Journal of Earth System Science ( IF 1.423 ) Pub Date : 2020-11-20 , DOI: 10.1007/s12040-020-01494-x Grace Nengzouzam, Sanayanbi Hodam, Arnab Bandyopadhyay, Aditi Bhadra
A high resolution (0.25°×0.25°) daily gridded dataset was analysed to study the spatial and temporal trend in various regions of India during the period of 1901–2013. The non-parametric Mann–Kendall (MK) test has been applied to the time series (1901–2013) rainfall data in order to detect the trends for both rainfall depth and number of rainy days. Sen slope estimator test was also used to find out the magnitude of the trend. In addition, Pettitt–Mann–Whitney (PMW) test was employed so as to determine the probable change year. The results of trend analysis of rainfall depth showed maximum number of grid points having increasing trend in summer months, while the dry months showed higher grid points with significant decreasing trend. Jammu and Kashmir, in particular, was found to have increasing trend in all months for all three levels of significances, i.e., 1%, 5%, and 10%, while NE (northeastern) region continuously reported significant decreasing trends. The annual trend analysis of the 30-year moving window of rainfall depth showed higher number of grid points with significant increasing trend during 1901–1950, decreasing trend during 1931–1980, and increasing trend again during 1961–2013. The PMW test for the rainfall depth and number of rainy days for 1901–2010 showed 1961 and 1974 as the most probable change point years with about 339 and 284 grid points agreeing upon it, respectively. The maximum increasing significant trends in rainfall was observed during south-west monsoon months. Recent years also reported an increase in rainfall intensity leading to probable increase of extreme events like floods and droughts.
MK test and Sen slope estimation were carried out to analyse the trend and magnitude of trends for rainfall depth over India for the period of 113 years (1901–2013) wherein the wet months were found to show positive trends while the dry months were characterized more by negative trends.
Pettitt–Mann–Whitney test was also carried out for both the rainfall depth and number of rainy days to detect the change year wherein the years 1961 and 1974 were found as the most probable change point years, respectively.
A 30-year moving window was used for both rainy days and rainfall depth to represent the climatic conditions of the region. Thus, the total 113 years daily rainfall data from 1901 to 2013 were divided into 10 sections, viz., 1901–1930, 1911–1940, 1921–1950, 1931–1960, 1941–1970, 1951–1980, 1961–1990, 1971–2000, 1981–2010, and 1991–2013.
Upon analysing the trend of rainfall depth, on seasonal scale, maximum significant increasing trends were reported during the periods 1961–1990 and 1971–2000 in all seasons except the post-monsoon months. This indicated that maximum change in trend took place during these periods justifying the outcome of PMW test carried out.
There might be an increase in the intensity of rainfall on the rainy days as opposed to prolonged dry periods leading to increase in extreme events like floods and droughts in the recent years. It is also clearly visible that the country is experiencing a huge shortage of rainwater during the recent years accentuating the need for conservation of water to satisfy the country’s increasing demand.