Morphological dynamics of gully systems in the sub-humid Ethiopian Highlands: The Debre Mawi watershed

Abstract

Abstract. Gully expansion in the Ethiopian highlands dissects vital agricultural lands with the eroded materials adversely impacting downstream resources, for example as they accumulate in reservoirs. While gully expansion and rehabilitation have been more extensively researched in the semi-arid region of Ethiopia, few studies have been conducted in the (sub) humid region. For that reason, we assessed the severity of gully erosion and identified gully-forming processes in 13 selected permanent gullies in the sub-humid Debre Mawi watershed, 30 km south of Lake Tana, Ethiopia. In addition, the rate of expansion of the entire drainage network in the watershed was determined using 50 cm resolution aerial imagery flown in 2005 and 2013. More than 0.7 million tons (or 155 t ha−1 yr−1) of soil was lost during this period due to actively expanding gullies. The net gully area in the entire watershed increased more than 4-fold from 4.5 ha in 2005 to 20.4 ha in 2013 (> 3 % of the watershed area), indicating the growing severity of gully erosion and hence land degradation in the watershed. Soil losses were caused by upslope migrating gully heads through a combination of gully head collapse and removal of the failed material by runoff. Collapse of gully banks and retreat of headcuts was most severe in locations where elevated groundwater tables saturated gully head and bank soils, destabilizing the soils by decreasing their shear strength. Elevated groundwater tables were thereby the most important cause of gully expansion. Additional factors that strongly relate to bank collapse were the height of the gully head and the size of the drainage area. Soil physical properties (e.g., texture and bulk density) only had minor effects. Conservation practices that address the most important controlling factors are principally the most effective ways of protecting gully development and expansion. These consist of lowering water table elevation and regrading the gully head and sidewall to reduce the occurrence of gravity-induced mass failures. Planting suitable vegetation on the gully face and around the boundary can also decrease the risk of bank failure by reducing pore-water pressures and reinforcing the soil. Best management practices affecting the runoff contributing catchment may decrease the overland runoff-induced gully head erosion.

Impact of Soil Depth and Topography on the Effectiveness of Conservation Practices on Discharge and Soil Loss in the Ethiopian Highlands

“…Sediment concentrations and loads decreased greatly in both watersheds after the implementation of conservation measures (Figure 7b,c). In the Debre Mawi watershed [22,28,48,49], in cases where gullies were not treated, we found that the sediment concentration at the outlet did not decrease after the installation of upland conservation practices because the unconsolidated soil in the gullies added the sediment back into the water that was taken out by the upland practices. Here, gullies were treated (Table 2) and sediment concentration decreased because gullies are the major sources of sediment [50][51][52].…”

“…Once the rills have formed, sediment concentrations decrease. When gullies are present downstream, any decrease in sediment is negated by unconsolidated sediments of the slipping banks that can easily be picked up by the flowing water in the gullies [48,49]. Under these circumstances, the peak sediment concentrations remain elevated throughout the rain phase, as seen in the Tikur-Wuha and the Guale watersheds, especially in 2010 before gully rehabilitation practices were in place (Figure 7a).…”

“…To discuss the differences in discharge and runoff coefficients between the two watersheds, we note that prior experimental studies have found that the main runoff generation mechanism in the (sub) humid Ethiopian highlands is saturation excess runoff [15,20,22,24,[45][46][47][48]. Saturated excess overland flow occurs at locations either where the soils become saturated above an impeding layer or where the regional groundwater reaches the surface.…”

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

“…The red rectangle is the location of Debre Mawi watershed. (b) Gully network in the Debre Mawi watershed in 2013 estimated as about 20 ha (Zegeye et al, ), the red rectangle shows the gully studied. (c) Photograph of the Debre Mawi watershed with the valley bottom lands in the middle of the picture, where the gullies are developing.…”

Developing Soil Conservation Strategies with Technical and Community Knowledge in a Degrading Sub‐Humid Mountainous Landscape

“…The study site is a sub‐watershed within the Debre Mawi watershed, in the northwestern part of the country (Figure ), which encompasses a total area of 523 ha and is located 30 km south of the city of Bahir Dar at 37°22′ east and 11°18′ north (Tilahun et al, ). This study site was selected because of its strategic importance for local crops ( Eragrostis tef ) and climatic representativeness (in the sub‐humid highlands), to contribute to ongoing research (Tebebu et al, ; Tilahun et al, ; Dagnew et al, ; Zegeye et al, ), and because of logistical partnerships with local research organizations. Slopes range between 1% and 30%, and the watershed is underlain by a shallow highly weathered and fractured basalt with compacted clays overlain (Tilahun et al, ).…”