Long-term changes in the Indian monsoon rainfall on regional and local scales have important social and economic consequences. To present a comprehensive picture of such changes, the monthly rainfall data at 306 stations, well spread over India, have been analysed over 114 years (1871-1984), for long-term trends. The trends are quantified by linear trend for monthly as well as seasonal rainfall for each station. Some broad contiguous areas showing statistically significant trends have been identified. Areas of increasing trend in the monsoon seasonal rainfall are found along the west coast, north Andhra Pradesh and north-west India, and those of decreasing trend over east Madhya Pradesh and adjoining areas, north-east India and parts of Gujarat and Kerala.The subseasonal patterns indicate that the excess or deficiency of the monsoon rainfall is more frequently realized in the later half of the season. Monsoon rainfall tends to be more concentrated in August, over the west coast and central India.

Sontakke and Singh ( The Holocene 6, 315—31, 1996) developed instrumental period summer monsoon (June—September total) rainfall series (1813—1995) for six homogeneous zones and all of India using 306 raingauge stations. This reconstruction has been revised and updated. Besides reconstructing backward and updating to 2005 the longest possible summer monsoon rainfall series (1813—2005), post-monsoon (October— December total) and annual rainfall series have also been developed for seven homogeneous zones: North Mountainous India (NMI), Northwest India (NWI), North Central India (NCI), Northeast India (NEI), West Peninsular India (WPI), East Peninsular India (EPI), South Peninsular India (SPI), and the whole country using data from 316 well-spread stations. The different series are reported here. The underlying mechanism of the possible cause of the recent decreasing trend in monsoon rainfall and increasing trend in post-monsoon rainfall is described.

Abnormalities in the performance of the Indian summer monsoon (June to September) rainfall, which provides 75-90 per cent of annual rainfall, have been studied during the period 1871 to 1984 over different meteorological subdivisions into which the country has been divided. Long homogeneous monsoon rainfall series of 29 subdivisions, prepared on the basis of a constant 306 rain gauges, have been tabulated for the users in view of their great importance. The criterion adopted in identification of droughtlflood over a subdivision is the percentage of rainfall departures from normal, as officially used in the India Meteorological Department.The worst drought years were

Long period June-to-September total rainfall series are vital for the study of summer monsoon/climatic variability over the Indian region. Owing to high spatial variability in rainfall, the representativeness of the summer monsoon rainfall series for the entire country is limited. In order to develop an effective system for monsoon rainfall studies, the country has been divided into six zones, named as North West India (NWI), North Central India (NCI), North East India (NEI), West Peninsular India (WPI), East Peninsular India (EPI) and South Peninsular India (SPI). Fluctuation, empirical orthogonal function (EOF) and cluster analyses of the summer monsoon rainfall of the period AD 1871-1984 of 306 raingauges are carried out for this classification. Updating the different zonal rainfall series on a real-time basis from all the gauges is a difficult task. In the present study, an objective technique is applied to select a subset of gauges whose mean showed the highest correlation with the all-gauges mean series; a total of 116 raingauge stations (19 for NWI, 27 for NCI, 15 for NEI, 18 for WPI, 14 for EPI and 23 for SPI) maintained by the India Meteorological Department (IMD) have been identified for updating the different zonal series. The different zonal series are extended as far as possible backward (prior to AD 1871) by applying an objective technique to the available gauges. The NEI and the EPI series are extended back to 1848, the NWI to 1844, the NCI to 1842, the WPI to 1817 and the SPI to 1813. Though of limited practical value, the all-India series is widely used in studies of long-range monsoon forecasting, teleconnections and large-scale climate dynamics. The all-India monsoon rainfall series from AD 1871 onwards is therefore calculated from the area-weighted mean of the six zones. The series is also extended back to AD 1813 by applying the objective technique on the available gauges. The different reconstructed series for the longest instrumental period are reported.

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The total volume of rain-water over India has been computed for each of the years during the period 1871-1978 on the basis of the rainfall data for a fixed network of 306 raingauges. The mean, the standard deviation and the coefficient of variation of the series of annual rain-water volume are 3143 km3, 300 km3 and 9.5 per cent, respectively. The series is homogeneous and random. The serial correlation is 0.013 which is too low to suggest any relationship between successive terms. Gaussian distribution gives a good fit to the series.The series giving the percentage area of the country with a deficiency of 20 per cent or more, as well as the percentage area with an excess of 20 per cent or more have been obtained. These bring out the variability in the deficiency and excess of rain-water over the country during the period. The years of well-marked rain-water deficiencylexcess over India have been identified by utilizing the criterion of 40 per cent or more area under rain-water deficiency/excess. The occurrence of such years has been found to be random and the number of such years in five-year periods appears to be distributed in accordance with the Binomial and the Poisson Probability models.The impact of marked deficiency/excess on the All India Index of food grain prices and A l l India Index of food grain production has been examined. It is found that the impact of well-marked deficiency can generally be seen clearly but that of well-marked excess is seen clearly in some cases only. This is understandable. Overall excess in a year may or may not result in floods which have the potentiality to damage crops. The impact of the well-marked deficiency of 1918 has been maximum.Remedial measures which would make the economy of the country less dependent on the variability of the annual rain-water have been discussed.

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