORATION
INFILTRATION
GROUNDWATER
Water Table
AQUIFER PROPERTIES
DARCY’S LAW
FLOOD FREQUENCY STUDIES
RECURRENCE INTERVAL
GUMBEL’S METHOD
FLOOD ROUTING
EVAPORIMETER

 

PRECIPITATION

The term precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew. Of all these, only the first two contribute significant amounts of water.

Rainfall being the predominant form of precipitation causing stream flow, especially the flood flow in a majority of rivers in India, unless otherwise stated the term rainfall is used in this book synonymously with precipitation.

The   magnitude   of   precipitation   varies   with   time   and   space. Differences in the magnitude of rainfall in various parts of a country at a given time and variations of rainfall at a place in various seasons of the year are obvious and need no elaboration.

It is this variation that is responsible for many hydrological problems, such as floods and droughts. The study of precipitation forms a major portion of the subject of hydrometeorology.

In this chapter, a brief introduction is given to familiarize the engineer with important aspects of rainfall and, in particular, with the collection and analysis of rainfall data.

For precipitation to form:

The atmosphere must have moisture

There    must   be   sufficient   nucleii   present    to   aid condensation

Weather conditions must be good for condensation of water  vapour to take place

The products of condensation must reach the earth.

Under proper weather conditions, the water vapour condensed over nucleii to form tiny water droplets of sizes less than 0.1 mm in diameter. The nucleii are usually salt particles or products of combustion and are normally available in plenty.

Wind speed facilitates the movement of clouds while its turbulence retains the water droplets in suspension. Water droplets in a cloud are somewhat similar to the particles in a colloidal suspension. Precipitation results when water droplets come together and coalesce to form larger drops that can drop down.

A considerable part of this precipitation gets evaporated back to the atmosphere. The net precipitation at a place and its form depend upon a number of meteorological factors, such as the weather elements like wind, temperature, humidity and pressure in the volume region enclosing the clouds and the ground surface at the given place.

FORMS OF PRECIPITATION

Some of the common forms of precipitation are:

Rain, Snow, Drizzle, Glaze, Sleet, Hail

Rain

It is the principal form of precipitation in India. The term rainfall is used to describe precipitations in the form of water drops of sizes larger than 0.5 mm. The maximum size of a raindrop is about 6 mm. Any drop larger in size than this tends to break up into drops of smaller sizes during its fall from the clouds. On the basis of its intensity,. rainfall is classified as

Snow

Snow is another important form of precipitation. Snow consists of ice crystals which.  usually combine  to  form flakes.  When  new,  snow  has an  initial density varying from  0.06 to 0.15 g/cm and it is usual to assume an average density of 0.1 g/cm In Jndia, snow occurs only in the Himalayan regions.

Drizzle

A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/h is known as drizzle. In this the drops are so small that they appear to float in the air.

Glaze

When rain or drizzle comes in contact with cold ground at around 00  C,the water drops freeze to form an ice coating called glaze or freezing rain.

Sleet

It is frozen raindrops of transparent grains which form when rain falls through air at subfreezing temperature. In Britain, sleet denotes precipitation of snow and rain simultaneously.

Hall

It is a showery precipitation in the form of irregular pellets or lumps of ice of size more than 8 mm. Hails occur in violent thunderstorms in which vertical currents are very strong.

TYPES OF PRECIPITATION

Anticyclones

These are regions of high pressure, usually of large area extent. The weather is usually calm at the centre. Anticyclones cause clockwise wind circulations in the northern hemisphere.

Winds are of moderate speed, and at the outer edges, cloudy and precipitation conditions exist.

Convective Precipitation

In this type of precipitation a packet of air which is warmer than the surrounding air due to localized heating rises because of its lesser density. Air from cooler surroundings flows to take up its place thus setting up a convective cell.

The warm air continues to  rise,  undergoes cooling  and  results  in precipitation.  Depending  upon  the  moisture,  thermal  and  other  conditions  light

showers to thunderstorms can be expected in convective precipitation. Usually the area extent of such rains is small. being limited to a diameter of about 10 km.

