Geomorphic Threshold Estimation for Gully Erosion in the Lateritic Soil of Birbhum, West Bengal, India

Abstract. It is a growing concern that accelerated soil erosion which aggravated by the processes of water erosion (rainsplash, inter-rill, rill, gully erosion and soil piping) in the tropical region and soil loss through crop harvesting represent a seriously threat to soil resource and the ecosystem services that it prevails. The present study examines a quantitative approach to predict critical conditions and locations where gully heads might develop in the lateritic terrain, located at eastern plateau fringe of Rajmahal Basalt Traps, India (Birbhum, West Bengal). The modern concept of geomorphic threshold is applied here on the issue of gully erosion hazard (specially in Indian context) to identify the critical slope of gully head (S) and upstream drainage area (A) with a core relationship of S = a A^−b. On the basis of 118 gully heads we have derived statistically significant relationships between slope and drainage area (r = −0.55), overland flow (Q) and slope length (L) (r = 0.69), relative shear stress (τ) and slope (r = 0.92), overland flow detachment rate (H) and eroding force of overland flow (F) (r = 0.98). The established S – A critical relationship, as geomorphic threshold line, is expressed as S = 17.419 A^−0.2517, above which gully initiation occurs on the laterites. This equation can be used a predictive model to locate the vulnerable un-trenched slopes (i.e. potential gully erosion locations) in other lateritic areas of West Bengal. The constant b value (0.2517) and Montgomery – Dietrich Envelope suggest a relative dominance of overland flow (52.51 % of gully heads) in the processes of gully erosion. The result of revised MMF model reflects soil loss of 2.33 to 19.9 kg m⁻² yr⁻¹ due to overland flow erosion.