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Agricultural sustainability and land degradation can be monitored through studying soil nutrient dynamics. This study was conducted to investigate the balance and stocks of nitrogen (N) and phosphorus (P) under major land use types and land management (LM) practices in three contrasting agroecological environments in Ethiopia. We employed a nutrient monitoring approach and assessed soil N and P inputs and outputs at watershed and plot scales. Field data were collected from 72 sites representative of the major land uses of the three watersheds for the watershed scale analysis. In addition to the watershed data, 16 experimental runoff plots were established and the response of nutrient balances to improved LM practices for the major land uses, including cropland with three treatments (control, farmyard manure [FYM] and soil bund), grazing land with three treatments (control, exclosure and FYM + exclosure) and bushland with two treatments (control and exclosure). Nutrient balances, flows and stocks varied significantly among agroecological settings, land-use types and LM practices. Severe and negative N balances were found in all agroecological environments, with midland (-70.8) > highland (-61.7) > lowland (-50.3 kg ha-1 yr-1), whereas slightly positive P balances were found in all agro-ecosystems. As a result, the midland (4.2% yr-1) and highland (2.1% yr-1) sites had greater N depletion rates than the lowland site (1.0% yr-1). Our results also demonstrated that applied LM practices improved N balances: by 32%-40% in cropland through FYM and mineral fertilizer, by 7%-18% in grazing land via FYM and exclosure, and by 6%-36% in degraded bushland via exclosure, implying that, implementing FYM and mineral fertilizer for cropland and implementing FYM with exclosure on non-croplands can reverse nutrient depletion in the study sites and in other areas having similar agroecological conditions. [ABSTRACT FROM AUTHOR]

The aim of this study was to determine how soil bunds affected important soil characteristics based on the soil's reflectance in Aba Gerima, an Ethiopian midland of the Abbay River Basin. Soil samples were collected from 0 to 30 cm of the soil depth in 48 plots for soil texture, pH, organic carbon (OC), total nitrogen (TN), available phosphorus (av. P), and available potassium (av. K). Using a spectroradiometer, we evaluated the reflectance of soils that had been air‐dried, pulverized, and sieved. To locate and forecast the change in soil parameters, we employed regression modeling. We identified and predicted soil properties using the models, which were evaluated by the coefficient of determination (R2) and root mean square error. Soils from bunded plots had less reflection. Higher amounts of clay (61%), OC (2.30%), av. P (13.72 ppm), and av. K (116.55 mg/kg) were found in bunded plots. For calibration data, R2 was 0.93, 0.83, and 0.82 for clay, sand, and OC contents, respectively. Spectroradiometry could therefore complement conventional soil analysis. More study is needed to enhance spectroscopy's prediction performance. Core Ideas: Soil bunds improved the clay, silt, organic carbon, and potassium contents of soils.Bunded soils recorded less reflectance.Low soil reflectance indices show increased soil quality.Spectroscopy could better characterize soils as compared to convectional one. [ABSTRACT FROM AUTHOR]

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Changes in peak flow response time governed by rainfall and physical configurations of watersheds have been the topic of many studies, but other factors are also important. We aimed to analyze the dynamics in the lag time (TL) of peak flow for a tropical watershed (Kecha) and its sub‐watersheds (Dokmit, Zenjero Maderia, and Wotit Minch) in Ethiopia, as influenced by land‐use changes from 1982 to 2017 and the implementation of soil and water conservation (SWC) practices since 2011. Layered maps of land use, soil type, and SWC practices were used to determine the curve number, and the Natural Resource Conservation Service hydrologic model was used to estimate TL. We compared the estimated values against measured median values of TL for 30 rainfall–runoff events. The estimated TL of 1982, 2005, and 2017 varied from 9 to 19 min, 8 to 18 min, and 10 to 22 min, respectively, in the Dokmit, Zenjero Maderia, and Wotit Minch sub‐watersheds. The smallest (8 min) and greatest (22 min) values of TL were observed during 2005 and 2017, respectively. These results are likely attributable to the increased amount of cultivated area at the expense of bushland and forest in 2005 as compared to 1982, and the implementation of SWC practices in 2011–2017. Dokmit had the lowest TL values of the three sub‐watersheds, most likely because of its greater coverage of grazing and cultivated lands and degraded Nitic Luvisol. The variation in TL values among the sub‐watersheds was related to changes in both land use and SWC practices but also to the presence of degraded areas inherited from past human activities. Overall, such spatiotemporal flow response time analysis can provide useful information for the proper design of sustainable development strategies for particular niches in tropical regions of Ethiopia and elsewhere. [ABSTRACT FROM AUTHOR]

