SUBMITED BY:-

1) Nikhil Holsamudrkar (BE11F01F017)

2)Suresh Hatkar(BE11F01F015)

3) Swapnil Dhakane(BE11F01F013)

4) Mamta Ingole(BE11F01F018)

GOVT.COLLEGE OF ENGINEERIG

AURANGABAD.

GUIDED BY:-PROF .K. A.PATIL

Topic :- MESUREMENT OF PRECIPITATION

PRECIPITATIONIntroduction

PRECIPITATION

All forms of water that reaches earth’s surface is known as precipitation.

It is expressed in terms of depth to which rainfall water would stand on an area if all the rain were collected on it.

In case of snowfall equivalent depth of water is considered as depth of precipitation.

Rain gauges are used for measurement of precipitation.

PRECIPITATION

In India ‘Indian Meteorological Department (IMD)’ is responsible for all weather and rainfall predictions.

It occurs due to:

1. Lifting of air mass2. Cooling of warm air3. Condensation

PRECIPITATION

Lifting of air occurs mainly due to three causes:

1. Cyclonic precipitation:- It is caused by lifting of an air mass due to pressure difference.

2. Convective precipitation:- It is caused due to the upward movement of air that is warmer than it’s surroundings. Generally this kind of precipitation occurs in tropics.

PRECIPITATION

3. Orographic precipitation:- It is most important precipitation and responsible for most heavy rains in India. It is caused by air masses which strike some natural topographic barriers such as mountains and can’t move forward hence rise up, causing condensation and precipitation.

RAINGAUGES

1. Recording type2. Non recording type

Most rain gauges used in India are recording type i.e. Symon's raingauge.

Recordings are taken at 8:30 am And if rainfall is more then intermediate

readings are taken at 5:30pm

PRECIPITATIONMeasurement, Estimation and Probability

PRECIPITATION DATA

Necessary for various fields Municipal Industrial Agricultural Forestry Flood prevention Recreation

1) Nonrecording gauge:-Symons’ Raingauge Extensively use in

India

Accuracy 0.1mm

At 8.30am

Capacity is10cm

Incase of Heavy rainfall

Low Maintenance

2)RECORDING RAIN GAGES

Weighing bucket type

Tipping bucket type

Natural-syphons type

TIPPING BUCKET TYPE 30.5 cm size as per us weather bureau.

water collect from Tip bucket to storage tank

least count of 1 mm and gives out one electrical pulse for every millimeter of rainfall

Electric circuit

TIPPING BUCKET TYPE

WEIGHING BUCKET TYPE

Weighing bucket typeIt consists of a storage

bin, which is weighed to record the mass. It weights rain or snow which falls into a bucket, set on a platform with a spring or lever balance. The increasing weight of the bucket and its contents are recorded on a chart. The record shows accumulation of precipitation.

FLOAT RECORDING GAUGES

FLOAT RECORDING GAUGES

RAINGAUGE NETWORK

Since the catching area of the raingauge is very small as compared to the areal extent of the storm, to get representative picture of a storm over a catchment the number of raingauges should be as large as possible, i.e. the catchment area per gauge should be small.

There are several factors to be considered to restrict the number of gauge: Like economic considerations to a large extent Topography & accessibility to some extent.

MINIMUM DENSITY OF RAINGAUGES ACCORDING TO IS 4987-1968

In plains : 1 station per 520 km2

In regions of avg. elevation of 1000m : 1 station per 260-390 km2

In predominantly hilly areas with heavy rainfall : 1 station per 130 km2

10% of total should be self recording raingauges

ADEQUACY OF RAINGAUGE STATIONS

RAINFALL ON A WATERSHED SCALE

3 common methods for estimating average rainfall.

1. Arithmetic Mean2. Thiesson

polygon method3. Isohyetal

method

i

ii

W

RWR

P2 = 2.15”P4 = 2.26”

P6 = 1.81”

P5 = 2.18”

P3 = 1.80”

P1 = 1.62”

Watershed boundary

Measured Rainfall at Six Rainfall Gages

ARITHMETIC MEAN METHOD

Pavg = [ Wi x Pi ] / Wi All gages given equal weight

Weight = 1 Pavg = (1.82 + 2.15 + 2.26 + 2.18 + 1.62 +

1.8) / 6 Pavg = 1.97 in.

