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Rainfall Seasons

Climatic Year/ Hydrological Year

Detailed studies on climatology of Sri Lanka has identified that “climatic year” or “hydrological year” of the island begins in March and not in January so that seasonal weather rhythm or more specifically the rainfall seasons ranges from March to February.

Rainfall Seasons

It is generally accepted that there are four rainfall seasons in Sri Lanka:

1 March – April First Inter Monsoon (FIM) rains
2 May – September South West Monsoon (SWM) rains
3 October – November  Second Inter Monsoon (SIM) rains
4 November – February North East Monsoon (NEM) rains

These rainfall seasons do not bring homogeneous rainfall regimes to the entire island, causing  immense agro-ecological diversity despite Sri Lanka’s relatively small aerial extent. Two consecutive combined rainy seasons make up the major crop growing seasons of Sri Lanka, namely Yala and Maha seasons.

Generally, Yala season is a combination of FIM and SWM rains. However, SWM rains are not effective over the Dry zone, therefore, Yala season in the Dry zone is restricted only from mid-March to early May. Being effective only for two months, the Yala season is considered as the minor growing season for the Dry zone. The major growing season of the whole country- Maha, begins with the SIM rains in mid-September/October and continues up to late January/February combined with the NEM rains.

Rice is grown under more diverse environmental conditions than any other major food crop in the world and the situation remains the same in Sri Lanka. Paddy is the most common land use in valley bottoms of the country. The only exception are almost all AERs in the Up country Wet and Intermediate zones as the low temperature during night becomes a limiting factor for rice growth in these regions.

Solar radiation is not a limiting factor for rice growth within the country. However, when all other conditions (such as water, nutrients and temperature) are non-limiting, the intensity of sunlight may determine the yield level depending on the location and season. For example, in the Wet zone, solar radiation may limit the rice yield during Yala season due to high cloud cover arising from the southwest monsoonal circulation whereas a similar situation could expect in the Dry zone during  Maha season due to overcast conditions that may result due to weather systems formed in the Bay of Bengal and northeast monsoonal circulation.