Consolidated sediment budget of Lake Tana, Ethiopia (2012–2016)

Abstract

Lake sediment budgeting is a vital tool to better understand catchment sediment fluxes. It allows assessing lake lifetime and can be used to design improved catchment management plans. Over the past few years, we collected sediment and runoff discharge data at fine spatial-temporal resolutions for the Lake Tana Basin (LTB), Ethiopia. Based on these data, this study presents a consolidated sediment budget of Lake Tana, following the proof of concept by Lemma et al. (2018). Sediment yield (SY) was quantified for 15 rivers and 2 lake outlets based on 12,671 suspended sediment concentration (SSC) samples, making this the most complete dataset for Lake Tana. In particular, we investigated seasonal and temporal patterns of sediment dynamics and the role of lakeshore floodplains. The latter was done by comparing the SSC measurements at upper and lower borders of the floodplains. Sediment rating curves were prepared that account for land cover conditions and rainfall seasonality. SY calculations for the main contributing rivers take bedload fluxes into account by means of a bedload rating curve. As a result, the gross annual sediment mass mobilized on the hillslopes of LTB was 33.30 Mt (9.93 Mt from ungauged rivers), dominantly from large river catchments in the southern and eastern parts of LTB. SSC and SY are generally larger at the beginning of the rainfed cropping calendar (May–June). By the end of rainfed cropping calendar (September–October), SY decreases. Floodplains store 2.28 Mt suspended sediment whereas annually an average SY of 31.02 Mt entered the lake. From this, 96.6% is suspended load and 3.4% is bedload. With ca. 30 Mt sediment deposited in the lake, its mean trapping efficiency equals 97%. Only 1.14 Mt of sediment leaves the lake through the two outlets. The rate of lake sedimentation (9.8 kg m−2 yr−1) is in line with the sedimentation rates of large lakes in the world. However, because Lake Tana is shallow, its expected lifetime is only 918 years. Moreover, this study is useful for managing freshwater ecosystems that requires understanding of sedimentation rate and associated reduction in storage capacity of lakes, especially lakes that are shallow and have experienced extensive catchment degradation.