- Rainfall linked to cyclones and existing soil moisture levels of river basins shape the severity of floods, says a study.
- The scientists studied the impact of tropical cyclones on flooding in four major Indian river basins in India’s east coast from 1981 to 2019.
- Cyclones that make landfall in the pre-monsoon season are less likely to cause severe flooding because of drier land conditions while flooding is more likely when the cyclone system encounters moisture-saturated land during monsoons and immediately after the monsoon.
- Experts bat for monitoring soil conditions, such as soil moisture, to predict flood severity from cyclones.
Flooding in India’s river basins is more likely when the rainfall-embedded cyclone system encounters moisture-saturated land during monsoons and immediately after the monsoon, says a recent study.
Cyclone-linked rainfall and the existing soil moisture levels of the basin, shape severity of floods, says the study by Indian Institute of Technology (IIT)-Gandhinagar researchers Vimal Mishra and Akshay Rajeev. The scientists examined the impact of tropical cyclones on flooding in four major Indian river basins (Subarnarekha, Brahmani, Mahanadi and Vamshadhara) in India’s east coast covering cyclones from 1981 to 2019.
Pre-monsoon season and cyclones
The researchers find that cyclones that make landfall in the pre-monsoon season are less likely to cause severe flooding because low soil moisture conditions of the basin prevent the cyclonic rain water from directly becoming runoff as the rainwater is sponged off by the dry land. And while India’s forecast of the cyclone tracks and landfall positions of the cyclones have “significantly improved” with time and human fatalities have reduced in the recent period, it is also crucial to monitor the basin characteristics, to minimise damage from cyclones.
“The timing of cyclones is extremely important. So, for instance, if cyclones occur before monsoon months (April-May) when land is very, very dry, and even when you have heavy rainfall, it doesn’t result in floods,” study’s corresponding author Vimal Mishra from IIT-Gandhinagar, told Mongabay-India.
“When cyclones overlap with monsoon or just after the monsoon season, the chances of cyclones causing floods, increase. Cyclone Phailin is an example. As you move forward in time, such as in the month of October, you have more chances of flooding if you have a cyclone that brings heavy rainfall, then it is likely to create floods because we have a lot of moisture stored in the land from the monsoon rains,” explained Mishra.
“But as you go further ahead to the months of November, December and January, the likelihood of cyclone-driven flooding goes down.”
An analysis of the past records
The scientists culled data on cyclone tracks from the Cyclone eAtlas of India Meteorological Department. The Cyclone eAtlas-IMD is an electronic atlas of cyclonic disturbances (CD) over the North Indian Ocean from 1891 onwards. “We also looked at extreme rainfall caused by the cyclones and tried to unravel whether that resulted in river basin floods,” Mishra adds.
In the Mahanadi basin, the August 1981 cyclone deluged a large part of the basin which is attributed to wet soil from the monsoons that generated a higher daily runoff during the cyclonic rains. Similarly, in the Vamshadhara basin, high soil moisture during the monsoon created the recipe for flooding in cyclone Pyarr in September 2005. “Hence, even though TCs (tropical cyclones) have low frequency and intensity during the monsoon period, any TC that does make landfall is likely to produce severe floods in the region as the antecedent moisture condition is high in each basin,” the paper says.
The 1999 Odisha supercyclone and cyclones Phailin (2013) and Hudhud (2014) round out the list of cyclones that led to severe flooding in the post-monsoon period. The Odisha supercyclone, that killed over 10,000 people, also showcases the impacts of clustering cyclones when two cyclones affected both the Brahmani and Mahanadi basins within a short period.
The 1999 Odisha cyclone caused extreme flooding even though it occurred towards the end of October when the land was drier. In this instance, two cyclones consecutively made landfall in the exact location, cooking up enough moisture in the soil to trigger extreme flooding in the basins.
“The first of them occurred during mid-October and affected all four basins but did not cause extensive flooding as antecedent soil moisture had started to decrease after the summer monsoon season. However, published reports show that the second TC affected the coastal region of Subarnarekha, Brahmani, and Mahanadi basins leading to extreme flooding in the two basins.”
Researchers explain that in addition to the severe rainfall associated with the cyclones, the first cyclone rain event may have driven up the soil moisture which created conditions amenable for the second cyclone event to add a double whammy – the rains from the second cyclone event in combination with the high soil moisture from the first event caused the excess water to runoff and flood. “It is also worth noting that the severity of the second tropical cyclone was stronger than the first one which may also have influenced flooding,” the authors write in the paper.
The need for data on soil features
Madhavan Rajeevan, the former secretary to the Ministry of Earth Sciences (MoES), explains that the study emphasises the importance of the processes on the land surface that can influence cyclonic flooding.
“The research re-emphasises the need for soil moisture data and assimilation of soil moisture data into the models so flood events can be predicted. Our MoES institutions are collecting soil observations including soil temperature and moisture, mainly for the agriculture sector. When we combine the high-resolution soil moisture data with satellite-based soil observations, our models will be better able to inform us on the rainfall associated with the cyclone landfalls, because generally, the models don’t do well in predicting rainfall associated with cyclones,” Rajeevan, who was not associated with the study, told Mongabay-India.
The north Indian Ocean (Arabian Sea and the Bay of Bengal) accounts for 6% of the global tropical cyclones annually but is responsible for more than 80% of the global fatalities due to cyclones, according to research. An analysis by Koll et al. shows that the intensity of cyclones has increased in the Arabian Sea by 20-40% in the last four decades and sea surface temperatures (SSTs) leading to cyclogenesis in the Arabian Sea are 1.2-1.4 degrees Celsius higher in the recent decades, compared to SSTs four decades ago. Rapid warming in the north Indian Ocean, associated with global warming, tends to enhance the heat flux from the ocean to the atmosphere and favour the rapid intensification of cyclones, according to Koll et al.
A recent Intergovernmental Panel on Climate Change (IPCC) report warns that unless there are immediate and large-scale reductions in greenhouse gas emissions, limiting global warming to close to 1.5 degrees Celsius or even 2 degrees Celsius over pre-industrial times will be beyond reach.
Banner image: The aftermath of Cyclone Phailin. Photo by EU/ECHO Samuel Marie-Fanon/Flickr.