THIESSEN POLYGON METHOD

First: Draw straight dashed lines between each rainfall gage

Second: Draw solid perpendicular bisectors to these lines so that watershed area associated with each gage is enclosed by bisector lines These enclosed areas are known as Thiessen

Polygons The area within each polygon is closer to the

rain gage enclosed than any other rain gage. The rainfall measured in the polygon is

assumed to be representative of the rainfall in the entire polygon

THIESSEN POLYGON METHOD

Third: Determine the area of each polygon The rain gage weight is the area of the polygon it is

located in Fourth: Calculate the average rainfall using:

Pavg = [ Wi x Pi ] / Wi

P2 = 2.15”

P4 = 2.26”

P6 = 1.81”

P5 = 2.18”

P3 = 1.80”

P1 = 1.62”

Watershed boundary

Step #1: Dashed Lines Between Each Rain Gauge

Watershed boundary

Step #2: Draw the Perpendicular Bisector Lines

Watershed boundary

Step #3: Determine the Area of Each Polygon

A2= 150 ac

A3= 136 ac

A4= 269 ac

A5= 216 ac

A6= 65 ac

A1= 56 ac

STEP #4: CALCULATE THE AVERAGE RAINFALL

Pavg = [ Wi x Pi ] / Wi

Pavg = [(65x1.81)+(150x2.15)+(269x2.26)+ (216x2.18)+(56x1.62)+(136x1.8)] / [65+150+269+ 216+56+136]

Pavg = 2.08 in.

ISOHYTAL METHOD

Plot gauge locations on map; Subjectively interpolate between rain

amounts between gauges at a selected interval;

Connect points of equal rain depth to produce lines of equal rainfall amounts (isohyets);

 

CALCULATION OF AVERAGE RAINFALL OVER CATCHMENT

COMPARISON BETWEEN METHODS FOR CALCULATING AVERAGE RAINFALL

Arithmetic mean method Assumes uniform rainfall distribution Very seldom occurs Easiest to use but least accurate

Thiessen polygon method Assumes linear variation Use when gages are not uniformly distributed Can use gages outside of watershed

Isohyetal method Theoretically the most accurate Most time consuming method Can use gages outside of the watershed

DAD CURVES

DAD stands for Depth Area Duration curve.

DAD curves exhibit the depth and the area covered by the rainfall with a particular duration.

There is a definite relation among depth, area and duration of rainfall.

The longer duration rainfall covers a wider area. Short time rainfalls normally cover small areas.

Rainfall rarely occurs uniformly over a large area.

A depth-area-duration curve expresses graphically the relation between progressively decreasing average depth of rainfall over a progressively increasing area from the center of the storm outward to its edges for a given duration of rainfall.

Purpose of DAD analysis of a particular storm is to determine the largest average depth of rainfall that fell over various sizes of area during the standard passage of time.

hydrologists and engineers require techniques whereby point rainfall amounts can be transformed to average rainfall amounts over a specified area

DAD CURVE FOR ONE DAY RAINFALL OVER THE AREA 5000 KM2

FREQUENCY OF THE RAINFALL

the frequency of the rainfall is the number of time that a given magnitude of the rainfall may occur in a given period.

The study of the probability of the occurrence of a particular extreme (such as 24-h maximum rainfall ) is of extreme important to determination of the design flood.

The probability of an event bring equaled by the following formulae

•California formula : Pro = m/N

•Hazen formula : Pro = 2m-1/2N

•Weibull formula : Pro = m/N+1

Where N= no of years of recordPro = probability

REFERENCES

• Introduction to Physical Hydrology, Martin R. Hendricks

• Hydrology and Floodplain Analysis, Bedient, Huber and Vieux

• National Geographic Magazine