Bihar: initially, soil erosion risk maps for the entire Koshi basin were developed and a study on claims

Patna: Heavy monsoon rains induced by climate change in the Himalayas, the watersheds of Koshi, one of the main rivers, are causing further soil erosion in parts of Bihar. Village after village, soil erosion is rapid, impacting livelihoods and causing displacement.

Nageshwar Mandal and Rameshwar Yadav, both marginal farmers, were displaced this month after their homes were engulfed by the swollen Koshi River in Madhepura district of Bihar. They are among thousands of people displaced after losing their homes and farmland due to erosion in the Koshi region, comprising the districts of Supaul, Saharsa and Madhepura.

Unexpected excess rainfall since mid-June in the watersheds of major rivers in the state and neighboring Nepal has resulted in flooding and the increasing incidence of erosion threatens the existence and survival of thousands of people who inhabit the area. these areas.

In, perhaps, the first assessment of the dynamics of erosion at the basin level, a final research study was carried out by a team of experts using remote sensing (RS) data and a geographic information system. (GIS) to estimate the spatial distribution of soil erosion across the entire Koshi Basin, and identify changes between 1990 and 2010.

The objective of the study, carried out by a team of researchers from the International Center for Integrated Mountain Development (ICIMOD), based in Kathmandu, is to develop a map of conservation priorities.

The study, “Estimation of the dynamics of soil erosion in the Koshi basin using GIS and remote sensing to assess priority areas for conservation», Indicates that the erosion risk class (divided into eight classes from very low to extremely high) remained unchanged between 1990 and 2010 over nearly 87% of the study area, but increased over more than 9% of the area and decreased in more than 3.8%, indicating an overall worsening of the situation.

According to the study, the high levels of water-induced erosion in transboundary Himalayan river basins are contributing to substantial changes in the hydrology and flooding of the basins.

“Soil erosion risk maps have been developed for the entire Koshi Basin using the Revised Universal Soil Loss Equation (RUSLE) in conjunction with GIS and remote sensing data. The study area was divided into eight erosion risk classes, from very low to extremely high, depending on the estimated erosion rates, ”he says.

The study found that the southern area of ​​the basin was less erodible and the central area was highly erodible. The differences in erosion levels between the northern, central and southern parts of the study area were mainly due to topography.

Areas of the very low erosion class were primarily located at lower elevations where the terrain is relatively flat. According to the study, the maximum average soil loss per hectare estimated occurs at altitudes between 1000 and 2000 MASL (minimum achievable soil loss) and the minimum at altitudes between 70 and 100 MASL.

The study further found that the estimated maximum average soil loss rate was 22 t / ha / year (ton / hectare / year – a measure of the mass of soil that has moved per unit area) at from barren land. The total loss of soil from barren land was estimated at 18 million tonnes in 1990 and 15 million tonnes in 2010. The total loss of soil from agricultural land was estimated at 10 million tonnes in 1990 and 14 million tonnes. tonnes in 2010. the loss in the entire Koshi basin was around 40 million tonnes (39 million tonnes in 1990 and 42 million tonnes in 2010).

“The Koshi Basin suffers from a very high level of erosion, which not only affects the land but also causes many negative impacts of downstream sedimentation,” the study said, adding that “it is important to design and implement erosion control practices for the basin, but the area is large.

The study therefore suggested that to maximize effectiveness, “erosion control measures should be targeted to the most vulnerable areas, where the impact is likely to be greatest”.

He pointed out that a high level of erosion leads to high levels of sedimentation which affects storage infrastructure (filling of dam lakes), can destroy agricultural land and contribute to downstream flu risks.

“Although it is well known that the high level of erosion and sedimentation is mainly the result of the young, fragile and tectonically active nature of the Himalayan mountains, little is known about the contribution of the different units and geological / geomorphic landscapes. This poses challenges for planning, in particular the planning of hydraulic infrastructure such as hydroelectric and irrigation systems, where knowledge of the potential risk of sedimentation is paramount, and planning to reduce the risk of erosion ”, a- he declared.

The study underlined that in order to be able to effectively control erosion, it was first necessary to have information on its spatial and temporal distribution.

“Erosion control also has an important role to play in reducing flood risk in the flood plains of Nepal and Bihar, where siltation from flooding is one of the main causes of land loss. useful agriculture, ”he added.

The study said it was first important to identify the areas most susceptible to soil erosion in the Koshi Basin, so that priority areas can be identified for conservation measures.

However, this could be difficult, as soil erosion management strategies in the Koshi Basin are limited by the scarcity and fragmented nature of the data available, he said, adding that “most studies erosion in the Koshi Basin focused on individual plots or watersheds. in the middle mountains of the Nepalese Himalayas, because the topography, the dynamics of land use and the high spatial and temporal variability of precipitation lead to higher levels of erosion.

The Koshi Basin stretches from the Tibetan Plateau in China, through Nepal, to the plains of the Ganges in India. It has diverse topography, geology and geomorphology, and a wide range of land use practices that have led to soil erosion, sediment transport, and land degradation.

The basin contains a rich biodiversity and is a source of valuable ecosystem services that support the lives and livelihoods of millions of people in China, India and Nepal.

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