Characterizing rainfall occurrence in India: Natural variability and recent trends

A deficit in rainy days can affect crop production and escalate groundwater abstraction leading to groundwater depletion, thus may threaten food and water security in the Indian subcontinent. The available researches on rainfall occurrence in India represent inconsistency in several fundamental considerations (spatial scale of analysis, the definition of rainfall classes, methods, dataset, study period, etc.) and lack of studies for non-monsoon seasons over large parts of the subcontinent. Here we attempt to deal with these inconsistencies by examining space-time patterns in seasonal count of rainfall events and their count in different rainfall classes during 1951-2015 for all meteorological seasons of India using high resolution (0.25 degrees) gridded daily rainfall dataset. The results show a widespread decline in monsoon rainy days over major crop-growing regions including north-central, central, and northeast India. A reduction in rainy days is also observed over parts of central India in premonsoon; over north-central and northeast India in postmonsoon; and over peninsular India in winter. These trends are mainly observed in events with rainfall <95th percentile (light to heavy rainfall events) in all the seasons. Occurrence of extreme events (above 95th and 99th percentile) exhibits contrasting trends over different zones with mainly an increase over central and south-central India and decline over central-north India. However, trends in extremes are highly dispersed in space and are conclusive only over a few regions. The observed trends substantially deviate from the patterns reported in past studies for different regions. Rainfall occurrence in different meteorological zones also represents strong interannual and decadal variability, however, the observed trends have emerged beyond the inherent natural variability. The present findings provide an essential foundation for several future explorations with improved understanding of change in rainfall frequency and its possible impacts. It would also assist in developing a comprehensive framework for year-wide management of water resources and agriculture, considering all crop seasons.

Automated summary

The study reveals a widespread decline in seasonal rainfall events in India over the past few decades, especially in major crop-growing regions. Light to moderate rainfall events have decreased in all seasons, while trends in extreme rainfall events show varying patterns of increase and decrease in different regions. The observed changes in rainfall frequency trends have exceeded natural variability.