rainfall climatology of rtff region chennai region

www.jbaconsulting.com

Rainfall Climatology of RTFF Region

Chennai Region –

Rainfall Climatology and Recent Rainfall

Trends

Y.E.A.Raj(Retd) Deputy Director General of Meteorology & Head

Regional Meteorological Centre Chennai

&

Meteorological Consultant

Real Time Flood Forecasting Project for Chennai

( TN Govt & SECON & JBA)

For the IMS one day seminar on Chennai Water

3 August 2019


We consider 3 districts collocated with Chennai city viz Chennai, Kanchipuram and

Tiruvallur as Chennai Region . The dams/lakes which supply water to Chennai City

and the catchment areas of such dams/rivers are located within these districts

The Nungambakkam and Meenambakkam observatories represent Chennai city &

Suburbs

The normal rainfall of these 3 districts & 2 observatories for all the 4 seasons are

presented seasonwise.

Inter annual variation of rainfall of Nbk & Mbk for the 50 year period 1969-2018

presented by means of charts

Trend analysis of Nbk & Mbk RF . Difference of rainfall means of I & II halves

analysed

Breaching of several RF records for Chennai Nbk & Mbk in recent years highlighted

Rainfall of the 3 districts & Nbk&Mbk for the years 2015,16,17,18 presented and

discussed

Importance of Evaporation data while interpreting the rainfall data

Outline of the presentation


Geographical extent of the region


Geography of rivers and water bodies


5 Basins of Chennai Region


India has four seasons. These are:

Seasons of India

Name Duration

Winter Jan-Feb

Pre Monsoon /

Summer

Mar-May

Monsoon Jun-Sep

Post / North East

Monsoon

Oct-Dec


For Tamil Nadu the most important rainfall season is the North East

monsoon season of Oct-Dec

However the South west monsoon season of Jun-Sep is also important

in the northern parts of Tamil Nadu and the Chennai Region as defined

above

The North East monsoon sets in over Tamil Nadu around 20 Oct which is

the normal date of onset with a Standard deviation of 7-8 days

The withdrawal takes place in the last week of Dec ( 30 Dec) with a std

deviation of 12-13 days

North East monsoon


Normal seasonal rainfall (in mm) of a few stations ( Source IMD, 1951-2000 normals)

situated in the Chennai Region

Normal Rainfall – Stations

Station Dist JF MAM JJAS OND Annual

Nungambakkam Chennai 38.4 65.0 442.6 856.7 1402.7

Minambakkam Kanchi 42.1 68.6 487.8 777.7 1376.2

Chengalpattu Kanchi 28.7 58.3 461.7 622.7 1171.4

Sriperumbudur Kanchi 24.5 71.1 459.7 637.1 1192.4

Kanchipuram Kanchi 22.1 80.8 558.4 531.9 1193.2

Tiruthani Tiruvallur 29.2 79.4 461.9 441.8 1012.3

R.K.Pet Tiruvallur 12.5 76.5 406.9 397.9 893.8

Poondi Tiruvallur 39.6 67.0 538.6 591.5 1236.7

RedHills Tiruvallur 39.0 62.7 404.8 750.5 1257.0

Ponneri Tiruvallur 39.0 55.2 427.5 757.1 1278.8

Arakkonam Vellore 23.9 79.9 504.8 509.5 1118.1


Normal Rainfall - Districts

Normal seasonal rainfall (in mm) of Chennai, Kanchi & Tiruvallur

districts

Data based on 1951-2000 IMD RF in mm

JF- Jan-Feb, MAM- Mar-Apr-May etc

Dist JF MAM JJAS OND Annual

Chennai 36.7 58.5 439.1 789.9 1324.2

Kanchipuram 29.1 66.0 490.8 641.8 1227.7

Tiruvallur 31.5 67.2 451.6 589.3 1139.6


Normal Rainfall - Districts

Normal seasonal rainfall (in mm) of Chennai, Kancheepuram & Tiruvallur districts

Data based on 1951-2000 IMD

JF- Jan-Feb, MAM- Mar-Apr-May etc.

