Frequency and Trend Analysis of Rainfall Data of Guwahati, Assam, India

Abstract

In this study, 35 years of daily rainfall data (1979–2013) from the Global Weather Data for SWAT were used to analyse rainfall characteristics for the selected station. The dataset was processed to determine the annual maximum and minimum rainfall, monthly rainfall variations, and key statistical indicators including standard deviation and coefficient of variation (CV). The analysis revealed that annual maximum rainfall over the period ranged from as low as 0.04 mm to as high as 1609.38 mm, indicating substantial inter-annual variability. Monthly rainfall assessment showed that October consistently received lower rainfall compared to other monsoon months throughout the period of record, whereas July and August exhibited the highest rainfall across all years analysed. In terms of variability, the lowest coefficient of variation was observed during July (47.34%), followed by June (50.03%), while November recorded the highest CV (140.63%), indicating large fluctuations during the post-monsoon season. Frequency analysis conducted using the Normal distribution and Log-Pearson Type III distribution showed that the maximum rainfall values at a 20% probability of exceedance (corresponding to a 5-year return period) occurred during the peak monsoon months of July to October.