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

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 Correspondence to: Z. M. Easton (zme2@cornell.edu) 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.

Improving efficacy of landscape interventions in the (sub) humid Ethiopian highlands by improved understanding of runoff processes

“…Zegeye et al (2010) and Tilahun et al (2015) showed that rill formation stopped in August, and soil loss was greatly decreased from the uplands. The correlation of soil loss with plant cover was very weak (Tebebu et al, 2010) excluding plant cover as the cause for the decrease. Assuming that all sediment loss is originating from the agricultural areas the sediment concentrations can be scaled as the quotient of the observed concentration and the fraction of agricultural land in the watershed.…”

“…Precipitation, discharge and storm event sediment yields are available from 1988-1994, 1996-2004, and 2008 (Table 1). 16901984-1993-2013Liu et al, 2008Guzman et al, 2013;Tilahun et al, 2013a,b;Elkamil, 2014;Tebebu et al, 2014;Bayabil et al, 2015 Maybar 113 Phaeozems and Phaeozems associated to Lithosols 12101988-19941996-2004Liu et al, 2008Bayabil, 2009;Bayabil et al, 2010;Guzman et al, 2013 Andit Tid 481 Andosols and Regosols 15001986-19921994-2001Liu et al, 2008Engda, 2009;Guzman et al, 2013;Tilahun et al, 2013b Debre Mewi 95 Nitosols and Vertisols 1240-2014Abiy, 2009Tebebu, 2009;Tebebu et al, 2010;Zegeye et al, 2010;Tilahun, 2012;Langendoen et al, 2013;Dagnew et al, 2014;Tebebu et al, 2014;Zegeye et al, 2014;Dagnew et al, 2015;Tilahun et al, 2015 The Andit Tid watershed, situated 180 km northeast of Addis Ababa at 39 • 43 ′ E and 9 • 48 ′ N, covers 481 ha ( …”

“…The change in flow paths requires the landscape to adjust to these increased direct flows by forming effective pathways to carry this water to the stream. These pathways are the gullies that have appeared all over the landscape a few years after the uplands were cleared and became more intensively used in the middle of the 14 year period of the communist regime from 1977 to 1991 in which the fertile bottom lands were taken over by the regime (Tebebu et al, 2010;Ayele et al, in press). …”

“…In many cases the saturated areas are also the location where gullies form (Tebebu et al, 2010). Once initiated these gullies expand rapidly, especially at locations where water tables are above the gully bottom.…”

“…However, incising channels such as gullies experience a predictable, non-reversible series of adjustment processes (Schumm et al, 1984). Upslope migrating head cuts increase gully depth and slope, destabilizing the gully banks, resulting in widening, and downstream deposition (Tebebu, 2009;Tebebu et al, 2010;Langendoen et al, 2013). Hence, it is important to note that gully stabilization should proceed from downstream to upstream by halting the headward migration of head cuts (Table 2), which will otherwise severely damage any improvement made on the hillslopes and headwaters.…”