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.

Variations in the HWS parameters do not indicate any particular long-term trend. Time series displays have been used to report the nature of recent trend. Sixteen basins across the country experienced multiple wet seasons in some years. Important characteristics of these wet seasons are discussed. The average HWS rainwater and Hydrological Rainwater Potential (HRWP) of the major basins has been calculated.

The mean wet season rainwater for the country amounts to 2911.064 billion cubic metres (bcm), of which 1856.947 bcm evaporates. Out of the remaining HRWP, 1054.117 bcm goes through different surface hydrological processes (soil moisture recharge, deep percolation, surface storage and streamflow). Over most of the minor basins in the CMRR in recent years a declining trend is seen in the HWS rainfall/rainwater and surplus rainfall/HRWP. The HWS rainfall for the country during 1965-2006 was 4.15% less as compared to the period 1912-1964, and the surplus rainfall/HRWP during 1978-2006 was 11.93% less as compared to 1915-1977.

Results of the present basin-scale rainfall analysis based on longest instrumental observations are expected to provide informative resources to planners and managers in water-related sectors, including those working for ‘interlinking of
rivers’ program of the country.

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