This study was conducted to investigate technical and socio-political attributes that lead to the underperformance of two selected irrigation schemes (Shina and Bebeks) in the Lake Tana floodplains, Ethiopia. Irrigation application efficiency (AE) at nine experimental fields showed a wide range, from 20 to 80%, but was mostly between 40 and 60%. Irrigation water-use efficiency (IWUE) varied from 1.9 to 7.2 kg m−3 for onion and 0.9 to 1.2 kg m−3 for maize. The lined and earthen canal conveyance losses in Bebeks were 0.037 and 0.047 l s−1 m−1, whereas in Shina they were 0.033 and 0.044 l s−1 m−1, respectively. The overall consumed ratio (OCR) of water was 0.58 for Bebeks and varied from 0.73 to 1.2 in Shina. Both schemes are performing below the standard based on technical performance indicators. Irrigation water user associations (WUAs) were not implemented, but irrigation committees (ICs), composed of local political leaders, are managing both schemes. Canal and reservoir sedimentation from erosion of upstream catchment areas during the rainy season was the major problem. [ABSTRACT FROM AUTHOR]

Highlights • Dangerously high increases in discharge have led to the drowning of 250 people. • Impacts on rural livelihoods are overshadowed by national development goals. • External costs are borne by local people, with low bargaining power. • The EIA of the Beles hydropower project is considered a formality and is unfindable. • In-depth post-analyses of hydropower projects should be mandatory. Abstract Despite public awareness of unintended impacts (1980s) and well-developed international standards (2000s), downstream impacts of large hydropower projects still very often are not properly assessed. Impacts of (hydropower-regulated) interbasin water transfers (IBWTs) are considered self-evidently positive, although they can have far-reaching consequences for hydrogeomorphological systems and consequently river-dependent communities. In this study, the downstream direct and indirect impacts of the Ethiopian hydropower-regulated Tana-Beles IBWT are evaluated in an interdisciplinary way. The components of the framework of rural livelihoods are considered and changing contexts, resources' availabilities and livelihood strategies are analysed. Mixed methods are applied, combining hydrogeomorphological field observations, GIS analyses, scientific literature, policy documents, and semi-structured interviews with local people and local to federal authorities. Results show that the IBWT drastically increased the Beles river's discharge (with an average release of +92 m3 s−1 at the outlet; *2 in rainy season and *12 in dry season 100 km downstream of the water release) and introduced dangerous situations for local communities (over 250 people drowned in the river). River bank erosion resulted in the uncompensated loss of farmland (163 ha) and the establishment of large-scale commercial farms increased the pressure on land and led to the impoverishment of displaced communities (4310 households). The project was implemented top-down, without any transparency, benefit sharing or compensation for external costs. This stresses the importance of downstream interdisciplinary impact assessments and highlights the need for decent in-depth ex post-analyses of hydropower projects. Environmental impact assessments should be taken seriously and cannot be considered a formality. In Ethiopia and in many developing countries, the hydropower industry is booming. Although dams and IBWTs can be the best solution for water-related problems in specific contexts, national development goals (such as the expansion of the electricity network) should not be at the expense of rural livelihoods. [ABSTRACT FROM AUTHOR]

