Temporal clustering of extreme rainfall events in relief transformation

This paper deals with extreme rainfall events, which are responsible for the transformation of slope and fluvial systems, and the clustering of the events in time. Several types of extreme rainfall events have been identified. Especially interesting and consequential events are those that have characteristics of more than one type. It is observed that the frequency and recurrence interval of such events depend upon the-climatic zonality as well as on the relaxation time of the disturbed system. The focus of this paper is on short-time events (1-3 years or less), which provide little or no time for relaxation to the slope and fluvial systems. Clusters including rare events are separated by longer periods. In the case of Cherrapunji the discussion relates to high frequency rainfall events rather than their clustering. Several examples of clustering are discussed in this paper. The response of slope or the channel systems to such clustering of extreme events is in the form of a new equilibrium condition. Such systems either show a tendency towards rejuvenation or a trend towards accelerated maturation. However, other factors such as rock resistance and neotectonic activity are also important and modify this picture. Historical and geological records provide evidence of such clustering of extreme events, especially during the Holocene phases with higher frequency like the Little Ice Age.