Morphological dynamics of gully systems in the subhumid 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 semiarid region of Ethiopia, few studies have been conducted in the (sub)humid region. For that reason, we assessed the severity of gully erosion by measuring the expansion of 13 selected permanent gullies in the subhumid 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 0.5 m resolution aerial imagery from flights in 2005 and 2013. About 0.6 Mt (or 127 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 heads and banks, destabilizing the soils by decreasing the shear strength. Elevated groundwater tables were therefore 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 factors controlling erosion are the most effective in protecting gully expansion. These consist of lowering water table and regrading the gully head and sidewalls to reduce the occurrence of gravity-induced mass failures. Planting suitable vegetation on the regraded gully slopes will in addition decrease the risk of bank failure by reducing pore-water pressures and reinforcing the soil. Finally, best management practices that decrease runoff from the catchment will reduce the amount of gully-related sediment loss.

Sediment detachment and transport processes associated with internal erosion of soil pipes

“…Tebebu et al () noted that gullies in the Ethiopian Highlands occurred in response to removal of indigenous vegetation which altered the hydrology of the region such that the water table rose and subsurface flow from hillslopes to valleys increased. Zegeye et al () noted that elevation of the shallow water table in this region was the main factor for a four‐fold increase in gully erosion. They attributed the effect of elevated water tables on gully erosion to be due to decreased shear strength and seepage‐induced erosion.…”

Assessment of Practices for Controlling Shallow Valley-Bottom Gullies in the Sub-Humid Ethiopian Highlands

“…The absolute error in measuring gully length and width were estimated at 0.1 m. In addition, the absolute measurement error of cross sectional area calculation was estimated at 1 m 2 . The estimated error of length and width measurements were related with the tape meter and handling [5]. To calculate the uncertainty of the gully head retreat, surface area and volume of the gully, we used the propagation error method [29].…”

“…Worldwide, gully erosion is the most severe form of soil erosion [1][2][3][4][5]. In Ethiopia, soil losses in watersheds with valley bottom gullies are in the range of 40 to 540 Mg ha −1 a −1 , which is equivalent to an annual soil loss of 100 to 1350 tons/acre [6][7][8].…”

“…It exceeds upland erosion many times, with a soil loss of less than 10 Mg ha −1 a −1 [9]. Contributions of valley bottom gullies to the total soil loss range from 28% in semi-arid Tigray [10] to 90% in the sub-humid highlands in Amhara [5,7]. Valley bottom gullies, therefore, are the main contributors of soil loss in Ethiopian watersheds.…”

Biophysical landscape interactions: Bridging disciplines and scale with connectivity

“…Another example where connectivity can be quantified between the various (sub)systems is the agricultural system in NW Ethiopia, (Tebebu et al, ; Zegeye et al, ) where soil erosion and sediment transfer to downstream reservoirs are an acute problem. Extensive soil and water conservation (SWC) measures have been introduced throughout the watershed to mitigate soil erosion.…”