Some features of the periods contributing specified percentages of rainfall to annual total in Kerala,India

Based on daily rainfall data over a period of 80 years (1901–1980) taken at 75 stations, this paper reports interannual and long term fluctuations of the following parameters of the periods contributing each of 2, 5, 10, 20–90 and 95% rainfall to annual total over each of North Kerala and South Kerala rainfall subregions of India:

(i)	the starting date,
(ii)	the ending date,
(iii)	the length of the rainfall period or duration,
(iv)	the total rainfall (which is a fixed percentage of the annual total) of the rainfall period, and
(v)	the rainfall time-distribution characteristic which has been quantified by computing Oliver's precipitation concentration index (PCI) using daily rainfall data of the concerned rainfall period.

For most of the rainfall periods, the time series of starting and ending dates and length are homogeneous and random, and tend to observe the normal probability distribution. The different PCI series of North Kerala and the annual rainfall series of South Kerala show significantly decreasing trend which are indicative of considerable change in the hydroclimatic environment of Kerala. Possible changes in the southwest monsoon circulation along the west coast of India and excessive deforestation in Kerala which might have caused these rainfall changes are discussed.