Impacts of Management Scenarios on Sediment Yield Simulation in Upper and Middle Awash River Basin, Ethiopia

ABSTRACT

Sediment transport is a complex hydrological phenomenon that influences water resources, ecosystems, soil and water quality. The evaluation of sediment yield and mitigation practices are the mechanism to enhance the water resource potential. The study was aimed at simulating the sediment yield and the impact of three best management scenarios in the Upper and Middle Awash River Basin (UMARB). The Soil and Water Assessment Tool (ArcSWAT 2012) was used to simulate and identify high sediment yield prone areas in the catchment by the implementation of three management practices such as filter strips, terraces, and grassed waterways. The model results indicated a good agreement between observed and simulated streamflow and sediment yield in the baseline scenario. The mean annual sediment yield was 21.43 ton/ha/yr. The implementation of 5 and 10 m filter strips reduced the mean annual sediment yield by 31.68% and 46.6%, respectively. Terraces and grassed waterways were showed a reduction of sediment yield by 19.56% and 16.64%, respectively. Filter strips were found to provide the highest reduction of sediment yield in each sub-basin and the outlet of the catchment. Besides, the combination of the three management scenarios has reduced the values of sediment yield by 50.88% from the baseline, and the mean annual sediment yield becomes 10.53 ton/ha/yr. In these results, the efficiency of soil conservation practices would increase by applying more than two sediment management scenarios in the study area. The study delivered important information for the reduction of sediment yield and implementation of sustainable watershed management in the UMARB.

Introduction

Simulation of sediment yield and its management scenarios is a device to estimate and mitigate sediment influx and outflux in the catchment, explicitly to assess how much sediment load is generated from certain soil and land cover types. Sediment transport is a complex hydrological phenomenon and the variables involved in soil erosion modeling make a challenging to measure (Kumar et al., 2015). Soil erosion and sediment yield have a significant impact on water and natural resources, water quality, sustainability of agricultural development, and loss of water storage in the reservoir (Abebe and Gebremariam, 2019; Gathagu et al., 2018; Malunjkar et al., 2015; Mishra et al., 2007; Tadesse et al., 2017; Xu et al., 2009). Recently, evaluating the impact of sediment yield and implementing soil and water conservation measures to manage sediment yield from different slopes and land used/cover areas are crucial issues of the world (Gathagu et al., 2018).

The simulation of sediment yield is the primary on-site of soil degradation processes in various dehydrated land ecosystems around the world (Foley et al., 2005; Lal, 1998; Yair and Kossovsky, 2002). It is sensitive to the impact of climate change, land degradation, vegetation cover, deterioration of soil quality, amount of potential water resources and environmental deterioration (Aksoy and Kavvas, 2005; Cotler and Ortega-Larrocea, 2006; Harries et al., 2021; Tesema and Leta, 2020). The results of soil erosion and land degradation are the most serious environmental risk for today's global ecosystems that creates a critical environmental hazard (Hua et al., 2012; Tolche et al., 2021; Yan et al., 2018; Zhang et al., 2019). Soil erosion is directly influencing the production of agriculture, increasing desertification in the catchment, siltation of waterways, aquatic ecosystems and insufficient food security (Abebe and Gebremariam, 2019; Schmalz et al., 2015).

Numerous studies have been done on the simulation of sediment yield and erosion of sediment transportation in different parts of Ethiopia (Ayana et al., 2012; Ayele et al., 2017; Molla et al., 2020; Tadesse et al., 2017; Tesema and Leta, 2020; Welde and Gebremariam, 2017; Yesuf et al., 2015). These studies reported that the north, eastern, central, western, and northeastern highlands of Ethiopia have been influenced by soil erosion and land degradation. Besides, the negative impact of soil erosion and land degradation influences water resource potential that could lead to an increase in drought events in the country. Ayele & Gebremariam (2020) quantified the high spatial and temporal variability of sedimentation in Bilate watershed in Rift Valley Lake Basin, Ethiopia. The spatial and temporal variability of sediment yield accounted to land use land cover, soil, and topographical features of the watershed. Besides, among four conservation measures used (i.e., grassed waterway, filter strips, terracing, and contouring) for reduction of sediment yield, terracing (63.26%) was found as the most suitable method in Bilate watershed. The studies conducted in Upper Blue Nile River Basin in Ethiopia reported that the basin is one of the sensitive areas of sediment transport and land degradations where soil erosion and sediment yields vary spatially and temporally (Ayele et al., 2017; Betrie et al., 2011; Easton et al., 2010; Yesuf et al., 2015). Choto & Fetene (2019) simulated streamflow and sediment yield in Gojeb watershed in the Omo-Gibe basin based on the variability of land use land cover. Among three conservation measures (i.e., filter strip, terrace (bund) and reforestation) practiced in the study area, the reforestation scenario was recommended as the most appropriate technique of controlling sediment yield in the watershed.

Conventionally, Awash River Basin is the backbone of the Ethiopian economy. In the basin, sugar factors and irrigation projects (Wonji Shoa sugar factor, Metahara sugar factor, Upper Awash Agro-industry, Fentale and Tibila Irrigation Project), water resource infrastructures (Koka and Tendaho dam), and other small and medium projects play a vital role in the economy of the country are found. For the best utility of these infrastructures, practicing soil and water conservation as well as finding the hotspot area of sediment yield (soil erosion and sediment transportation) is paramount (Tesema and Leta, 2020).

Several studies were addressed the soil erosion and sediment transportation of different watersheds in the Awash River Basin (Abebe and Gebremariam, 2019; Bishaw and Kedir, 2015; Jilo et al., 2019; Molla et al., 2020; Tesema and Leta, 2020; Zeberie, 2020). Hence, most of the studies were limited to a certain watershed with a small landscape. In this paper, UMARB covers more than 28,000 Km² which is 23.4% of the basin landscape were covered. Besides, in the downstream of the study area, the amount of flood and sediment transportation are high and have more impact on Tendaho sugar factory and Tendaho dam, Dubti and Assayita town (Jilo et al., 2019; Tilahun et al., 2017). Therefore, simulation and mitigation of sediment load in the UMARB are very important for land and water management in the catchment as well as for the downstream section. Moreover, the earlier studies in Awash River Basin have highlighted the application of sediment management practice (Tesema and Leta, 2020; Zeberie, 2020). But, before implementation, the efficiency of each management practice needs to be established as the efficacy of any conservation practice varies in the area due to land uses, slope, and climatic conditions. Therefore, the study is aimed to (i) simulate sediment yield in the UMARB and (ii) evaluate the impact of three management scenarios and their combination on sediment yield reduction in UMARB by using the ArcSWAT2012 hydrological model.