Application of Standard Precipitation Index (SPI) to Assess Rainfall variability in a Major Agricultural Area in Dry Zone of Sri Lanka

Agriculture in the dry zone of Sri Lanka is highly constrained by the inadequacy of irrigation water. The small village tanks located in these areas help farmers to cultivate their lands during dry periods by storing rainwater. However, the rainfall variability has made a considerable threat to functioning of these tanks and in providing expected benefits to the farmers. The objective of this study was to assess the climatic variability using 30-year (1989 to 2018) daily rainfall data from six rain gauging locations (Anuradhapura, Mahagalkadawala, Hingurakgoda, MahaIlluppallama, Girithale and Diyabeduma) in a small tank dominated area in the Dry Zone. Standard Precipitation Index (SPI) was calculated to assess the temporal variability of dry and wet extremes. Mann Kendal Trend Test (MK test) was used to analyze the trend of SPI and Sen’s Slope Estimator to assess the magnitude of the trend. The results of Anuradhapura, Diyabeduma and Mahagalkadawala show significantly increasing trend of SPI 12/ annual rainfall. According to the SPI, there were increasing trends in First Inter Monsoon in Anuradhapura and MahaIlluppallama, which have resulted an increasing rainfall trend in Yala season. Additionally, there was an increasing trend in Second Inter-Monsoon in Anuradhapura. These changes highlight that there are extreme rainfall events occurring in some seasons without making a significant impact in the annual rainfall pattern of the area.