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

Abstract

Land use changes in many landscapes result in gully formation, carving up agricultural land and playing a large role in filling up downstream reservoirs by connecting uplands with rivers. This includes the Ethiopian highlands. Our objective is to begin investigating the interaction of upland and gully erosion and to quantify the portion of eroded sediment originating from a gully to prioritize erosion control practices. For this purpose, a 5‐m deep valley bottom gully of the 13‐ha catchment in the Debre Mawi watershed in the subhumid Ethiopian highland near Lake Tana was selected. The upstream and downstream gully discharge and sediment concentrations were measured over a 2‐year period. The results show that the sediment concentration at the outlet was about 10 times greater than at the inlet. The sediment budget calculation showed that about over 90% of the sediment at the outlet originated from within the gully. Hysteresis analysis of the sediment concentration discharge relationship showed that sediment supply from the upland was limited, but sediment was always available to be eroded and transported in the gully because of bank failures and headcut retreat. Thus, to reduce sediment loads in rivers and consequent adverse downstream impacts, designing cost‐effective measures to treat gullies should be a priority in the subhumid Ethiopian highlands.

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

“…This decrease in sediment concentration for a given discharge during the rain phase does not occur because the upland runoff (low in sediment concentration) flowing through the gully will pick up the unconsolidated sediment from the failed gully banks (Figure 7(b), (c), and (f)). In agreement with the results of Zegeye et al (2018), the suspended sediment concentration before the water entered the gully did not determine the concentration at the outlet due to the pickup of unconsolidated sediment in the gully.…”

“…In agreement with the results of Zegeye et al . (), the suspended sediment concentration before the water entered the gully did not determine the concentration at the outlet due to the pickup of unconsolidated sediment in the gully.…”

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

“…One of the reasons is the emphasis on upland erosion control practices [38] by action agencies based mainly on the traditional view of watershed rehabilitation that one starts at the top of the watershed and then subsequently moves downstream. However, as shown by [39], the sediment pickup in gullies from failed banks can negate any positive effect of hillside soil and water conservation practices upstream. At the same time, as we saw in our study for gullies RV1 in 2014 and C5 in 2016, fesses built in the uplands for carrying of excess rainfall from agricultural land directly contribute to gully advancement (Table 2).…”

Development of the Edge of the Gully in the Zone Adjacent to the Field

“…In order to prevent further development of the investigated gully, and, for example sediment loads in rivers related to this, designing cost-effective measures to treat gullies should be a priority [Zegeye et al 2018;Zhao et al 2013]. Above all, particular attention should be paid to the use of its margins.…”

“…This type of erosion is recognized as a large source of soil loss and a substantial contributor to the sedimentation of rivers, small ponds and large reservoirs [Fox et al 2016]. As a rule, gullies form as a result of a systematic process of washing out soil particles on lines of periodic, concentrated water runoff lasting for hundreds of years [Maruszczak 1988; Zegeye et al 2018;Zgłobicki et al 2014]. Large forms of gully may also develop rapidly, during a single catastrophic rain or thaw runoff [Buraczyński and Wojtanowicz 1974;Cerdan et al 2002; Onyelowe et al 2018; Rodzik 2008].…”