Quantifying the impacts of spatiotemporal land use and land cover changes on soil loss across agroecologies and slope categories using GIS and RUSLE model in Zoa watershed, southwest Ethiopia

Background
Soil erosion in Ethiopian highlands is highly consistent with land use/land cover (LULC) changes that are associated with deforestation and a decline in biodiversity. However, identifying soil erosion-prone areas and
quantifying soil loss in rugged terrains and various agroecologies due to LULC changes have not been dedicated to scientific studies. Therefore, we quantified the impacts of spatiotemporal LULC changes on soil loss across agroecologies and slope categories using GIS and RUSLE model from 1985 to 2021 in Zoa watershed, southwest Ethiopia. Moreover, prioritizing erosion severity risks at sub-watersheds and quantifying temporal sediment yield is essential for better conservation planning. Landsat images, rainfall, Digital Elevation Model, and soil data were obtained from field observations and secondary sources.
Results
Bareland and farmland have been expanding at the expense of other land use types. The annual soil loss in the watershed ranged from 0 to 113.21 t ha−1 year−1, 0 to 163.16 t ha−1 year−1, and 0 to 194.58 t ha−1 year−1 with amean annual soil loss of 21.07, 29.35 and 40.93 t ha−1 year−1 in 1985, 2000, and 2021, respectively. Among LULC classes, the highest soil loss was generated from bareland (31.73 t ha−1 year−1 ) and farmland (27.08 t ha−1 year−1 ) in 1985 later upsurged to 35.52 t ha−1 year−1 and 59.91 t ha−1 year−1 in 2021, respectively, due to the maximum susceptibility of soil erosion risks from unprotected surfaces. The results also revealed that the lowland agroecology generated the highest mean soil loss of 24.05 t ha−1 year−1 in 1985, 39.74 t ha−1 year−1 in 2000, and increased to 57.55 t ha−1 year−1 in 2021. Considering the slope categories, the highest and most excruciating soil loss was engendered from steep (35.55–60.78 t ha–1 year–1) and very steep (52.48–72.69 t ha−1 year−1 ) slope terrains during 1985–2021. The northwestern part of the watershed is the most erosion-prone area which is now expanding to the central and western parts of the watershed. The sediment yield increased at the fastest rate at the watershed outlet, from 39.3% in 1985 to 94.26% in 2021.
Conclusions
The results of this study indicated that the conversion of other LULC categories into farmland was the most detrimental to a watershed in terms of soil loss, which necessitates the implementation of appropriate soil and water conservation measures with effective design by considering spatial variability to reduce soil erosion hazards.