Indian Institute of Tropical Meteorology (IITM)

The paper discuses climatological and fluctuation features of important parameters of the Hydrological Wet Season (HWS), viz. starting and ending dates, duration, seasonal rainfall/rainwater and surplus rainfall/rainwater potential individually for the eleven major and thirty six minor river basins, the West Coast Drainage System (WCDS) and the rest of the country. It is based on reliable monthly rainfall data from a well spread network of three hundred and sixteen rain gauge stations from 1813 upto 2006.

The paper observes that the average period of the HWS is in close conformity with the southwest Monsoonal Rainy Season (MRS) over the basins in the Core Monsoon Rainfall Regime (CMRR). In the Mixed Monsoon Rainfall Regime (MMRR) covering south peninsula, northeast and extreme north, the HWS is of longer duration compared to the MRS due to extra-monsoonal rainfall over there. The mean starting date of the HWS is May 30, ending October 11 and the duration is 135 days for the entire country.

The map shows that there is a decreasing trend of rainfall over most of peninsular India (barring Maharashtra) including the western part of the Gangetic plain and the north eastern states apart from Assam.

An increasing trend of rainfall has been observed for the eastern part of the Gangetic plain, Assam, Telengana and all coastal regions of the country.