We have investigated the relevance of the notion of “peripheralism” in the Beles basin. In this lowland border area of Ethiopia, important investments require an evaluation of their socio-economic and ecological impacts in the light of Ethiopia’s Climate-Resilient Green Economy (CRGE) strategy. We contrasted literature of different periods with field observations. In the middle and lower parts of the basin, the Gumuz people traditionally practised shifting cultivation. Resettlement of highlanders is particularly linked to water and land resources. A large irrigation project was initiated in the 1980s, but vegetables and fruits face post-harvest losses. Large water transfers from Lake Tana since 2010 affect the movement of people, the hydrogeomorphology, and ecology of the river. In several parts of the basin, the settlers’ economy now dominates. Many Gumuz became sedentary but maintained their agricultural system, particularly in the south of the lower basin. Land titling allowed allocation of “vacant” areas to transnational or domestic investors. As a result, the semi-natural vegetation is frequently replaced by open cropland, leading to decreased carbon storage and increased soil erosion. This and water abstraction for irrigation jeopardise hydropower production, in contradiction with the CRGE objectives. Despite the recent developments, the contrasts in economic activity make the core-periphery dichotomy to remain actual in the Beles basin. The resettlements and permanent cropping tend to make the upper basin part of the core. However, the installation of a transit road and commercial farms in the lower basin do not allow to consider that a non-peripheral integration has taken place. [ABSTRACT FROM AUTHOR]

Abstract: Over the last three decades, many soil and water conservation projects have been implemented in various parts of eastern and southern Africa to control land degradation, and improve land productivity, especially under ‘catchment approach’ initiatives of the 1980s. In Ethiopia, many of these soil conservation projects were implemented following the severe drought of 1974. To capture long-term impacts of these initiatives, a study was conducted in Anjenie Watershed of Ethiopia, assessing fanya juu terraces and grass strips constructed in a pilot project in 1984, and which are still functional 25 years later. Data were collected from government records, field observations and questionnaire surveys administered to 60 farmers. Half of the respondent had terraced farms in the watershed former project area (with technology) and the rest were outside the terraced area (without technology). The crops assessed were teff, barley and maize. Cost–benefit analyses were used to determine the economic benefits with and without terraces, including gross and net profit values, returns on labour, water productivity and impacts on poverty. The results indicated that soil and water conservation had improved crop productivity. The average yields on terraced fields for teff, barley and maize were 0.95tha−1 (control 0.49), 1.86tha−1 (control 0.61), and 1.73tha−1 (control 0.77), respectively. The net benefit was significantly higher on terraced fields, recording US$ 20.9 (US$ −112 control) for teff, US$ 185 (US$ −41 control) for barley and US$ −34.5 (US$ −101 control)ha−1 yr−1 for maize, respectively. The returns on family labour were 2.33, 1.01 and 0.739US$man-day−1 for barley, teff and maize grown on terraced plots compared to US$ 0.44, 0.27 and 0.16man-day−1 for without, respectively. Using a discount rate of 10%, the average net present value (NPV) of barley production with terrace was found to be about US$ 1542 over a period of 50 years. In addition, the average financial internal rate of return (FIRR) was 301%. Other long-term impacts of terracing included farmers’ growing of maize on terraced fields as a result of water conservation. Currently, farmers also grow barley on terraced fields for two crop seasons per year unlike the experiences on farms without terraces. Household incomes and food security had improved and soil erosion drastically reduced. Many farmers had adopted terracing doubling the original area under the soil conservation pilot project and consequently improving environmental conservation in the watershed. [Copyright &y& Elsevier]

