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Title: Gullies, a critical link in landscape soil loss: A case study in the subhumid highlands of Ethiopia

Author
item ZEGEYE, ASSEFA - Cornell University
item Langendoen, Eddy
item GUZMAN, CHRISTIAN - Cornell University
item DAGNEW, DESSALEGN - Bahir Dar University
item AMARE, SELAMAWIT - Bahir Dar University
item TILAHUN, SEIFU - Bahir Dar University
item STEENHUIS, TAMMO - Cornell University

Submitted to: Land Degradation and Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2017
Publication Date: 4/16/2018
Citation: Zegeye, A.D., Langendoen, E.J., Guzman, C.D., Dagnew, D.C., Amare, S.D., Tilahun, S.A., Steenhuis, T.S. 2018. Gullies, a critical link in landscape soil loss: A case study in the subhumid highlands of Ethiopia. Land Degradation and Development. 29(4): 1222-1232. DOI: 10.1002/ldr.2875.
DOI: https://doi.org/10.1002/ldr.2875

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 as sediment concentrations in the rivers have remained high. 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 and sediment concentrations in the Debre-Mewi watershed, about 30 km south of Lake Tana for the 2013 and 2014 rainy seasons. Measurements of sediment concentrations and runoff showed that 92% of the total sediment carried by runoff originated in the gully and only 8% was contributed by sheet and rill erosion. The gully erosion was mainly caused by gravity-induced failures of the gully head and sidewalls when groundwater table was elevated. Gully erosion control measures should therefore focus on stabilizing head cuts and gully sidewalls by to reduce the occurrence of gravity-induced mass failures.

Technical Abstract: Soil and water conservation programs in the humid Ethiopian highlands have not been effective in lowering sediment concentrations, partly due to the formation of gullies in the periodically saturated valley bottom lands. Despite the major effect of gully erosion on sediment loads, its contribution to the amount of soil loss and sediment concentration have not been well quantified. Our objective is to better understand sediment contributed by a gully and explore effective control measures. The 608 ha Debre-Mawi watershed, 30 km south of Lake Tana, was selected for this purpose. We measured the discharge and sediment concentrations upstream and downstream of one selected, 5-m deep gully located in a valley bottom and at the outlet of a 95 ha catchment for a two-year period. These measurements showed that about 92% of the total sediment carried by the runoff originated within the gully and only 8% of sediment was contributed from upslope sources. In addition, we installed headcut control structures in the gully to halt headcut migration and assess the potential of reducing soil losses and therefore sediment concentrations. The gully head control structure was only partially successful in arresting the headcut (and reducing gully expansion by 20%), thereby indicating that in the (sub) humid Ethiopian highlands, where the soil saturates periodically arresting gully headcuts requires more research. Thus, in order to reduce sediment loads in rivers, designing cost effective measures to treat deep gullies located in the saturated valley bottom lands should be a priority for future research.