What is the best technique to estimate topographic thresholds of gully erosion? Insights from a case study on the permanent gullies of Rarh plain, India

The Rarh plain in the Lower Ganga Basin in India, best known for its lateritic landscape and gullied tracts, faces grave problems brought about by all types of soil erosion, of which gully erosion is the most conspicuous. The present study uses data collected through field measurements at 110 gully heads in ten sites of the Rarh plain, to first assess the applicability of various methods used to construct critical topographic threshold lines, and secondly, to characterise topographic threshold conditions of the permanent gullies in the Rarh plain as well as to identify main factors that promoted gully initiation in this region. It is concluded that thresholds defined though orthogonal regression are more appropriate than manual fitting of threshold line or employing quantile or nonlinear regression techniques for the same. The critical topographic threshold conditions of gullying in the Rarh plain, expressed by the relationship S = 0.118A ^−0.111 between the gully head slope gradient (S) and upslope catchment area (A) can be used to map areas under risk of gully head development in the lateritic terrain, mainly in the deforestation fronts. The value of the exponent (0.111) suggests that subsurface processes and mass failures are the main processes of gully growth at present, and it is suspected that gully erosion in this region is not a recent phenomenon, judging by rather small gully head catchment areas as well as the low AS ² range of 2–170 m ². Although statistical evidence in favour of distinct site-specific thresholds is not found, results of this study indicate that gullies under eucalyptus stands have a significantly lower threshold than those in other land cover types. Gully erosion in this region was most likely triggered by massive changes in land cover and land use that commenced from the middle of 19th century and continued into the first few decades of 20th century. However, gullies observed under eucalyptus stands are much younger, judging by their larger upslope catchment areas compared to gullies found in other land covers. Albeit it is observed that the lateritic terrain of Rarh plain offers more resistance to gully initiation than croplands or Mediterranean badlands, the threshold of gully head development is much lower than that of the primary laterites of the adjacent Chhotanagpur plateau fringe as well as other comparable soil types from around the world.