Analysis of rainfall characteristics of northeast region of Bangladesh

Abstract

The aim of this study is to summarise the changing behaviour of rainfall in space and time over the northeast region of Bangladesh. During the study various rainfall parameters were investigated and mathematical relationships were established. Updated isohyetal maps of mean monthly, monsoon (June-September) and annual rainfall were constructed for the study region. Spatial distribution of mean monthly, monsoon and annual number of rainy days were investigated. Normal monsoon rainfall of selected 32 stations within the region have been estimated and normal annual rainfall chart has been updated. Temporal variability of rainfall and number of rainy days over the region has been investigated. Moving average and percent departure of rainfall for every year with reference to the normal rainfall were worked out for this purpose. Attempts were made to establish mathematical relationships between number of rainy days and monsoon rainfall total and between mean daily rainfall intensity and monsoon rainfall total. Various forms of equations were examined. Analytical probability distributions were fitted to 1-, 2-, 3-, 5-, 7- and lO-day maximum rainfall values. Normal, lognormal and Gumbel (EV1) distributions were examined and all of them were found good for maximum rainfall values. Gumbel distribution was found to be better. Pattern of Gumbel parameters over the region for 1-, 3-, 5- and 7-day maximum rainfall were presented on the base maps. Binomial and Poisson discrete probability models were tested to monsoon and annual number of rainy days. Normal approximation to Poisson distribution was found suitable to describe monsoon and annual number of rainy days. HYMOS (HYdrological MOdelling System), developed at Delft Hydraulics, the Netherlands, has been extensively used for probability distribution and other analysis in performing the study.