Watershed scale hydrological and biogeochemical models rely on the correct spatial-temporal prediction of processes governing water and contaminant movement. The Soil and Water Assessment Tool (SWAT) model, one of the most commonly used watershed scale models, uses the popular curve number (CN) method to determine the respective amounts of infiltration and surface runoff. Although appropriate for flood forecasting in temperate climates, the CN method has been shown to be less than ideal in many situations (e.g. monsoonal climates and areas dominated by variable source area hydrology). The CN model is based on the assumption that there is a unique relationship between the average moisture content and the CN for all hydrologic response units (HRUs), and that the moisture content distribution is similar for each runoff event, which is not the case in many regions. Presented here is a physically based water balance that was coded in the SWAT model to replace the CN method of runoff generation. To compare this new water balance SWAT (SWAT-WB) to the original CN-based SWAT (SWAT-CN), two watersheds were initialized; one in the headwaters of the Blue Nile in Ethiopia and one in the Catskill Mountains of New York. In the Ethiopian watershed, streamflow predictions were better using SWAT-WB than SWAT-CN [Nash-Sutcliffe efficiencies (NSE) of 0·79 and 0·67, respectively]. In the temperate Catskills, SWAT-WB and SWAT-CN predictions were approximately equivalent (NSE > 0·70). The spatial distribution of runoff-generating areas differed greatly between the two models, with SWAT-WB reflecting the topographical controls imposed on the model. Results show that a water balance provides results equal to or better than the CN, but with a more physically based approach. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article provides information on a study that presented a simple rainfall-discharge analysis for the Andit Tid, Anjeni, and Maybar watersheds of northern Ethiopia. A hydrologic data collected from the three research stations in the Amhara National Regional State was used in the study. It was found that temporal dynamics play an important role in the hydrologic behaviour of the watersheds. It shows that the saturation excess mechanism, despite the different rainfall patterns between the three watersheds, is the main reason that the runoff behaviour is similar.

The Lake Tana Basin, comprising the largest natural lake in Ethiopia, is the source and the uppermost part of the Upper Blue Nile Basin. In this review paper, research papers, mainly on the rainfall-runoff modeling and lake water balance, and on the hydrogeology, have been reviewed. The earlier water balance estimation attempts used simple conceptual and statistical approaches and calculate on a monthly timescale. More recent research has been using advanced semi-physically or physically based distributed hydrological models. Accordingly, mean annual precipitation over the lake was estimated in the range 36.1–53.1%; lake evaporation at 45.3–57.5%; river inflow (all gauged and estimated ungauged) at 43.6–63.9%; and river (lake) water outflow at 0–9.2%. With the few isotope studies, groundwater inflow and outflow are found insignificant. Different studies had estimated groundwater recharge, ranging from 57 mm to 850 mm. The basin has a heterogenous aquifer system consisting of different volcanic rocks and alluvio-lacustrine sediments. Generally, groundwater with low TDS, Ca–Mg–HCO3 type, isotopically relatively enriched, and high TDS, Na–HCO3 type, isotopically relatively depleted, water types have been identified. In this paper, major research gaps such as aquifer hydraulic characterization, surface-groundwater interaction, groundwater flow and groundwater balance have been identified. Hence, future research shall focus on the groundwater resources, so that existing surface water studies are updated and future water usage options are explored. [ABSTRACT FROM AUTHOR]

Land degradation is primarily caused by accelerated soil erosion rate, which results in a loss in soil fertility and agricultural production. An improved understanding of the effect of soil bund spacing is a precondition for sustainable land management practices on sloping highlands. We examined the effects of bund spacing on soil loss, runoff, and soil water content on teff (Eragrostis tef [Zucc.] Trotter) cropland in an area of 9 % slope on Luvisol soil in the Aba Gerima (micro) watershed of the Lake Tana Basin, Ethiopia. We measured these parameters during 2018 and 2019 on five adjoining runoff plots (each 30 m long × 3 m wide) separated by metal sheets 0.35 m high. We used a plot with no bunds as a control. In the other four plots, we installed bunds at spacings of 27.2, 12.7, 7.87, and 5.45 m. Both runoff and soil loss were considerably higher in the control plot than in the plots with bunds. Narrower bund spacings resulted in substantial reductions of runoff and soil loss. The two-year average runoff depths in the control, 27.2, 12.7, 8.97, and 5.45 m bund spacing plots were 517, 454, 365, 306, and 243 mm, respectively. Average soil loss for the control plot was 25.7 ton ha−1, whereas it was 17.9, 11.8, 6.5, and 4 ton ha−1 for the 27.2, 12.7, 8.97, and 5.45 m bund spacing plots, respectively. The efficiency of soil bunds in enhancing soil water content increased with reduced soil bund spacing. Over the course of the measurement period, average soil water levels for soil bund spacings of 27.2, 12.7, 8.78, and 5.45 m were 0.30 kg kg−1, 0.31 kg kg−1, 0.32 kg kg−1, and 0.32 kg kg−1, respectively. Further cost–benefit analyses and statistical tests are needed to determine optimum bund spacings in the Ethiopian highlands. [Display omitted] • We quantified the effects of soil bund (SB) spacing on soil loss and runoff. • Soil bunds reduced runoff and soil loss. • Runoff and soil loss reduction were inversely related to the spacing between SB's. • Soil water content was higher in plots that had SBs. [ABSTRACT FROM AUTHOR]

