Abstract:
Rainfall is the major climatic parameter that affects the Sri Lankan agriculture. In
order to improve the water security by maximizing the rainfall usage for paddy
cultivation in Sri Lanka, an analysis was carried out using rainfall and evaporation
data for the last 33 years (1976-2008) representing different agro-climatic zones.
*The selected stations are namely Jaffna, Anuradhapura, Ampara,
Angunukolapelessa, Lunuwila, Monaragala; Colombo, Ratnapura, Badulla, Matale,
Bandarawela and Sitaeliya. Histogram analysis of mean weekly rainfall for four
durations was used fo find the onset of yala and maha. Probability plot analysis was
done to ensure the accuracy in addition to the standard deviation values. Moreover
Hargreaves Moisture Availability Index (MAI) at 75%, 70%, 65%, 60% probability
rainfall was used to find the onset for both seasons.
Trends Of changes , of annual rainfall for all selected stations are not in significant
level. Typical bimodal rainfall pattern couldn’t be identified clearly in Jaffna and
Ampara. Literature reveals that the rainfall onsets for yala and maha seasons are 13 th
and 40th weeks respectively. Based on histogram analysis and Hargreaves MAI
method with different dependable rainfall probability levels, rainfall onset occurred
on 13th-14th week for yala and 38th-42nd week for maha. The results indicated that
the duration of rainfall in yala is not sufficient to cater the crop water demand for
paddy while the rainfall duration in maha is adequate in most areas in the country
for rain-fed paddy cultivation. There was an evidence of overlapping of maha
harvest with the onset of yala with 40-50% probability for wet zone and below 35%
probability for dry and intermediate zones.
