Forest cover affects gully expansion at the tropical watershed scale: Case study of Luzinzi in Eastern DR Congo

This study aimed at assessing the land use and land cover change and its effect on the gully expansion in an agricultural and woodland tropical watershed. We used the case study of Luzinzi watershed in South-Kivu, eastern Democratic Republic of Congo (DRC) where gullies are in development. We used very high-resolution (VHR) images downloaded from Google Earth and unmanned aerial vehicles (UAV) images. ArcGIS 10.7 software helped for spatial analysis and images georeferencing while ENVI 5.3 tools were used for classification. The gullies were then digitized and characterized to determine their width, length, depth, surface and volume. The digital elevation model (DEM) was used to determine the contributing area as well as the slope at the gully headcut. The land use at the headcuts and in the gully was extracted from the different land uses obtained from classified images and validated by field measures. Results showed significant changes in land use and land cover throughout the watershed; changes that affected gully expansion. From 2011 to 2020, number of gullies passed from 38 to 201. These gullies were increasing not only in number but also in characteristics such as gully length and headcut. Their volumes increased from year to year in the same trend as woodland and forest reduction. Forest played important role in gully stabilization: gullies located in forest presented a linear retreat rate (~4.6 m) than those in other land uses (~2.4 m) from 2010 to 2020. The forest cover reduced gully surface at linear rate while the depth rate still increased. A forest cover of ~10% led to a gully expansion of ~700 m3 yr−1 and reduced to 300-400 m3 yr−1 with 30% forest coverage. Thus, forest cover at the gully headcut and in the contributing area helped to stabilize gullies. The permanent maintenance of forest and woodland covers as well as the reduction of anthropogenic activities in gullies are to be promoted at the watershed scale along with other measures to contribute to effective land resource management in the region.