Onset and Retreat of Rainy Season in the Dry Zone

Abstract

Sri Lanka is an island in the Indian Ocean, located off the southern coast of India. The climate of Sri Lanka is considered to be warm and tropical. The mean daily temperature ranges from about 16 °C in the central highlands, to a maximum of 33 °C in low-altitude coastal areas. There are predominantly two climatic zones in Sri Lanka, the central and the south-western part of the country, known as the wet zone and the southeast, east, and northern parts of the country, known as the dry zone. When considering the wind pattern, the year can be divided into four climate seasons, namely the two monsoons (South-West from May to September and North-East from December to February) and the two inter monsoons. There is high variability of rainfall in different parts of the country during these four seasons. Since Sri Lanka’s economy depends on agriculture production, understanding temporal variation in climate in different parts of the country, especially the variations in the start and end of the rainy seasons is important to enhance agricultural productivity. A number of methods have been adopted to determine the onset and retreat of rainy seasons (Odekunle, 2006). Studies carried out in Africa show that the percentage cumulative mean rainfall is one of the frequently used methods. Since rainfall is a readily available measurement, there are advantages in using it rather than using an associated variable. Since patterns of dry spells and wet spells at the start and end of rainy seasons are critical for crop growth, there is an advantage of developing an index to determine the start and end of rainy seasons based on the temporal patterns of rainfall events (Cook, G.D., Heerdegen R.G. 2001). The main objective of this study is to look into the feasibility of determining the onset and the retreat dates of the rainy season in the dry zone.