Frequency Analysis of Rainfall Data for Roorkee, Uttarakhand, India

Abstract

Daily rainfall data for 20 years (2005–2024) were obtained from the Global Weather Data portal for SWAT modelling applications. The dataset was processed to determine the annual maximum and minimum daily rainfall values, as well as to examine monthly rainfall variability. In addition, the standard deviation and coefficient of variation (CV) were computed to evaluate the consistency and variability of rainfall during the study period. The analysis revealed that the maximum daily rainfall during the 20-year period ranged from 18.8 mm to 632.5 mm, indicating a substantial inter-annual fluctuation in extreme rainfall events. Monthly rainfall assessment showed that October consistently received the lowest rainfall among the monsoon-related months, whereas July and August recorded the highest rainfall across all years. The lowest coefficient of variation was observed in July (50.3%), followed by September (53.6%), signifying relatively stable rainfall during peak monsoon months. Conversely, the highest CV was recorded in June (77.4%), reflecting significant variability in early-monsoon rainfall. Frequency analysis was carried out using Normal, Log-Normal, and Gumbel extreme value distributions to estimate rainfall magnitudes at a 20% probability of exceedance (return period ≈ 5 years) for the months of July to October. The results indicate that the Gumbel distribution produced comparatively higher design rainfall estimates, followed by Log-Normal and Normal distributions, making it more suitable for extreme event analysis in the study region.