Geological constraints on the spatial distribution and morphology of Erosion gullies in the black soil region of Northeast China: a case study of the Heihe area

Gully erosion threatens soil resources and agricultural production in the black soil region of Northeast China, but geological effects are difficult to separate from terrain and land use. We combined 5407 erosion gullies from the First National Water Resources Census with 1:250,000 lithological and soil-forming parent material maps, a 12.5 m digital elevation model, and land-use data for the Heihe area. We calculated area-normalized and cropland-restricted gully densities, compared five morphometric variables, and used generalized additive models to account for terrain, land cover, spatial structure, and subwatershed clustering. Gully length density differed by more than two orders of magnitude among geological units. Pleistocene basalt had the highest density among major units, whereas Late Triassic–Early Jurassic granites were nearly gully-free even within cropland. The apparent concentration of gullies in the Neogene Sunwu Formation mainly reflected its large outcrop area. Adding lithology to terrain- and cropland-adjusted grid models increased explained deviance by 1.27 percentage points at 1 km and 0.98 percentage points at 2 km. In contrast, lithology or parent material added no more than 0.24 percentage points of deviance to terrain-based models of gully morphometry. Lithology and parent material were nearly redundant, and geological units accounted for much of the elevation and relief variation. Geological substrate therefore provides a small additional signal for where gullies concentrate, whereas its association with gully form is expressed mainly through geologically inherited terrain. These results support using substrate information to refine terrain-based gully erosion risk zoning.