Normal Annual Rainfall = 1324.2 mm Normal Annual Rainfall = 1227.7 mm

Normal Annual Rainfall = 1139.6 mm


Lon variation of cum rainfall over Chennai & Nbhd

During NE monsoon – DWR estimated RF


Inferences from the Tables & Figures

Normal rainfall of RG stations in the Chennai Region varies from:

13-42 mm in winter (JF)

58-111 mm in Summer (MAM)

405-556 mm during SW monsoon season (JJAS)

398-857 mm during NE monsoon season (OND)

894-1403 mm for the entire year (Jan-Dec)

The North East monsoon seasonal rainfall is heaviest along the coast

and decreases westwards inland

Chennai Nungambakkam has the highest Oct-Dec normal RF of 857 mm

and highest annual RF of 1403 mm

Inferences


Geographical location- Nbk & Mbk


Rainfall – Chennai Nbk and Mbk

Normal seasonal rainfall (in mm) and statistics of Nungambakkam observatory

based on 1969-2018 data ( 50 years)

CV- Coeff of Variation NRD – No of rainy days

Station JF MAM JJAS OND Annual

Nbk Mean mm 33.9 58.0 461.8 840.0 1393.6

CV % 208.1 144.2 37.0 41.6 27.2

0-398.7 0-423.7 197.2-

1178.3

311.5-

2105.7

738.1-

2562.8

Range mm

NRD 1.6 2.7 26.2 27.3 57.8


Rainfall – Chennai Nbk and Mbk

Normal seasonal rainfall (in mm) and statistics of Meenambakkam observatory

based on 1969-2018 data ( 50 years)

CV- Coeff of Variation NRD – No of rainy days

Station JF MAM JJAS OND Annual

Mbk Mean mm 36.1 71.5 484.4 850.0 1442.1

CV % 175.1 118.5 28.6 38.2 23.0

0-322.6 0-363.1 176.0-

772.4

350.3-

1879.1

873.1-

2453.6

Range mm

NRD 1.9 3.0 27.6 27.3 59.7


Rainfall – Chennai NBK and MBK

Normal seasonal rainfall (in mm) and statistics of Nungambakkam (NBK) observatory based on

1969-2018 data ( 50 years)

Annual Mean Rainfall = 1393.6 mm


Rainfall – Chennai NBK and MBK

Normal seasonal rainfall (in mm) and statistics of Meenambakkam (MBK) observatory based on

1969-2018 data ( 50 years)

Annual Mean Rainfall = 1442.1 mm


Nbk rainfall JJAS & OND 1969-2018


Nbk rainfall Annual 1969-2018


Mbk rainfall JJAs & OND 1969-2018


Mbk rainfall annual 1969-2018


Nbk and Mbk RF Trend


Inferences from the Tables

When rainfall of Nbk and Mbk observatories are considered several

Rainfall records have been breached recently. Details…

Nbk observatory (1901-1943 & 1951 to now)

Nbk JJAS 1961 712.5 mm

1996 1178.3

Nbk OND 1913 1417.4 mm

1997 1570.7

2005 2105.7

Nbk Annual

1903 1946.2 mm

1996 2438.6

2005 2562.8

Breaching of Rainfall Records


Inferences from the Tables

When rainfall of Nbk and Mbk observatories are considered several

Rainfall records have been breached recently. Details…

For Meenambakkam ( Opened in 1943)

Mbk OND

1946 1546.8 mm

2005 1859.9

2015 1879.1

Mbk Annual

In 1977 1798.1 mm,

1985 2214.0

2005 2394.8

2015 2453.6

Breaching of Rainfall Records


Inferences from the Rainfall Normals, 1969-2018 RF data

and from trend analysis

The 3 districts of Chennai Region receive annual rainfall of 115-135 cm which is

quite a decent figure

Both the observatories in Chennai City receive nearly 139-144 cm of annual

rainfall

At Nbk Annual RF increased from 134 to 144 cm an increase of 10 cm during

the recent 25 years compared to 25 years before

Increase in Mbk is from 142 to 146 cm

These increases are welcome for a water stressed city such as Chennai

Statements such as Chennai does not receive any rain, Chennai

hardly receives any rain, Chennai rainfall is decreasing etc are

obviously fallacious and unfounded

Infrences


Less than 100 cm of annual rainfall

Nbk received less than 100 cm annual rainfall in 50

years 1969-2018:

1973,1974,1982,1986,1998, 2003 & 2018 ( Freq 7/50 years)

Mbk received less than 100 cm annual rainfall in

1974,1982 and 2003 ( Freq 3/50 years)