Orographic Precipitation

The moist air masses may get lifted-up to higher altitudes due to the presence Of mountain barriers and consequently undergo cooling, condensation and precipitation Such a precipitation is known as Orographic precipitation.

Thus in mountain  ranges  the  windward  slopes  have  heavy precipitation  and  the  leeward slopes light rainfall.

Other links:

HYDROLOGIC CYCLE
RAIN GAUGE
EVAPORATION
INFILTRATION
GROUNDWATER
Water Table
AQUIFER PROPERTIES
DARCY’S LAW
FLOOD FREQUENCY STUDIES
RECURRENCE INTERVAL
GUMBEL’S METHOD
FLOOD ROUTING
EVAPORIMETER

 

HYDROLOGIC CYCLE

HYDROLOGIC CYCLE definition and the process are explained in detail below.

Definition of  HYDROLOGIC CYCLE

HYDROLOGIC CYCLE is the series of conditions through which water changes from vapor in the atmosphere through precipitation upon land surface or water surfaces and ultimately back into the atmosphere as a result of evaporation and transpiration.

Water occurs on the earth in all its three states, viz. liquid, solid and gaseous, and in various degrees of motion.

Evaporation of water from water bodies such as oceans and lakes, formation and movement of clouds, rain and snowfall, stream flow and ground water movement are some examples of the dynamic aspects of water.

The various aspects of water related to the earth can be explained in terms of a cycle known as the hydrologic cycle.

HYDROLOGICAL CYCLE

Figure 1 is a schematic representation of the hydrologic cycle.

A convenient starting point to describe the cycle is in the oceans. Water in the oceans evaporate due to the heat energy provided by solar radiation.

The water vapour moves upwards and forms clouds. While much of the clouds condense and fail back to the oceans as rain, a part of the clouds is driven to the land areas by winds.

There they condense and precipitate onto the land mass as rain, snow, hail, sleet, etc.

A part of the precipitation may evaporate back to the atmosphere even while falling.

Another part may be intercepted by vegetation, structures and other such surface modifications from which it may be either evaporated back to atmosphere or move down to the ground surface.

A portion of the water that reaches the ground enters the earth’ s surface.through infiltration, enhance the moisture content of the soil and reach the groundwater body.

Vegetation sends a portion of the water from under the ground surface back to the atmosphere through the process of transpiration.

The precipitation reaching the ground surface after meeting the needs of infiltration and evaporation moves down the natural slope over the surface and through a network of gullies, streams and rivers to reach the ocean.

The groundwater may come to the surface through springs another outlets after spending a considerably longer time than the surface flow.

The portion of the precipitation which by a variety of paths above and below the surface of the earth reaches the stream channel is called runoff.

Once it enters a stream channel runoff becomes stream flow.

The sequence of events as above is a simplistic picture of a very complex cycle that has been taking place since the formation of the earth.

It is seen that the hydrologic cycle is a very vast and complicated cycle in which there are a large number of paths of varying time scales.

Further, it is a continuous recirculating cycle in the sense that there is neither a beginning nor an end or a pause.

Path of the hydrologic cycle

Each path of the hydrologic cycle involves one or more of the following aspects

• Transportation of water
• Temporary storage
• Change of state.

For example

• The process of rainfall has the change of state and transportation
• The groundwater path has storage and transportation aspects

The quantities of water going through various individual paths of the hydrological cycle can be described by the continuity equation known as water budget equation or hydrologic equation.

The  hydrological  cycle  has  important  influences  in  a  variety  of  fields including  agriculture,  forestry,  geography,  economics,  sociology  and  poitical scene.

Engineering applications of the knowledge of the hydrologic cycle, and hence of the subjects of hydrology, are found in the design and operation of projects dealing with water supply, irrigation and drainage, water power, flood control, navigation, coastal works, salinity control and recreational uses of water.

Other links:

PRECIPITATION
RAIN GAUGE
EVAPORATION
INFILTRATION
GROUNDWATER
Water Table
AQUIFER PROPERTIES
DARCY’S LAW
FLOOD FREQUENCY STUDIES
RECURRENCE INTERVAL
GUMBEL’S METHOD
FLOOD ROUTING
EVAPORIMETER