Influences of Basin Morphometry Controls on Gully Development in the Ofu Catchment, Nigeria (Published)
Gully erosion is a major form of land degradation whose occurrence and development are strongly influenced by basin morphometry through its control on runoff concentration, drainage efficiency, slope gradients, and sediment transport. This study assessed the influence of basin morphometric characteristics on gully formation and development in the Ofu River Catchment, Nigeria. A 30-m Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model was processed within a Geographic Information System to delineate the catchment, extract the drainage network, and derive key morphometric parameters. Field investigations identified 42 gully erosion sites where gully dimensions and the Gully Severity Index (GSI) were measured. Pearson and Spearman correlation analyses were used to examine relationships between morphometric variables and gully characteristics, while multiple regression evaluated the collective influence of selected predictors on gully severity. The Ofu River Catchment covers 1,641.74 km² and is characterized by a fourth-order drainage network with a total stream length of 264.00 km. Morphometric analysis revealed a drainage density of 0.161 km km⁻², stream frequency of 0.041 streams km⁻², bifurcation ratio of 3.97, form factor of 0.116, circularity ratio of 0.201, elongation ratio of 0.385, relief ratio of 0.0029, and mean basin slope of 2.86°. These indices indicate an elongated basin with localized runoff concentration along drainage pathways. Correlation analysis showed that drainage position had a strong positive relationship with gully length (r = 0.643, p < 0.001), while slope significantly influenced gully depth (r = 0.502, p = 0.001), catchment surface area (r = 0.528, p < 0.001), and gully severity (r = 0.316, p = 0.042). GSI also increased significantly with drainage position (r = 0.423, p = 0.005). The regression model was statistically significant (R² = 0.384, Adjusted R² = 0.335, F = 7.891, p = 0.0003), indicating that morphometric factors explained 38.4% of the variation in gully severity, while model validation produced an AUC of 0.769 and an accuracy of 71.4%. The findings demonstrate that basin morphometry exerts a significant control on gully formation and expansion through the interaction of slope, drainage position, and contributing area. Although the catchment exhibits low overall drainage density and gentle relief, localized topographic conditions create favorable environments for runoff concentration and gully enlargement. The study highlights the importance of incorporating morphometric indicators into watershed management strategies, gully stabilization programs, and erosion-risk planning within the Ofu River Catchment.
Keywords: Basin morphometry, Gully Severity Index, Nigeria, Ofu River Catchment, drainage network, gully erosion, watershed management