Assessment of Continuous Wet Day Rainfall Frequency Distribution in the Intermediate Zone of Sri Lanka

Abstract

Changes in rainfall distribution patterns around the world have caused fluctuations in thoseclimate regimes. Agriculture is particularly vulnerable to climatic conditions and it is chiefamong the systems that are expected to undergo changes as a consequence of climate change.This study analyses the climate change variability through the fluctuation in precipitationpattern and evaporation, in low country intermediate zone (IL1a) agro ecological region basedon Gumbel’s frequency distribution analysis on wet day rainfall data. Focusing on a cascadeagricultural system in the Katupotha divisional secretariat division of the Kurunegala districtthat is located in the IL1a, the climate change variability was analysed using daily rainfall dataof fifty years (1961-2010) together with long term evaporation data. Experimentalmethodology considered a day to be wet only if the rainfall exceeded its evaporation. Thiswas considered as the precipitation amount that could potentially be used by the crops.

Gumbel’s frequency analysis was employed to derive the variability in continuous wet dayrainfall distribution at 25 year time intervals (1961-1985 and 1986-2010). The totalcontinuous wet day rainfall and seasonal total continuous wet day rainfall frequencydistributions were analysed using the technique. Results of the analysis show an increasingtrend in magnitude of the total continuous wet day precipitations from 1961-2010. Thisincreasing trend is observed beyond 2.2 years recurrence interval. At longer recurrenceintervals (20 years) a 50 mm increase is experienced during the last 25 years (1986-2010)while at shorter recurrence intervals (2.2 years), a 6 mm increase is evident. A distinctincreasing trend in the total continuous wet day precipitation from 1961-2010 was observedduring the second inter-monsoon which was a 165 mm increase at a 10 year recurrenceinterval. In contrary, the south-west monsoon, first inter-monsoon and north-east monsoonrecorded decreasing trends in the total continuous wet day precipitation from 1961-2010.