A dissertation submitted in partial fulfillment of the requirements for the degree of Engineering

Thesis (Ph.D.) - Asian Institute of Technology, 1973

This research deals with a study of the areal and time distributions of tropical daily rainfall using the probabilistic approach. Records of daily rainfall, over an area in the central part of Thailand, are analyzed. The rainy season is divided into three periods and it is assumed that the rainfall characteristics are stationary within each period. The marginal distribution of daily rainfall at a station is found to be a mixed variate Gamma distribution. The conditional distributions of rainfall at a distance from a control station, given the rainfall at the control station, also follow the mixed-variate Gamma distribution. The relations of the parameters of the conditional distribution to distance and the given rainfall at the control station are determined to describe the areal distribution of daily rainfall. The degree of dependence of rainfall at two stations decreases as the distance between them increases. The sequence of occurrence and non-occurrence of daily rainfall are assumed to follow the simple Markov chain. The distribution function of total rainfall in n rainy days is obtained from the distribution function of rainfall in one rainy day. The conditional distribution functions of total rainfall in N days are analyzed and compared with the observed data. It is found that the probability of rainy days to follow a rainy day is higher than the probability of rainy days to follow a non-rainy day. It is also found that the probability is higher of having a heavy rainfall in N days if it is given that the preceding day is a rainy day rather than a non{u2014}rainy day.