Study finds an increase in the rate of soil erosion post the extreme floods in August 2018.
In August 2018, the people of Kerala saw one of the worst floods since the great floods of 1924. The state received 96% excess rainfall than predicted, setting off massive floods in most regions. Agriculture, housing, fisheries, animal husbandry and other businesses, and the natural flora and fauna of the state, faced huge losses. The State Land Revenue Department reported around 330 landslides, and the projected economic loss crossed INR 31,000 crore. More than 100 people were reported dead due to the landslides. The landslides also resulted in crop loss, with more than 300 acres of coffee and tea plantations in the state affected.
The erosion of the topsoil is common during extreme floods. This leads to further erosion of the underlying soil and can trigger huge landslides. Studies have shown that flood-related soil erosion events have been increasing across India, especially in states like Kerala, which receives heavy rainfall and has steep gradients and undulations in its topography. In a study, published in the Journal of the Indian Society of Remote Sensing, researchers from the Indian Institute of Technology Bombay (IIT Bombay) and Rural Data Research and Analysis (RuDRA), Mumbai, assessed the soil erosion rates before, during and after the Kerala 2018 floods. This data helped in understanding the extent of damage caused by the floods to the land and soil system in the state and can guide the relief measures needed to improve the overall conservation of natural resources.
The frequency of extreme floods and natural calamities has increased in the last few decades. Several reasons have been cited for this, none more frequently than climate change. “Reasons for the increase in flood events depends on many factors varying from climate change- induced factors, and use, land cover change, anthropogenic stressors [such as mismanagement] and antecedent hydrological conditions [such as heavy precipitation rate],” says Prof Pennan Chinnasamy from the Center for Technology Alternatives for Rural Areas (CTARA), IIT Bombay.
The researchers studied the factors affecting soil erosion rates using data such as precipitation and rainfall, digital soil map, elevations in the land and satellite imagery obtained from remote sensing platforms. They also analysed the variation in soil erosion rates across the state. The results were startling.
The average soil erosion rate increased by 80% during the 2018 Kerala floods compared to January 2018. This is in line with the Government of Kerala study conducted in 2018, which estimated that up to 71% of the state’s total area was eroded during the floods.
The districts of Kerala surrounding the Western Ghats received higher amounts of rainfall, with Idukki being the highest. The mountain ranges lift moist hot air, leading to condensation and eventually causing rainfall, a phenomenon known as orographic precipitation. The Rainfall Erosivity Factor, estimating the soil's susceptibility to erosion, ranged from 133 to 179, indicating high rainfall and associated soil erosion from June to August and a huge variation between the districts.
“Rainfall in Kerala has always been higher than the average values due to the orographic effect from the Western Ghats, which is due to the undulating topography,” said Prof Chinnasamy. “With such higher rainfall rates in Kerala, it is necessary to protect the land from excess soil erosion as the rainfall quantitatively represents the impact on the soil surface,” he added.
The Soil Erodibility Factor, estimating the susceptibility of the soil to erosion, was on the higher end, with the highest in Alappuzha district, indicating that the sandy soil present across the state is prone to erosion. This factor also pointed towards the highly undulating topography of the Western Ghats. Such a topography with curves and slopes and a loose topsoil increases the erosion.
The Cover Management Factor analysis showed that the vegetation cover decreased by around 63% from January 2018 to August 2018. The vegetative land was converted to near barren condition due to the floods, making it further susceptible to soil erosion.
Similarly, the average Conservation Practice Factor, which quantifies the lack of impact of the soil and water conservation measures on minimising soil erosion, increased from 0.8 in January 2018 to 0.89 in August 2018. The highest increase, from 0.5 to 1, was seen in Idukki, Kottayam, Ernakulam and Thrissur. The increase points towards cropland damage due to waterlogging and washing away of topsoil due to the floods.
The researchers found that the Idukki district had an increase of 220% in the soil erosion rate during the floods, while Ernakulam showed an increase of 95%. Both districts have the dual distinction of being bordered by the Western Ghats, causing heavy rainfall, and a huge rise in urbanisation. A study conducted in 2017 found that forest cover in Idukki had fallen by over 44% between 1925 and 2012 while the settlements increased by 400%.
On similar lines, the rate and total amount of sediment deposited increased dramatically during the 2018 floods, with Idukki being the worst affected. The sediment comes from the soil carried by the rivers and other water streams from the land into the ocean. Since the district and the surrounding area are home to many rivers and water streams, high erosion rates also cause downstream erosion as more water gets accumulated and flows with higher velocities, thereby increasing the amount of deposition.
Soil erosion rate before, during and after the 2018 Kerala floods. [Source]
The data also shows that the high erosion rates are not only due to the heavy rainfall but also because of the unsustainable conversion of natural land into human settlements. “In some regions, there is a monsoon shift observed or an increase in the concentration of rain events, which has led to less water storage in the watershed and more floods. In other regions, we have noticed a drastic change in land use and land cover patterns which alters the hydrological regime leading to flash floods,” says Prof Chinnasamy.
It is, therefore, necessary to understand and improve the methods of human settlement and soil management towards a more sustainable, climate-friendly future. “Setting up monitoring stations for collection of sediment data and soil nutrient data and educating the locals on the need for soil conservation activities are much needed to tackle these sensitive issues,” Prof Chinnasamy concludes.
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