Spatial Patterns and Temporal Trends of Rainfall Seasonality in Sri Lanka

Abstract

Understanding and knowledge of rainfall variability is necessitated for agricultural planning, flood mitigation activities, and water resources planning and management. Like other rainfall characteristics seasonality of rainfall is also spatio-temporally specific which has not been evaluated to Sri Lanka. In this study, seasonality in rainfall over Sri Lanka was analyzed using the seasonality index (SI) proposed by Walsh and Lawler. Rainfall data at 39 raingauge stations for the period 1988-2017 were collected to obtain the annual monthly rainfall cycles. The SI, a measure of annual rainfall distribution, was used to identify different rainfall regimes. Southwest and central highlands covering the stations Baddegama, Pelawatte, Kudawa, Deniyaya, Mawarella, Mapalana and Beausejour (lower) were identified as “equable with a definite wetter season”. Skirting to the definite wetter season was the “rather seasonal with a short drier season” regime surrounding the Colombo, Ratmalana, Nuwara Eliya, and Bandarawela. The region centered on Mannar received the most rain in three months or less. Markedly seasonal with a long drier season is in the eastern extending from Pottuvil to Trincomalee and in the northern part of the country above Puttalam and Maha Illuppallama except the surrounding of Mannar. Intermediate region to SI classes “rather seasonal with a short drier season” and “markedly seasonal with a long drier season” was designated as “seasonal”. However, the seasonal rainfall contribution, i.e., in NEM (Dec.-Feb.), IM1 (March-April), SWM (May-Sep.), and IM2 (Oct.-Nov.), and the annual monthly rainfall profiles confirmed the presence of sub-regimes within the identified rainfall regimes. Non-parametric Mann-Kendall test and Sen’s slope were applied to identify the temporal changes in SI. Approximately, half of the country showed strong trends in the SI. Sixty one percent of the area including the northern part of the country surrounding Jaffna and the definite wetter region in the southern corresponds to the decreasing trend in seasonality.