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Trends in extreme events of rainfall in low country wet zone of Sri Lanka

HERATH H.M.L.K.SIHILELLA J.P.W.P. ABEYSINGHE D.C.DHANUSHIKA M.P.

Introduction

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• Climate is changing

• Climate change affects both growth and the production of plants

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• Rubber and tea are main revenue earners among plantation crops in Sri Lanka

• Most of rubber plantation and Low grown tea plantations; Mainly confined in low country wet zone region of the Island

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• Both rubber and tea plantations have direct relationships with the climate

• Require well spread-out rainfalls Tea - 1000 to 1250mm

Rubber - 1650 to 3000mm

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6

Changes in the amount and the distribution of rainfall in Sri Lanka are not very apparent Jan Fe

bMar Apr

May Jun JulAug

Sep

Oct NovDec

0

100

200

300

400

500

600

Rain

fall

(mm

)

7

Variability in climatic parameters: Variability due climate change may overwhelmed by the random variability

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

15002000250030003500400045005000

Year

Rain

fall

(mm

)

Objective

to identify trends in extreme events of rainfall in low country wet zone of the Island

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Materials and Methods

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Daily rainfall data from 8 Agro meteorological stations for the period of 1941-2010 were used

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Agro ecological region, elevation, latitude and longitudes of Agro meteorological stations under investigation

Agro- met station

AER Elevation (m) Latitude Longitude

Awissawella WL1 134 6057’11’’N 80013’6’’E

Dartonfield WL1 152 6031’0’’N 80010’0’’E

Ratnapura WL1 259 6058’N 80046’E

Ambanpitiya WL2 297 7014’0’’N 80020’0’’E

Galle WL3 105 7046’N 80017’E

Hanwella WL4 81 6054’23’’N 8005’23’’E

Kalutara WL4 15 6035’24’’N 79057’36’’E

Matara WL4 2 14041’3’’N 39024’52’’E

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At each location,

• Annual, monthly and weekly rainfalls

• Number of rainy daysA rainy day is defined as a day receiving ≥0.5mm rainfall

• Wet spells

• Dry spells 12

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• Extreme events were identified using the boundary line technique

• Regression techniques to identify trends

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Boundary Line Approach

Upper boundary

Lower boundary

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Results and Discussion

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Amount and the distribution of rainfall Linear trend and the respective R2 values of total

annual rainfalls at each location

Location Trend R2 valueAmbanpitiya -10.43 0.155Awissawella -3.99 0.023Dartonfield -4.334 0.011Galle -7.127 0.101Hanwella -1.705 0.005Kalutara -1.33 0.003Matara -5.825 0.073Ratnapura -3.166 0.017

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Trends of all the locations were not significant

But the signs of the trends were negative

The temporal variations of daily rainfall variability were random in all the locations

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Temporal variation of annual rainfall in Ambanpitiya

19411946

19511956

19611966

19711976

19811986

19911996

20012006

0500

1000150020002500300035004000

Ambanpitiya

Linear (Ambanpitiya)

Year

Rain

fall

(mm

)

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Location Trend R2

Ambanpitiya -65.8 * 0.35

Awissawella -29.2 0.07

Dartonfield -18.7 0.03

Galle -26.6 * 0.24

Hanwella -12.0 0.05

Kalutara 1.2 0.00

Matara -34.4* 0.17

Ratnapura 10.3 0.06

Boundary line analysis on annual rainfall

Trends and the R2 values for the lower boundary

* Significant at p<0.05

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Temporal variations of lower boundary of annual rainfall in Ambanpitiya

1943

1948

1953

1958

1963

1968

1973

1978

1983

1988

1993

1998

2003

2008

0

500

1000

1500

2000

2500

3000

Minimum Linear trend

year

Rain

fall

(mm

)

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The temporal variation of maximum and minimum occurrences of annual rainfalls events in some locations were statistically significant

This is an indication of having some years with comparatively low annual rainfalls

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Temporal variations in wet and dry days

Location Trend R2

Ambanpitiya -1.026*** 0.54Awissawella -0.611*** 0.22Dartonfield -0.211 0.05Galle -0.327** 0.156Hanwella -0.458** 0.142Kalutara -0.524** 0.132Matara -0.827*** 0.421Ratnapura -0.004 0.001

** Significant at p<0.01, ***Significant at p<0.001

Linear trend and the respective R2 values of wet days at each location

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19411945

19491953

19571961

19651969

19731977

19811985

19891993

19972001

20052009

0

50

100

150

200

250

Wet days Linear trend

Year

No.

of W

et d

ays

Temporal variation of annual wet days together with its linear trend in Ambanpitiya

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Trends in lower and upper boundary of annual wet days were statistically significant

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Upper boundary line the trend fitted to annual wet days in Ambanpitiya

1943

1948

1953

1958

1963

1968

1973

1978

1983

1988

1993

1998

2003

2008

0

50

100

150

200

250Upper boundary Linear trend

Year

No.

of w

et d

ays

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lower boundary line the trend fitted to annual wet days in Ambanpitiya

1943

1948

1953

1958

1963

1968

1973

1978

1983

1988

1993

1998

2003

2008

020406080

100120140160

Lower boundary Linear trend

Year

No.

of w

et d

ays

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significant decline in number of wet days

A risk of aggregating heavy showers

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Temporal variation of maximum dry run

Location Trend R2

Ambanpitiya 0.58** 0.35Awissawella 0.36** 0.31Dartonfield -0.41 0.12Galle 0.08 0.01Hanwella 0.38** 0.26Kalutara 0.25 0.07Matara 0.73** 0.40Ratnapura 0.14* 0.13

* Significant at p<0.05, **Significant at p<0.01

Linear trend and the respective R2 values of annual maximum dry run in each location

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1941

1945

1949

1953

1957

1961

1965

1969

1973

1977

1981

1985

1989

1993

1997

2001

2005

2009

010203040506070 Max. dry run

Linear trend

Year

Max

imum

dry

run

(day

s)

Temporal variation of maximum dry run with its linear trend in Ambanpitiya

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Risk of having lengthy dry spells in some locations in the Low country wet zone

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No significant change in annual rainfall (Cumulative rainfall)

Significant tendency of having incidences of aggregated heavy showers in the low country wet zone

Signs of lengthy dry spells

Conclusions

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Adverse impacts on some operations like crop establishment and fertilizer application

Favorable for latex exploitation

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Acknowledgements• Director of the Rubber Research Institute of Sri Lanka

(RRISL)

• Dr. (Mrs.) Wasana Wijesekara, Head of the Biometry Section of RRISL

• Mr. Vidura Abeywardena, technical officer of the Biometry Section of RRISL

• Department of Meteorology

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THANK YOU