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Impacts of time trends in colombo rainfall pattern
on design rain events


A.K.D.Y. Abeywickrama

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University of Moratuwa, Sri Lanka.

Dr. T.M.N. Wijayaratna

University of Moratuwa, Sri Lanka














Keywords: time trends; Colombo rainfall; pattern
analysis, non-parametric methods; design rain           events



Sri Lanka is an island
which is extends between 5°55? to 9°51?N and 79°42? to 81°53?E and rice
cultivation plays a major role in country’s economy as in the case of most
countries in South and Southeast Asia. Irrigated agriculture contributes to 22%
of Sri Lankan exports while 75% of exports are powered from the national grid
electricity which 65% is generated via hydropower. So as country which its economy is heavily depends on the rainfall and the
availability of water resources, studies in rainfall pattern analysis is really
important for Sri Lanka. Also these pattern analysis is far more
important to manage, to plan and to predict the rainfall related adverse
effects such as high floods due to excessive rainfall and more accurate
rainfall forecasting will come in handy in crop cultivation managing. Design of
hydraulic structures such as dams, spillways, culverts, sluice gates etc. also
depend on the time trends of rainfall pattern. Even though the country has a
considerably high average rainfall, due to its seasonal and spatial variability
there is some shortage of water for agricultural, hydropower and domestic use
from time to time. Since this is a tropical country, air temperature doesn’t
varies much throughout a year, except in the upcountry area. Therefore the main
visible climate change is related to annual rainfall and due to Sri Lanka’s
location it is identified that this rainfall is governed by the seasonal
varying monsoon system in Indian Ocean. The mean annual rainfall can be high as
4500-5000mm in high lands and some areas such as southeast and northwest in the
country it can be low as 800-1200mm. The four monsoon seasons were identified

1st Inter
monsoon – from March to April

Southwest monsoon – from
May to September – YALA agricultural season

2nd Inter
monsoon – from October to November

Northeast monsoon –
from December to February – MAHA agricultural season

However over the past
few decades due to the extreme environmental pollution, excessive population
growth, rise of greenhouse gasses, development projects related to irrigation
and agricultural projects the balance of the nature was pushed off a cliff and
the regular rainfall pattern of Sri Lanka was affected in a bad manner. Over
the past few years Sri Lanka was suffered from several severe flood situations
which damaged several hydraulic structures as well as immeasurable damages for
the public properties. Therefore many believed that the annual precipitation
was increased. However those disastrous heavy rainfalls were normally followed
by heavy droughts which lasts for months. So, actually what happened was even
though the number of rainfall events were reduced, the intensity of the
occurred rainfalls were massive and this couldn’t tolerate by the hydraulic
structures which was designed without considering the time trends in rainfall



As the capital of the
country Colombo has a far more importance than the other cities since it is the
economical centre of the country. Colombo metropolitan area has a population of
5.6 million people in average and over 1 million people come to the city in a
day for various purposes. According to the data from the Department of
Meteorology, Colombo approximately gets an average rainfall of 2400mm per year
and 200mm per month. Furthermore May is the wettest month with an average of
382mm of rain and the driest month is January with a rainfall of 62mm. Also the
hottest and the coldest months are April (29°C) and January (27°C)
respectively. In recent past Colombo was frequently flooded even for a 2-4 hour
rainfall and it caused a lot of property loss and disabled the whole economy in
Sri Lanka. So, a proper knowledge in rainfall pattern in Colombo will far more
helpful in flood mitigation process in the city.

surface temperature (SST) in Indian Ocean is directly influenced to the precipitation
in Sri Lanka. High sea surface temperatures in western Indian Ocean, Indian
Ocean Dipole will cause large convergence in lower troposphere which will eventually
cause enhanced rainfall in Sri Lanka. Most studies conducted for the long-term
variations of rainfall in country was influenced by monthly tools. So, an analysis
which based on daily rainfall data and observe how seasonal rainfall totals
reflect the frequency of daily totals can be more effective in rainfall pattern
analysis since it allows to determine the number of times totals exceed a given
threshold in a given period of time. Also evaporation is lead to the cool down
of Sea Surface Temperatures, which cause to the reduction of convection of the
following year.














































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