Surface and subsurface flow effect on permanent gully formation and upland erosion near Lake Tana in the northern highlands of Ethiopia

T Y Tebebu,A D Zegeye,H E Dahlke,A S Collick,S Kidnau,A Z Abiy,F Dadgari,Z M Easton,T S Steenhuis,S A Tilahun,S Moges

doi:10.5194/hess-14-2207-2010

Abstract

Abstract. Gully formation in the Ethiopian Highlands has been identified as a major source of sediment in water bodies, and results in sever land degradation. Loss of soil from gully erosion reduces agricultural productivity and grazing land availability, and is one of the major causes of reservoir siltation in the Nile Basin. This study was conducted in the 523 ha Debre-Mawi watershed south of Bahir Dar, Ethiopia, where gullies are actively forming in the landscape. Historic gully development in a section of the Debre-Mawi watershed was estimated with semi structured farmer interviews, remotely sensed imagery, and measurements of current gully volumes. Gully formation was assessed by instrumenting the gully and surrounding area to measure water table levels and soil physical properties. Gully formation began in the late 1980's following the removal of indigenous vegetation, leading to an increase in surface and subsurface runoff from the hillsides. A comparison of the gully area, estimated from a 0.58 m resolution QuickBird image, with the current gully area mapped with a GPS, indicated that the total eroded area of the gully increased from 0.65 ha in 2005 to 1.0 ha in 2007 and 1.43 ha in 2008. The gully erosion rate, calculated from cross-sectional transect measurements, between 2007 and 2008 was 530 t ha−1 yr−1 in the 17.4 ha area contributing to the gully, equivalent to over 4 cm soil loss over the contributing area. As a comparison, we also measured rill and interrill erosion rates in a nearby section of the watershed, gully erosion rates were approximately 20 times the measured rill and interrill rates. Depths to the water table measured with piezometers showed that in the actively eroding sections of the gully the water table was above the gully bottom and, in stable gully sections the water table was below the gully bottom during the rainy season. The elevated water table appears to facilitate the slumping of gully walls, which causes the gully to widen and to migrate up the hillside.

Highlights

Soil erosion is one of the most challenging global environmental problems
Since 1985 the government of Ethiopia has run an ambitious soil and water conservation (SWC) program supported by donors and nongovernmental organizations and backed by the largest food-for-work
In the research reported here we explored the interaction of hydrological factors on gully formation, and compared gully erosion rates to better understood rill and interrill erosion rates in the Debre-Mawi watershed

Summary

Introduction

Soil erosion is one of the most challenging global environmental problems. Loss of soil has both on-site effects, such as loss of soil fertility and lowered water holding capacity, and off-site effects, such as siltation of reservoirs and lakes (Tamene and Vlek, 2007). In the Ethiopian Highlands, erosion has increasingly received greater attention since the 1980’s following the development of the Soil Conservation Research Program (Hurni, 1988; Moges and Holden, 2008). Since 1985 the government of Ethiopia has run an ambitious soil and water conservation (SWC) program supported by donors and nongovernmental organizations and backed by the largest food-for-work. Researchers from Switzerland and Belgium in cooperation with Ethiopian researchers have made great advances in both understanding and control of upland erosion (Beshah, 2003; Bewket and Sterk, 2005; Herweg and Ludi, 1999; Hurni et al, 2005; Nyssen et al, 2004; Shiferaw and Holden, 1998). Gully erosion is still, at best, poorly understood, and little has been done in practice to control gully erosion

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