Probability and Trend Analysis of Rainfall Data of Shillong District

Abstract

This study analyses 15 years (2002–2016) of daily rainfall data for Shillong, located in northeastern India, obtained from the Global Weather Data repository developed for the SWAT model. The primary objective was to evaluate the variability, trends, and statistical characteristics of annual and seasonal rainfall. Descriptive statistical analysis was carried out to determine annual maximum and minimum rainfall, monthly rainfall variability, standard deviation, and coefficient of variation. The annual rainfall exhibited substantial variability, ranging from 0.04 mm to 1609.38 mm. The lowest mean monthly rainfall occurred in 2006 (115.3 mm), followed by 2013 (123.4 mm) and 2005 (151.2 mm), whereas the highest was recorded in 2004 (255.3 mm). The computed standard deviation (326.6) and coefficient of variation (127.9%) indicated a high degree of variability in rainfall distribution. Returnperiod analysis classified Shillong under normal rainfall conditions, with the maximum rainfall event corresponding to a return period of 24 years. Trend analysis based on slope (m) values revealed an increasing rainfall trend during August (18.257 mm/year) and September (11.537 mm/year), while June (–19.338 mm/year), July (–25.535 mm/year), and October (–13.815 mm/year) exhibited declining trends. The annual slope value (–28.894 mm/year) indicated an overall decreasing trend in annual rainfall over the study period.