Spatiotemporal Changes of Extreme Rainfall Events and Associated Human Fatalities across Indian States (1970–2019)

Rain does not fall evenly across the planet, and it rarely falls the same way from one year to the next. In places like India, where the annual monsoon season dictates the rhythm of life, the difference between a normal shower and a deluge can mean the difference between a bountiful harvest and a flooded village. For decades, scientists have watched the weather, tracking how much rain falls and where. But as the world warms, the question has shifted from simply measuring the water to understanding how the character of that water is changing. Are storms becoming more frequent, or just more intense? And crucially, how do these shifts in the sky translate to danger on the ground for the millions of people living in river valleys and coastal plains? This is the territory of hydroclimatology, a field dedicated to mapping the relationship between water in the atmosphere and the hazards it creates for human societies.

A team of researchers set out to answer these questions for India, a country where the monsoon is the lifeblood of the economy and the primary source of water. They looked at fifty years of history, from 1970 to 2019, examining every state and union territory to see how the pattern of extreme rain has evolved. They focused on three specific categories of rainfall intensity: heavy rain, very heavy rain, and extreme heavy rain. By cross-referencing detailed rainfall records with official reports of human fatalities, they built a clear picture of where the rain is changing and where those changes are costing lives. Their work reveals that while the total amount of rain might be shifting in complex ways across the country, the most dangerous storms are becoming more frequent in specific regions, creating a stark map of vulnerability where the weather meets the population.

The researchers began by gathering daily rainfall data from thousands of weather stations across the country, organizing this information into a grid that covers every corner of India. They then sorted the rain into buckets based on how much fell in a single day. A "heavy" rain event is defined as a day with between 64.5 and 124.4 millimeters of rain. A "very heavy" event exceeds 124.5 millimeters, and an "extreme heavy" event is anything above 224.5 millimeters. With these definitions in place, they tracked how often each type of event occurred in every state over the last half-century. They also pulled data on human deaths directly caused by these weather events, such as drownings in floods or victims of landslides, to see if there was a direct link between the intensity of the rain and the loss of life.

The results show that India's rainfall is not changing in a uniform way. Some states are seeing more rain, while others are seeing less, and the intensity of the storms is shifting differently in each place. States along the Western Ghats, such as Kerala and Karnataka, and the northeastern states like Meghalaya and Assam, consistently experience high rainfall. These areas are also seeing an increase in the frequency of the most severe storms. In contrast, states in the northwest like Rajasthan, Haryana, and Punjab generally receive less rain, though they are not immune to extreme events. The study highlights a clear divide: while some regions are becoming wetter, others are becoming drier, but the storms that do occur are often more violent.

When the researchers looked at the trends over the fifty-year period, they found a worrying shift in the nature of the rain. The number of days with standard "heavy" rainfall has actually decreased slightly across the country. However, the days with "very heavy" and "extreme heavy" rainfall are becoming more common. This suggests that while the total number of rainy days might be dropping, the rain that does fall is arriving in more intense, dangerous bursts. It is as if the sky is holding back the water for longer periods and then releasing it all at once, rather than distributing it evenly. This intensification is particularly evident in states like Uttar Pradesh, Bihar, and Maharashtra, where the frequency of these extreme events has risen significantly.

The human cost of these changes is stark and varies greatly from state to state. The study found a direct connection between the frequency of extreme rainfall and the number of deaths. Uttar Pradesh emerged as the most affected state, with over 10,000 people losing their lives to rainfall-related disasters between 1970 and 2019. Bihar, West Bengal, and Maharashtra also recorded high death tolls, with thousands of lives lost in each. The researchers calculated a "human loss rate" for each state, which measures how many people died per 100,000 residents. Uttar Pradesh had the highest rate, followed by Bihar and West Bengal. These states are often home to major river basins and dense populations, meaning that when a river overflows due to intense rain, the impact is catastrophic.

In contrast, states like Kerala, while experiencing a high number of extreme rain events and significant flooding, had a human loss rate of 0.32, which is higher than many other states such as Karnataka (0.11) and Meghalaya (0.03). The study notes that the geography of a place plays a huge role in its vulnerability. Areas at the foot of the Himalayas, such as Uttarakhand, or along the steep slopes of the Western Ghats, are prone to landslides when heavy rain strikes. The 2013 floods in Uttarakhand, for instance, were triggered by a single day of rain that was nearly four times the normal amount, leading to nearly 6,000 deaths. Similarly, the coastal state of Kerala has seen massive floods in recent years, driven by consecutive days of heavy rain that overwhelmed rivers and drainage systems.

The researchers also examined how these trends have changed over time, breaking the fifty-year period into decades. They found that the variability of rainfall is increasing. Some states, like Madhya Pradesh and Bihar, saw a sharp decline in rainfall during the 2000s, only to see it return to normal levels in later years. This unpredictability makes it difficult for farmers and city planners to prepare. The study confirms that the risk is not just about how much rain falls, but when and how it falls. The increasing frequency of extreme events in river basins and densely populated areas suggests that the current infrastructure and disaster management systems are struggling to keep pace with the changing climate.

Ultimately, the study paints a picture of a country where the weather is becoming more volatile and more dangerous for those living in the path of the storms. The link between extreme rainfall and human fatalities is undeniable, with the most severe losses occurring in states where major rivers flow through crowded cities and towns. The findings suggest that as the climate continues to warm, the pattern of heavy and extreme rainfall will likely persist or worsen, particularly in the river basins of the Ganges and Brahmaputra and along the western coast. The authors conclude that to protect lives, India needs to strengthen its early warning systems and develop disaster management plans that are specific to the unique risks of each state. The data shows that the threat is real and growing, and the only way forward is to adapt to a new reality where the rain falls harder and more unpredictably than it did in the past.