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

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 lowers crop yields, reduces 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 of 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 estimates 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 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 quick bird 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 between 2007 and 2008 was 530 t ha<sup>-1</sup>yr<sup>-1</sup> 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 inter-rill erosion rates in a nearby section of the watershed, gully erosion rates were approximately 20 times the measured rill and inter rill 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 facilitates the slumping of gully walls, which causes widening and up-migration on the hillside

A multi basin SWAT model analysis of runoff and sedimentation in the Blue Nile, Ethiopia

“…The model indicates that later in the rainy season on the receding limb of the sediment hydrograph, sediment export is dominated by channel processes. However, as noted earlier, gully erosion is also a large sediment source later in the season, as interflow causes the soil to saturate and increases the hydrostatic pressure in the gully (e.g., a water table forms above the gully bottom) (Tebebu et al, 2010). Based on watershed outflow measurements, we cannot discriminate between these mechanisms since both signals appear at the same time.…”

“…While the SWAT model cannot predict the formation of gullies, the SWAT-WB model can indicate where the formation of gullies is probable. In most cases gullies form where the soil is saturated either from a large contributing area for water to accumulate or where slopes flatten and the effective hydraulic conductivity is reduced (Tebebu et al, 2010). These areas tend to occur at the bottom of long slopes in the wetter valley bottom areas, which, not surprisingly also support intensive animal agriculture.…”

“…The high sediment yield areas are generally predicted to occur at the bottom of steep agricultural slopes, where subsurface flow accumulates, and the stability of the slope is reduced from tillage and or excessive livestock traffic. Note also that these are the areas that gully formation is likely (Tebebu, 2009;Tebebu et al, 2010).…”

“…This determination of runoff volume before soil water volume is an inappropriate approach for all but the most intense rain events, particularly in monsoonal climates where rainfall is commonly of low intensity and long duration and saturation processes generally govern runoff production. Several studies in the Blue Nile basin or nearby watersheds have suggested that saturation excess processes control overland flow generation (Liu et al, 2008;Collick et al, 2008;Ashagre, 2009;Engda, 2009;Tebebu, 2009;Tebebu et al, 2010;White et al, 2010) and that infiltration-excess runoff is rare (Liu et al, 2008;Engda, 2009).…”

An Assessment of the Influences of Surface and Subsurface Water Level Dynamics in the Development of Gullies in Anambra State, Southeastern Nigeria

“…than the developed countries. Many of these countries lack the needed financial, technical, and institutional capacity to tackle these problems head on, thus exacerbating the severity of natural disaster-induced problems (Tamene and Vlek 2007;Tebebu et al 2010). Although Nigeria is a third-world country, it is comparatively free from the destructions that usually accompany the incidence of the more devastating natural disasters (e.g., earthquakes and tsunamis) (Akpan and Yakubu 2010).…”