Impacts of time trends in colombo rainfall patternon design rain events A.K.D.
Y. AbeywickramaUniversity of Moratuwa, Sri Lanka.Dr.
T.M.N. WijayaratnaUniversity of Moratuwa, Sri Lanka Abstract: Keywords: time trends; Colombo rainfall; patternanalysis, non-parametric methods; design rain events 1.
IntroductionSri Lanka is an islandwhich is extends between 5°55? to 9°51?N and 79°42? to 81°53?E and ricecultivation plays a major role in country’s economy as in the case of mostcountries in South and Southeast Asia. Irrigated agriculture contributes to 22%of Sri Lankan exports while 75% of exports are powered from the national gridelectricity which 65% is generated via hydropower. So as country which its economy is heavily depends on the rainfall and theavailability of water resources, studies in rainfall pattern analysis is reallyimportant for Sri Lanka. Also these pattern analysis is far moreimportant to manage, to plan and to predict the rainfall related adverseeffects such as high floods due to excessive rainfall and more accuraterainfall forecasting will come in handy in crop cultivation managing. Design ofhydraulic structures such as dams, spillways, culverts, sluice gates etc. alsodepend on the time trends of rainfall pattern.
Even though the country has aconsiderably high average rainfall, due to its seasonal and spatial variabilitythere is some shortage of water for agricultural, hydropower and domestic usefrom time to time. Since this is a tropical country, air temperature doesn’tvaries much throughout a year, except in the upcountry area. Therefore the mainvisible climate change is related to annual rainfall and due to Sri Lanka’slocation it is identified that this rainfall is governed by the seasonalvarying monsoon system in Indian Ocean.
The mean annual rainfall can be high as4500-5000mm in high lands and some areas such as southeast and northwest in thecountry it can be low as 800-1200mm. The four monsoon seasons were identifiedas;1st Intermonsoon – from March to AprilSouthwest monsoon – fromMay to September – YALA agricultural season2nd Intermonsoon – from October to NovemberNortheast monsoon –from December to February – MAHA agricultural season However over the pastfew decades due to the extreme environmental pollution, excessive populationgrowth, rise of greenhouse gasses, development projects related to irrigationand agricultural projects the balance of the nature was pushed off a cliff andthe regular rainfall pattern of Sri Lanka was affected in a bad manner. Overthe past few years Sri Lanka was suffered from several severe flood situationswhich damaged several hydraulic structures as well as immeasurable damages forthe public properties. Therefore many believed that the annual precipitationwas increased. However those disastrous heavy rainfalls were normally followedby heavy droughts which lasts for months. So, actually what happened was eventhough the number of rainfall events were reduced, the intensity of theoccurred rainfalls were massive and this couldn’t tolerate by the hydraulicstructures which was designed without considering the time trends in rainfallpattern. 2.
BodyAs the capital of thecountry Colombo has a far more importance than the other cities since it is theeconomical centre of the country. Colombo metropolitan area has a population of5.6 million people in average and over 1 million people come to the city in aday for various purposes. According to the data from the Department ofMeteorology, Colombo approximately gets an average rainfall of 2400mm per yearand 200mm per month. Furthermore May is the wettest month with an average of382mm of rain and the driest month is January with a rainfall of 62mm. Also thehottest 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 hourrainfall and it caused a lot of property loss and disabled the whole economy inSri Lanka. So, a proper knowledge in rainfall pattern in Colombo will far morehelpful in flood mitigation process in the city. Seasurface temperature (SST) in Indian Ocean is directly influenced to the precipitationin Sri Lanka. High sea surface temperatures in western Indian Ocean, IndianOcean Dipole will cause large convergence in lower troposphere which will eventuallycause enhanced rainfall in Sri Lanka. Most studies conducted for the long-termvariations of rainfall in country was influenced by monthly tools.
So, an analysiswhich based on daily rainfall data and observe how seasonal rainfall totalsreflect the frequency of daily totals can be more effective in rainfall patternanalysis since it allows to determine the number of times totals exceed a giventhreshold in a given period of time. Also evaporation is lead to the cool downof Sea Surface Temperatures, which cause to the reduction of convection of thefollowing year.