Estimating the Sediment Flux and Budget for a Data Limited Rift Valley Lake in Ethiopia

Abstract

Information on sediment concentration in rivers is important for the design and management of reservoirs. In this paper, river sediment flux and siltation rate of a rift valley lake basin (Lake Ziway, Ethiopia) was modeled using suspended sediment concentration (SSC) samples from four rivers and lake outlet stations. Both linear and non-linear least squares log–log regression methods were used to develop the model. The best-fit model was tested and evaluated qualitatively by time-series plots, quantitatively by using watershed model evaluation statistics, and validated by calculating the prediction error. Sediment yield (SY) of ungauged rivers were assessed by developing and using a model that includes catchment area, slope, and rainfall, whereas bedload was estimated. As a result, the gross annual SY transported into the lake was 2.081 Mton/year. Annually, 0.178 Mton/year of sediment is deposited in floodplains with a sediment trapping rate of 20.6%, and 41,340 ton/year of sediment leaves the lake through the Bulbula River. The annual sediment deposition in the lake is 2.039 Mton/year with a mean sediment trapping efficiency of 98%. Based on the established sediment budget with average rainfall, the lake will lose its volume by 0.106% annually and the lifetime of Lake Ziway will be 947 years. The results show that the approach used can be replicated at other similar ungauged watersheds. As one of the most important sources of water for irrigation in the country, the results can be used for planning and implementing a lake basin management program targeting upstream soil erosion control.

An Alternative Empirical Model to Estimate Watershed Sediment Yield Based on Hydrology and Geomorphology of the Basin in Data-Scarce Rift Valley Lake Regions, Ethiopia

“…SWAT does not separate The model results were calibrated and validated by using historical stream flow and sediment flow data. The daily stream flow data was obtained from MoWIE and the sediment data was generated by using sediment-discharge rating curve developed by [40]. SWAT does not separate sediment load into suspended load and bed load, and the simulated total sediment load is supposed to include both.…”

“…To validate the applicability of newly developed model, one of the sub basin of Lake Ziway Katar was used. Lake Ziway has two tributary rivers (Maki and Katar) and for both of them, the daily river flow rate was available in MoWIE and the daily sediment flow rate was determined by using a sediment rating curve developed for the sites by [40]. Hence, the empirical model developed for sub basin Maki was checked inside of the Karat sub-basin.…”

Impact of Land-Use Changes on Sediment Load and Capacity Reduction of Lake Ziway, Ethiopia

“…Due to the lack of long term observed sediment data in the study area, flow-sediment rating curve established by Alemu et al [14] for Meki at Meki station, Katar at Abura station and Bulbula River station used to generate the long-term sediment yield to estimate the sediment yield discharged to the lake.…”

Sediment deposition (1940–2017) in a historically pristine lake in a rapidly developing tropical highland region in Ethiopia

“…Lakes are under continuous threat by sedimentation worldwide. Examples are large shallow lakes in the Yangtze River Basin in China (Xu et al, 2017), lakes in Europe (Gąsiorowski, 2008;Rose et al, 2011), and African lakes (Aga et al, 2019;Gorgas & Wilkens, 2002;Maina et al, 2018;Olago & Odada, 2007). Although sediment deposition is a natural process when enhanced by anthropogenic effects such as soil erosion from land degradation (Mekonnen et al, 2015;Muleta et al, 2006;Yesuf et al, 2015), it threatens lake ecosystem functioning (Vijverberg et al, 2009) by reducing the storage capacity (Kim et al, 2014;Syvitski et al, 2005), increasing nutrient levels, obstructing navigation and endangering income sources for its beneficiaries (Elsahabi et al, 2018).…”