A Study on Rainfall Trends in Kokrajhar District of Assam, India
Journal Title: International Journal of Research in Chemistry and Environment (IJRCE) - Year 2013, Vol 3, Issue 1
Abstract
An analysis of 40 years rainfall data from 1971 to 2010 to discern the trends of the rainfall pattern over Kokrajhar district of Assam in India was performed by applying different statistical methods. For the interest of a comparative study, a study on the mean variation of state rainfall was also accommodated. Interannual variations of the seasonality indices were analyzed by Walsh and Lawler approach and Markham technique was applied to obtain average time of occurrence of seasonality. Rainfall series were tested for normality by Brazel and Balling technique and then Cramer’s test was used for analysis of sub-periodical variations. Trends were detected by Mann-Kendall test and their magnitudes were determined by Sen slope estimator, then their equations were obtained by Theil-Send procedure. At last, the rainfall anomalies were analyzed. Rainfall of the district had higher variability than that of the state. Occurrences of wetter and drier conditions of two annual rainfalls were not concurrent. The mean seasonality indices of the district and state rainfalls were 0.88 and 0.76. The “markedly seasonal with a long dry season” and “seasonal” regimes were more pronounced in the district and the state respectively. The average time of occurrence of seasonality was 188.400 for the district and 187.960 for the state, i.e. the month of July for both. Cramer’s test for 30-year overlapping sub-periods displayed significant change of April rainfall of the district (tk= -4.44) and September rainfall of the state (tk= 2.48) for the period 1971- 2000, while t-test for 20-year non-overlapping sub-periods demonstrated significant deviations in April and September rainfalls of the district and July, September and Summer rainfalls of the state. Mann-Kendall test for determining the months (seasons) with significant trends in rainfall exhibited that the April rainfall of the district had a significant increasing trend, while July, September, Summer and Annual rainfalls had decreasing trends. July, September, December, Summer and Annual rainfalls of the state had decreasing trend. Decreasing rate of annual rainfall of the district was higher than that of the state (10.25 mm/yr and 6.68 mm/yr respectively). The standardized anomaly indices of annual rainfalls smoothened by 5-year running mean exhibited that the district rainfall was more fluctuating in character than the state rainfall. There is a tendency of summer and winter seasons of rainfall of the district to shift forward by a month each
Authors and Affiliations
Nath C. , Mwchahary D.
Keywords
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