Analysis of long-term rainfall trends and change point in West Bengal, India

In this study, an attempt has been made to analyze long-term annual and seasonal rainfall trends along with change point of annual rainfall in West Bengal, India for 102 years (1901 to 2002) using monthly rainfall data of 18 rainfall stations. The Mann-Kendall test is used to identify trend in rainfall time series and Theil-Sen's slope estimator to assess the magnitude of this trend. Trend-free pre-whitening method is used to eliminate the influence of significant lag-1 correlation from the series. Change in magnitude is derived in terms of percentage change over mean rainfall. Pettitt-Mann-Whitney and standard normal homogeneity test have been used to identify change point of annual rainfall. The results show that significant trend is found at five stations in annual rainfall, six stations in monsoon rainfall, and eight stations in postmonsoon rainfall. Maldah station has recorded highest negative change in magnitude in annual (-14%) as well as monsoon rainfall (-20.48%). South 24 Parganas rainfall station exhibits highest positive change in magnitude in annual (+13.98%) and monsoon (+13.27%) rainfall. Postmonsoon rainfall portrays positive change in magnitude at 16 rainfall stations with highest change in Birbhum station (+40.07%). Three most probable change point years of annual rainfall, viz. 1956, 1967, and 1952 have been observed for the rainfall stations situated in northern, southern, and western part in West Bengal. In the post change point period, the number of rainfall stations with decreasing trend has risen in northern and western part whereas it has lessened in southern part.