Magnitude of floods decreased between 1970 and 2010: Study
The magnitude of floods decreased in most parts of India between 1970 and 2010 as compared to the previous 100 years
The magnitude of floods decreased in most parts of India between 1970 and 2010 as compared to the previous 100 years even as flood intensity increased in north-western Himalayan and Malabar regions in the pre-monsoon and early monsoon season in the same period, according to a new study based on water flow data from 173 gauges across India.
The study, “The changing magnitude and timing of riverine floods in India”, published last week in peer reviewed journal, Nature, comes at the time when the flash floods caused by extreme rainfall have caused the death of 148 people in Himachal Pradesh (78) and Uttarakhand (70) since the onset of monsoon on June 1.
The Central Water Commission, India’s water regulatory body, has installed gauges, also known as water sensors, across river systems to measure the flow of water for hydrological purposes including flood warning.
The study, by the Indian Institute of Technology (IIT) Delhi and IIT-Roorkee said, flash floods in the higher reaches of Ganga and Indus regions are now happening in the early monsoon period, a change that could have “catastrophic outcomes” since the entire flood management system needs to be redesigned to change in the flooding pattern.
A possible reason for higher frequency of early flash floods is climate change induced alteration in the rain-snow transition zone in the higher reaches which could advance spring/pre-monsoon flood peaks by several weeks in snowmelt-dominated regions, the study added.
Another region where the flood intensity has increased (by 8% per decade) during the study period was in the west flowing rivers from Tadri to Kanyakumari (Chaliyar, Periyar, Bharathapuzha, Vamanapuram, etc.) in the Malabar coast , the result of intense pre-monsoon rainfall.
“Increasing discharges and early floods on the Malabar coast in the pre-monsoon season may have severe implications for agriculture, as it is a significant crop harvesting time,” the study added.
The magnitude of flooding refers to its severity without any assessment of the damage; flood intensity refers to depth and volume of inundation.
It is likely that the situation has worsened since 2010. Hindustan Times’ own analysis have shown that monsoon rains have become increasingly erratic in recent years, with instances of extreme rainfall increasing.
The monsoon season accounts for 80% of India’s annual rainfall and floods are a common occurrence in this season. Floods have killed 113,390 people between 1975 and 2015, averaging 2,765 deaths per year. In this study, 85% of the floods identified across all stations were recorded during the monsoon season between June and September.
The study, which did not cover north-eastern India, said that 74% (128) of the 173 gauging stations studied showed a declining trend in flood magnitude with 47 showing “significant” decrease. In contrast, only 26% (45) of the gauging stations showed an increasing trend and only four stations showed a “significant” increase.
In the main Ganga basin, which is home to about 40% of India’s population, the study recorded a 17% per decade decline in flood magnitude.
“The inference is that the 100-year flood discharge has decreased by more than half during these 40 years at many locations in the western and central Ganga basin,” the study said.
The authors said that the decreasing flood magnitudes in the Ganga basin was “strongly” linked to declining soil moisture and monsoon precipitation influenced by Indian Ocean warming and atmospheric aerosols in the region.
The flood magnitude has also declined in Narmada basin that covers a large part of Gujarat and Madhya Pradesh primarily because of the “construction of dams” in this region during the study period.
The study found that in the Marathwada region of the Deccan plateau, which has been experiencing severe droughts in recent times, river flows were found to be decreasing at an average rate of 8% per decade during the monsoon season and 31% in pre-monsoon.
The study found a clear relationship between the catchment area and the observed percentage change in flood magnitude per decade.
The flood magnitudes over larger catchments are decreasing at higher rates, and as the catchment size decreases, the rate of change is also decreasing, the study said, highlighting the role the dams (which are mostly on big rivers) play in reducing the flood intensity.
“The construction of large dams and reservoirs, which are more prevalent in larger catchments…has been shown to attenuate flood peaks in existing studies,” the study said. However, the study cautioned that improperly operated dams may aggravate flooding.
The study said if these changes in the flood magnitude and timing trends are not fully understood, significant economic and environmental consequences could emerge, given that reservoir filling, ecosystems, and communities are tuned to typical seasonal flooding patterns.
For instance, diminished flood magnitudes can lead to lower reservoir fillings, adversely impacting water supply, irrigation, and hydropower generation, the study said, highlighting that similar shifting in flooding has been recorded in other monsoon areas such as Mekong, Peninsular Malaysia, Sumatra and Yangtze river valley in China.
