Submitted to: Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2016
Publication Date: 9/7/2016
Publication URL: https://handle.nal.usda.gov/10113/5763094
Citation: Zegeye, A.D., Langendoen, E.J., Stoof, C., Tilahun, S.A., Dagnew, D.C., Zimmale, F.A., Guzman, C.D., Yitaferu, B., Steenhuis, T.S. 2016. Morphological dynamics of gully systems in the subhumid Ethiopian Highlands: The Debre Mawi watershed. Soil. 2(3):443-458.

Interpretive Summary: Gully erosion is a major cause of land degradation in northern Ethiopia. Reducing gully erosion has been fairly successful in the semi-arid regions of Ethiopia such as Tigray, however not in the humid Amhara region. For developing more effective gully rehabilitation measures for humid northern Ethiopia, scientists at the USDA, ARS, National Sedimentation Laboratory in collaboration with researchers at Cornell University and Bahir Dar University, Ethiopia, have investigated the gully evolution processes in the Debre-Mewi watershed, about 30 km south of Lake Tana for the 2013 and 2014 rainy seasons. Data were obtained directly from field topographic surveying, measuring flow and sediment transport, shallow ground water sampling, and digitizing aerial imagery. The result showed that saturation of soil was the major cause of gully incision and expansion. The total soil loss was estimated as 140 tons per hectare per year. Gully erosion control measures should focus on stabilizing head cuts and gully sidewalls by lowering water table elevations through subsurface drainage, and regrading the gully head and sidewall to reduce the occurrence of gravity-induced mass failures.

Technical Abstract: Gully expansion in the Ethiopian highlands dissects vital agricultural lands with the eroded materials adversely impacting downstream resources, for example as they accumulate in reservoirs. While gully expansion and rehabilitation have been more extensively researched in the semi-arid region of Ethiopia, few studies have been conducted in the (sub) humid region. For that reason, we assessed the severity of gully erosion and identified gully-forming processes in 13 selected permanent gullies in the sub-humid Debre Mawi watershed, 30 km south of Lake Tana, Ethiopia. In addition, the rate of expansion of the entire drainage network in the watershed was determined using 50 cm resolution aerial imagery flown in 2005 and 2013. More than 0.7 million tons (or 144 t ha-1 yr-1) of soil was lost during this period due to actively expanding gullies. The net gully area in the entire watershed increased almost five fold from 4.5 ha in 2005 to 20.4 ha in 2013 (> 3% of the watershed area), indicating the growing severity of gully erosion in the catchment. Soil losses were caused by upslope migrating gully heads through a combination of gully head collapse and removal of the failed material by runoff. Collapse of gully banks and retreat of headcuts was most severe in locations where elevated groundwater tables saturated gully head and bank soils, destabilizing the soils by decreasing their shear strength. Elevated groundwater tables were thereby the most important cause of gully expansion. Additional factors that strongly relate to bank collapse were the height of the gully head and the size of the drainage area. Soil physical properties (e.g., texture and bulk density) only had minor effects. Conservation practices that address the most important controlling factors are principally the most effective ways of protecting gully development and expansion. These consist of lowering water table elevation through subsurface drainage, and regrading the gully head and sidewall to reduce the occurrence of gravity-induced mass failures. Planting suitable vegetation on the gully face and around the boundary will also decrease the risk of bank failure by reducing pore-water pressures and reinforcing the soil. Best management practices affecting the runoff contributing catchment may decrease the overland runoff-induced gully head erosion.