Mapping of effect of rainfall pattern on cultivation of lowland annual crops (paddy) in Sri Lanka

Abstract

The study was conducted to identify the relationship between current rainfall pattern and paddy cultivation in Sri Lanka with relation to time periods of Yala, Maha onset and Maha harvesting. The analysis was carried out using rainfall data for 33 years (1976- 2008) and Pan evaporation data at different agro-meteorology and meteorology stations, namely Jaffna, Anuradhapura, Ampara, Angunukola Pelessa, Lunuwila, Monaragala, Colombo, Ratnapura, Badulla, Matale, Bandarawela and Sitaeliya. Histogram analysis of mean weekly rainfall for four durations was used to identify the onset for Yala and Maha. Probability plot analysis was applied to ensure the accuracy in addition to the standard deviation values. Moreover, Hargreaves Moisture Availability Index (MAI) at 75 percent, 70 percent, 65 percent, and 60 percent probability rainfall was used to identify the onset for both seasons. Literature reveals that the rainfall onset for Yala and Maha seasons are 13th and 40th weeks respectively. Based on Histogram analysis, rainfall onset occurred at 13th -14th week for Yala and 38th -42nd week for Maha. Onset for both seasons in some stations was not identified using this methodology. Based on Hargreaves MAI method, dependable rainfall at 75 percent probability level noted, no onset for Yala was identified in Monaragala. When probability level of dependable rainfall was reduced, onset for Yala was varied between 13th -15th week. Most stations, rainfall onset for Maha was taken place at the 38th-42nd week at 75 percent probability level. The results indicated that the duration of rainfall in Yala was not sufficient and rainfall duration in Maha was adequate in most areas for rain-fed paddy cultivation. Probability level of overlapping of rainfall onset of Yala with harvesting of paddy in Maha at 8th week was less than 50% in all selected stations. When the dry zone was concerned it was lower than 35%. Regionalization was done based on the results obtained by the research and maps were produced accordingly.