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Statistical and trend analyses of rainfall: a case study of Bharathapuzha river basin, Kerala, India

This study aims to determine trends in the annual and seasonal total rainfall over Bharathapuzha catchment of Kerala using 33 years (1976–2008) of monthly rainfall data at four raingauge stations (Eruthempathy, Mangalam dam, Thrithala, and Malampuzha dam). The procedure is based on the nonparametric Mann–Kendall test for the trend and the nonparametric Sen’s method for the magnitude of the trend. From Mann–Kendall analysis for annual rainfall and southwest monsoon rainfall, a positive trend was observed for Eruthempathy and Malampuza dam stations. However, at all four stations considered, a negative trend was observed for the northeast monsoon rainfall, positive trend for nonmonsoon rainfall and premonsoon except at Eruthempathy. From Sen’s analysis, it was observed for an annual rainfall series a maximum increase of rainfall at Malampuza dam by 1.55 mm/year and a maximum reduction at Thrithala station by −5.80 mm/year. For southwest monsoon, a maximum increase of 2.09 mm/year at Eruthempathy station, and a maximum reduction of −9.81 mm/year observed at Thrithala station. On the other hand, a maximum reduction of −2.63 mm/year observed at Mangalam Dam station for northeast monsoon. For nonmonsoon, a maximum increase of 0.85 mm/year observed at Malampuza Dam and 2.20 mm/year at Thrithala. A similar trend observed from liner regression analysis, existed for annual and seasonal rainfall series estimated by Mann–Kendall Z statistics and Sen’s Slope method. Overall trend analysis for annual rainfall reveals stations in the north and east shows an increasing trend and stations in the south and west a decreasing trend. Current study will direct the future planning and management of water resources at Bharathapuzha river basin.