Evaluating erosion control practices in an actively gullying watershed in the highlands of Ethiopia

Abstract

Soil erosion, with significant contributions from gullies, is a serious problem in the Ethiopian highlands. The objective of this paper is to examine patterns of discharge and sediment transport in the Ethiopian highlands, and to provide an initial assessment of whether soil and water conservation practices (SWCP) can reduce sediment loads in watersheds with actively eroding gullies. The study was conducted in the 414‐ha Ene‐Chilala watershed with a unimodal sub‐humid monsoon climate and actively eroding gullies in the valley bottoms. In 2013 and 2014, the local community was mandated to install upland infiltration furrows and farmers voluntarily rehabilitated six gully heads and protected 16 m of eroding stream banks. Discharge and sediment concentration were measured in two upslope watersheds and at the outlet. Since median infiltration capacity in the uplands was always greater than the rainfall intensity, saturation excess and interflow were main runoff pathways. After 175 mm cumulative rainfall, the groundwater table reached the surface in the valley bottoms, restricting infiltration, and runoff was generated as saturation excess overland flow and flowed through active gullies out of the watershed. Upland rill erosion on ploughed land early in the rain phase, and gully erosion in saturated valley bottoms thereafter, were sources of sediment in the rivers. The mandated infiltration furrows installed on the contour overtopped and damaged cropland. The off‐contour furrows increased streamflow. Gully rehabilitation of an upland gully effectively reduced stream sediment concentration in the upland weir. However, there was little benefit at the watershed outlet since the stream picked up the unconsolidated sediment from the failing banks in the downstream porting of the watershed. Therefore, soil conservation programs that in addition to installing upland practices, rehabilitate the main sediment source (gullies) appear to be the most effective approach to reducing in‐stream suspended sediment concentrations. © 2018 John Wiley & Sons, Ltd.

The Effect of Landscape Interventions on Groundwater Flow and Surface Runoff in a Watershed in the Upper Reaches of the Blue Nile

“…At the same time, for equal rainfall amounts, the GWFI is less (average of 62%) before implementation than after (average 64%) ( Figure 6, Table 4). Although many studies in Ethiopian highlands have noted either no change [29] or a decrease in total discharge [17,32,34,62] due to soil and water conservation practices, to the best of our knowledge, there is only one other study that found that soil and water conservation practices increase base flow [63]. Thus, while overall these structural practices do not increase the biomass from the watershed [36], biomass production during the dry phase might potentially increase downstream because more base flow is available for irrigation.…”

“…These land management practices consisted of infiltration furrows, micro basins, and soil bunds. They improve the recharge to shallow groundwater and decrease surface runoff [30,32]. In the PED model, this is modeled as a decrease in the fraction of degraded hillside lands with limited infiltration capacity in the subsoil [29,52,61].…”

“…Research from a paired watershed study near Gondar indicated that a watershed with soil and water conservation structures consisting of stone and earthen bunds significantly had less direct runoff and soil erosion than a watershed without a conservation practice [31]. In the Ene-Chilala watershed in the northcentral highlands, 50 cm deep contour furrows decreased runoff, however runoff overtopped the bunds and caused damage to nearby crops; off-contour furrows increased the overland flow and did not cause damage to the crop [32]. In the Debre Mawi watershed south of Lake Tana, infiltration furrows decreased direct runoff, however sediment concentrations remained initially constant due to active gully formation [17].…”

Gully erosion processes, disciplinary fragmentation, and technological innovation

“…Total soil losses attributed to ephemeral gully erosion can range from 10 to 97% (Bennett et al, 2000;Poesen et al, 2003;Capra, 2013). Rates of soil loss due to permanent gullies significantly exceed losses observed in agricultural areas, which can be the leading cause of landscape degradation worldwide (Castillo and Gómez, 2016;Ayele et al, 2018). Hillslope incision due to gully erosion can also increase landscape dissection and fragmentation.…”

“…The program featured 44 presentations from researchers representing state and federal governments, consultancies, and universities from 15 different countries. Keynote presentations were given by Richard Cruse (Iowa State University; Gelder et al, ), Irene Marzolff (Goethe University; Marzolff and Pani, ), Tammo Steenhuis (Cornell University; Ayele et al, ), and Norm Widman (USDA‐NRCS), each of whom contributed to the symposium's theme: ‘Integrating Processes, Management, and Prediction.’ The symposium also featured technical workshops for hands‐on training of erosion prediction technology and remote sensing as well as tours of an instrumented soil erosion site on a local farm and the USDA‐ARS National Soil Erosion Laboratory. Lastly, selected papers written in support of this symposium are published in this special issue, addressing a broad spectrum of topics from across the globe.…”