SEASONAL TRENDS OF CLIMATE IN THE WET ZONE AND DRY ZONE OF SRI LANKA

Abstract

Climatic change is an important factor especially in view of current global warming scenarios.Study of climatic changes in Sri Lanka is relevant, particularly, as any changes in the seasonalityof rainfall can affect the country's agricultural productivity. It is also important in selectingspecies for forestry. Hence, the present study attempts to separately ascertain whether there havebeen changes in seasonal rainfall and temperature in the 'dry' and 'wet' zones during the 1870-1999.

As there are no cyclic changes or linear trends observed during the study period, it was dividedinto four sub-periods: 1870-1909, 1910-1939, 1940-1969 and 1970-1999.Simple regressioncoefficients were calculated for those sub-periods, considering the year as the independentvariable and temperature and rainfall respectively as the dependent variables. Dry zone rainfalland temperature were estimated by averaging the amounts recorded in the meteorologicalstations of Puttalam, Anuradhapura, Trincomalee and Hambantota. Similarly, wet zone rainfalland temperature were estimated by averaging the amounts recorded in the meteorologicalstations of Galle, Ratnapura, Nuwara Eliya and Kandy. Averaging the data from differentlocations reduces measurement errors and fluctuations. Moreover, as monthly data do notnecessarily related to seasonality, only typical months for each of the four seasons were selectedand the data were averaged for the respective seasons: Southwest monsoon season wasrepresented buy the average of June and July data. Northeast monsoon was represented by theaverage of December and January data. Likewise the first and second inter-monsoons wererepresented by April and October data.

The analyses reveal the following trends: A decrease of rainfall during both monsoons and anincrease of rainfall in the two inter-monsoons .As for temperature there has been a slightincrease allover the island (with the exception of a slight decrease in the dry zone in the 1910-1939 period.)

The first period (1870-1909) is a wetting period indicating a remarkable increase of rainfall allover the island, due to an increase in the rainfall in both inter-monsoon seasons. However, theincrease is higher in the dry zone. The second (1910- j 939) appears a drying and cooling periodwith a decrease of rainfall in both monsoons as well :1S in the second inter-monsoon. This periodwas also a cooling episode for the dry zone. The third period (1940-1969had a wetting trend inthe dry zone with a drying trend in the wet zone. This contrast was remarkable in three seasonsexcept the first inter-monsoon. The fourth period (I970-1999) was a recovering period with awetting trend in the wet zone and a drying trend in the dry zone