Utilizing over 100 years of rainfall records in 15 meteorology stations, an analysis was carried out to extract the trends of annual rainfall depth in Sri Lanka over the last century. A statistically significant increasing trend of rate 3.15 mm/year was observed at Colombo and decreasing trends were observed at Nuwara Eliya and Kandy with rates of 4.87 mm/year and 2.88 mm/year respectively. Since no coherent increase or decrease of rainfall in any group of stations in the wet or dry zones was observed, the possibility of large scale change over the past century was ruled out. However, more recent data records (1949 onwards), revealed a decreasing trends in 13 of the 15 stations. Thus, traces of a temporal change seem to be apparent in the rainfall records over the last half century. In general, the downward trends in recent decades are steeper than the long term variations. For the recent data records, the largest downward trend of 11.16 mm/year was observed at Batticaloa. 9 data records maintained by the Department of Meteorology, Sri Lanka [2]. They have applied the necessary quality control checks on the data and estimated missing values using difference and ratio methods suggested by Thom [3]. The recent data records were obtained from the IRI monthly climatic data (IRI/LDEO Climate data Library) and Statistical abstracts from the Department of Meteorology as well as through direct purchase. Table 1 shows a summary of the selected rainfall stations used in this work.

Abstract:Two different spectral analysis methods, the multitaper method (MTM) and the maximum entropy method (MEM) were applied to investigate the presence of low-frequency periodicities in precipitation records of 14 climatological stations in Sri Lanka. The spectral analysis revealed statistically significant periodicities in the range of 2 -3 year and 3 -6 year periods in all parts of Sri Lanka irrespective of the climatic variability. The 2 -3 year band corresponds to the Quasi-Biennial oscillation (QBO), while the 3 -6 year band corresponds to the El-Nino/Southern oscillation (ENSO) higher and lower frequency bands. Cross spectrum analysis showed statistically significant (at 5 %) coherencies for the Indian Ocean dipole (IOD) and Southern oscillation index (SOI) in the 2 -3 year band and the 3 -6 year band, respectively for most of the regions. Thus, it is concluded that the IOD and SOI play important roles as modulators of precipitation in Sri Lanka.

Daily rainfall data recorded at 13 stations were analyzed to study the spatial patterns of rainfall in the dry zone of Sri Lanka. Principal component analysis was utilized to classify the dominant spatial regions. The first 2 eigenvectors accounted for 70.2% (the first eigenvector 54.8% and the second 15.4%) of the total variation, which clearly supports the commonly used major climatic division of Sri Lanka into wet and dry zones. Both the inverse distance weighting method and kriging successfully estimated weekly average rainfall in the North Central dry zone of Sri Lanka. For both methods, high correlation coefficients of 0.88 and 0.91 were observed for the southwest and northeast monsoon periods, respectively, with slightly lower values for intermonsoon periods. For inter-monsoon periods, the inverse distance weighting method produced better results than kriging. This work shows that the strength of the predictions depends on the rainfall seasons as well as the geometrical placement of the stations in the dry zone.