Rainfall Trend and Drought Analysis in Magadh Region Bihar

Abstract

Climate change is increasingly influencing regional hydrology, and Bihar reflects this sensitivity through contrasting climatic patterns. While northern Bihar regularly experiences floods, the southern part, including the Magadh division, is witnessing declining rainfall and recurring droughts. This study examines longterm rainfall trends in the five districts of Magadh division—Arwal, Aurangabad, Gaya, Jehanabad, and Nawada, using rainfall records from 1970 to 2020. The Mann–Kendall test and Sen’s slope estimator were applied to analyze annual, seasonal, and monthly precipitation trends. The results indicate a significant decrease in rainfall in Nawada, Aurangabad, and Jehanabad. Nawada, in particular, shows a notable reduction in both monsoon (4.8 mm/year) and annual rainfall (5.8 mm/year), suggesting increasing hydro-climatic stress. Frequent drought events, occurring every 2-4 years, were also observed across the region. To assess drought severity, the standardized precipitation index (SPI) was computed by fitting the long-term rainfall data to a probability distribution and normalizing it. Positive SPI values indicate wetter-than-normal years, while negative values represent rainfall deficits. The SPI results reveal extreme drought occurrences in Nawada, Aurangabad, Gaya, and Jehanabad, with severe drought recorded in all districts. Nawada experienced the highest drought frequency (four severe events), followed by Aurangabad and Gaya (three each), Arwal (two), and Jehanabad (one). These findings underscore the growing vulnerability of southern Bihar and highlight the need for climate-resilient planning and drought-mitigation strategies.