A multi-method approach to analyse changes in gully characteristics between 2009 and 2018 in southeast Nigeria

Gully erosion is the dominant environmental problem in southeast Nigeria and has led to loss of human and material resources. In this study, we evaluated changes in gully characteristics in southeast Nigeria and their potential drivers between 2009 and 2018 using a multi-method approach: analysis of high-resolution satellite imagery (2–5 m) and focus group discussions. Gully numbers increased from 26 to 39 and mean gully length increased from 0.39 to 0.43 km. We found that land adjacent to rivers had the highest concentration of gullies which is associated with an increase in slope angle from 10 to 58% up to 500 m from rivers. Regarding potential gully-drivers, land-use changes were observed. Non-vegetated lands increased from 58.6 to 144.7 km2 between 2009 and 2018, while reductions in fallow lands from 281.2 to 57.8 km2 were observed. Results from focus group meetings indicate there was no gullying in the area before the Nigerian civil war (1967–1970). War-time activities such as digging trenches and increased population density were said to have led to the formation of the oldest gullies in the area. Although war-time activities have ceased, meeting attendees believed that present land-use changes have increased the volume of surface runoff and thus enhancing gully erosion. Incorporating local knowledge in this study has therefore provided a valuable understanding on the key drivers of gullying that pre-dates the availability of freely available high-resolution satellite data.