This study highlights the hydro-climatic features of the five wet periods contributing in different percentages to the annual rainfall total over major river basins in India.Spatial and temporal variations in the parameters such as starting date,duration and rainfall intensity of these wet periods throughout India have been discussed using daily gridded rainfall data for the period 1951 –2007.An attempt is also made here,to assess the impact of global SSTs on the start and duration of the wet periods in Indian river basins.

It is observed that,for almost all river basins in India,the 10%wet period occurs in the months of July/August with an average duration of 1 –3 days and rainfall intensity varying from 44 to 89 mm/day.The duration of the wet period contributing 90%to the annual rainfall varies from 112 days in the central parts of India to 186 days in the northern parts of the country.Signi ficant increase in the rainfall intensity has been observed in the case of some river basins of central India. The late start of 75%wet period along the West Coast and in peninsular river basins has been observed with increase in Nino 3.4 SSTs (MAM),while increase in the duration of the 75%wet period over the Krishna basin is associated with increase in Nino 3.4 SSTs (concurrent JJAS).

Estimation of extremely high rainfall (point or areal) is one of the major components of design storm derivation. The estimation of Probable Maximum Precipitation (PMP) involves selection of heavy rain-storms and its maximization for the moisture content during the rainstorm period. These heavy rain-storms are nothing but the widespread heavy rainfall exceeding a certain threshold value. The present study examines the characteristics of heavy rainstorms in the Indus basin selected from present climate and future scenarios simulated by the regional climate model. Such information on heavy rainfall forms the basis for the hydrologic design projects and also for the water management of a river basin. Emphasis is given to severe rainstorms of 1-day duration covering an area of at least 40,000 km² with spatial average rainfall of at least 5cm. This analysis also provides the information on the temporal changes in the storm factors such as shape, orientation, and movement, and shows that the model can well simulate the rainstorm pattern in terms of its intensity, orientation, and shape of the rainstorm, but overestimates the frequency of such heavy rainstorms. The future scenario indicates increase in rainfall intensity at the center of the rainstorm with decreasing areal spread. Decrease in the frequency of rainstorms is projected under the global warming conditions.