Causes and controlling factors of valley bottom gullies

Authors: AMARE, SELAMAWIT - Wageningen University And Research Center; KEESSTRA, SASKIA - Wageningen University And Research Center; VAN DER PLOEG, MARTINE - Wageningen University And Research Center; Langendoen, Eddy; STEENHUIS, TAMMO - Cornell University; TILAHUN, SEIFU - Bahir Dar University

Submitted to: Land
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2019
Publication Date: 9/17/2019
Citation: Amare, S.D., Keesstra, S., Van Der Ploeg, M., Langendoen, E.J., Steenhuis, T.S., Tilahun, S.A. 2019. Causes and controlling factors of valley bottom gullies. Land. 8(9):141. https://doi.org/10.3390/land8090141.
DOI: https://doi.org/10.3390/land8090141

Interpretive Summary: Gully erosion has been found to cause soil losses that far exceed those caused by sheet and rill erosion. Gullies form on both hillslopes and the valley bottomlands. The factors that govern gully erosion on hillslopes and valley bottomlands are generally different, and have not been appropriately cataloged. Researchers from USDA, ARS, Oxford, MS in collaboration with scientists from Wageningen University and Research, The Netherlands, Cornell University, and Bahir Dar University, Ethiopia, conducted a comprehensive review of anthropogenic and natural factors controlling gully erosion in the valley bottomlands. The review indicates that onsite and offsite anthropogenic and/or natural factors that increase water delivery to the valley bottom, in the form of both surface and subsurface flow, are the major determinants of gully formation and erosion in the bottomlands. The findings of this review provide watershed managers, natural resource agencies, and other agencies active in rehabilitating degraded landscapes guidelines on the most important factors that have to be controlled through conservation measures.

Technical Abstract: Valley bottom (VB) land provides diverse agricultural and ecosystem benefits. However, due to a number of on- and off-site factors, VB lands are more vulnerable to gully erosion than hillslope areas. They are also geomorphologically and hydrologically different from hillslope areas, which can change the local governing erosion processes and their control. Still, studies undertaken so far did not clearly reveal the need for a separate perspective for gullies located at VBs. Therefore, our aim is to show why VB lands are vulnerable for severe gully erosion, and to present deficiencies in existing rehabilitation measures and limitations in applicability of existing topographic threshold predictors. Hence, a literature review on existing valley bottom gully (VBG) erosion studies was undertaken. The review identified various offsite (e.g., decline in uphill vegetation and conservation measures that increase surface runoff and elevate groundwater at the VB) and onsite factors (e.g., large drainage area, and thick alluvium and colluvium soil deposits) that presently make VB land more vulnerable to formation and growth of gullies than hillslope lands. The integral impact of these factors negatively affects soil erosion-resistance and stability, leading to combined fluvial and mass wasting gully erosion processes. Due to intensive agricultural activities more VBGs are reported on grazing and cultivated land use types. The dominant process of VB gully erosion differs between arid (Hortonian overland runoff erosion) and humid (saturation excess runoff erosion) regions. VBG reclamation measures seem effective in areas where gullies are caused by Hortonian overland runoff, whereas for saturation excess runoff erosion remedial actions were generally not successful. Topographical threshold predictors for gully erosion worked well for Hortonian overland runoff driven VBG, whereas for VBG with dominant subsurface erosion processes the use of this threshold predictor might be flawed. In summary, VB lands are more susceptible to gully erosion than hillslope lands. Existing gully erosion topographical threshold predictors, for humid and sub-humid regions, should be re-evaluated to better define the combined surface and subsurface drainage areas into VBGs. This will ensure the implementation of both improved prevention and more effective rehabilitation measures. For successful implementation of rehabilitation measures, especially for humid regions, understanding the erosion process is fundamental, and an integrated, landscape approach that takes into account the combined subsurface and surface drainage into the VBG must be followed.