2015-2018 rainfall

In the forth coming slides we present the rainfall figures

Both actual and percentage departure from normal of

Chennai, Kanchipuram and Tiruvallur districts

and also of

Chennai Nbk & Mbk observatories

for each year of 2015-19 for JJAS , OND seasons & Annual


2015 Rainfall

Rainfall of Chennai , Kanchi & Tiruvallur Districts JJAS , OND &

Annual in mm

JJAS : Jun-Sep, OND: Oct-Dec, Annual :Jan-Dec RF in mm

A- Actual PDN- % departure from normal

Year Dist JJAS

A

JJAS

P

OND

A

OND

PDN

Ann

A

Ann

PDN

2015 Chennai 370.0 -16 1614.5 104 2057.6 55

2015 Kanchi 345.0 -30 1815.1 183 2256.6 84

2015 Tiruvallur 455.1 1 1469.9 149 2019.2 77


2016 Rainfall


Annual in mm



Year Dist JJAS

A

JJAS

P

OND

A

OND

PDN

Ann

A

Ann

PDN

2016 Chennai 495.8 13 342.1 -57 1036.6 -22

2016 Kanchi 484.2 -1 334.8 -48 992.5 -19

2016 Tiruvallur 406.8 -10 247.8 -58 814.3 -29


2017 Rainfall


Annual in mm



Year Dist JJAS

A

JJAS

P

OND

A

OND

PDN

Ann

A

Ann

PDN

2017 Chennai 449.8 2 937.8 19 1393.8 5

2017 Kanchi 493.9 1 672.5 5 1197.2 -2

2017 Tiruvallur 503.3 11 677.9 15 1203.1 6


2018 Rainfall


Annual in mm



Year Dist JJAS

A

JJAS

P

OND

A

OND

PDN

Ann

A

Ann

PDN

2018 Chennai 395.3 -10 353.6 -55 754.9 -43

2018 Kanchi 397.0 -19 417.1 -41 833.0 -32

2018 Tiruvallur 364.3 -20 343.2 -42 733.4 -35


2015-2019 Rainfall of Chennai

Rainfall ( in mm) of Chennai , Nungambakkam and

Meenambakkam

Nbk Nbk Nbk Mbk Mbk Mbk

Year JJAS OND Ann JJAS OND ANN

2015 407.7 1663.9 2094.6 427.9 1879.1 2453.6

2016 526.3 324.6 1060.7 522.5 459.1 1228.8

2017 508.7 978.5 1494.8 638.2 960.6 1609.5

2018 432.3 388.7 826.8 441.1 563.9 1018.2


2015-2018 Rainfall of Chennai

Rainfall ( in mm) of Chennai , Nungambakkam and Meenambakkam


Inferences from the Rainfall figures of 2015-18 for

the 3 Districts

2015 Below normal SWM rain followed by exceptionally heavy NE

monsoon rain

2016 Poor NE monsoon rain but preceded by normal SWM rain

2017 Normal SWM rain , good NEM rain, annual rainfall normal

2018 SWM below normal & NE Monsoon failure. Annual RF between

73- 83 cm

2015-18 rainfall figures


Inferences from the Rainfall figures of 2015-18 for

Chennai City Nbk and Mbk

2015 Normal SWM rain followed by exceptionally heavy NE monsoon

rain resulting in flooding & inundation

2016 Good SWM rains followed by NEM failure.

Still both the observatories received 100+ cm of annual rainfall

2017 Good SWM rain followed by good NEM rain 150-160 cm annual

rainfall

2018 SWM Near normal & NE Monsoon failure. Still annual RF

crossed 100 cm at Mbk

2015-18 rainfall figures


Evaporation ( Pan ) over India – Chennai region 200 cm annual


RWH in all constructions especially in residential/coomercial projects

carried out in more than 10 grounds area ( i.e 24000 sq ft)

In larger housing projects recycling of waste water to be made

mandatory. Recycled water could be used for toilet flushing and

gardening

Proper management of Lakes and rivers and also streams taking water

to dams, by removing encroachments

Is it possible to reduce Evaporation by deepening of large lakes and

dams and reducing the surface area without reducing the total

storage?

Recommendations OR points for discussion


Thank You!

rainfall climatology of rtff region chennai region

Documents