Gully erosion is an important sediment source and causes severe environmental degradation, particularly in the drought-prone regions of Ethiopia. We investigated the morphological characteristics of gullies, the topographic thresholds of gully formation, and estimated headcut retreat rates in three agro-ecologies of the Upper Blue Nile basin, Ethiopia: highland (Guder), midland (Aba Gerima), and lowland (Dibatie). Gully morphological characteristics were analyzed using data measured in the field, whereas the topographic thresholds were estimated using slope and upslope drainage area. Average annual linear (R l) and volumetric (V e) headcut retreat rates were estimated by visual interpretation of very high resolution (0.5–1.5 m) satellite images (QuickBird, IKONOS, SPOT-7, and Pleiades) in a GIS environment. A significant power relationship, fitted between gully volume (V) and length (L) for the three sites, is V = 8.097 L1.032 (R 2 = 0.902, n = 94). The average annual R l , V e , and soil loss rates were estimated at 0.76 m yr−1, 6.77 m3 yr−1, and 8.73 t ha−1 yr−1 in Guder, 2.09 m yr−1, 19.58 m3 yr−1, and 20.76 t ha−1 yr−1 in Aba Gerima, and 3.42 m yr−1, 42.16 m3 yr−1, and 49.33 t ha−1 yr−1 in Dibatie. The higher gully headcut retreat rate in Dibatie is mainly related to the highly erodible nature of Vertisols. The coefficients of the slope (S)-drainage area (A) relationship S = a A− b were a = 0.219 and b = −0.139 in Guder, a = 0.133 and b = −0.234 in Aba Gerima, and a = 0.113 and b = −0.216 in Dibatie, indicating that topographic thresholds for gully initiation varied among the agro-ecologies. The results of this study can be used to estimate gully erosion rates and identify areas for gully initiation, thereby supporting the planning of appropriate gully control measures in the study sites and other areas with similar environmental settings. Unlabelled Image • Gully erosion is highly dynamic erosion process in northwest Ethiopia. • We characterized 94 gullies for 10 morphological parameters in three environments. • Gully length is the most dynamic parameter in all agro-ecologies. • A higher gully initiation threshold was found in the highland agro-ecology. • High gully head retreat rate was observed in the Vertisol-dominated lowland site. [ABSTRACT FROM AUTHOR]

Gully erosion is one of the main causes of land degradation, particularly in the drought-prone regions of Ethiopia. This study assessed spatio-temporal changes of gully length and density in watershed pairs in Guder, Aba Gerima, and Dibatie sites, which are representative highland, midland, and lowland agro-ecologies in the Upper Blue Nile basin of Ethiopia. Aerial photographs (1957, 1982) and very high resolution satellite images (QuickBird, IKONOS, Worldview-2, SPOT-7, and Pleiades) of the six watersheds, along with field survey results, were used in the analyses. The aerial photographs were scanned and orthorectified using ENVI 4.3 image analysis software, and gullies were mapped by visual image interpretation in the ArcGIS environment. Rates of increase in gully length in Guder (36.9 m yr−1) and Aba Gerima (33.6 m yr−1) were almost double the rate in Dibatie (17.8 m yr−1) from 1957 to 2016 or 2017, and over the same period, gully density similarly increased by 5.9, 5.4, and 3.7 m ha−1 in Guder, Aba Gerima, and Dibatie, respectively. The higher rates in Guder and Aba Gerima reflect the long history of cultivation and human settlement in those sites, whereas agricultural activity became widespread in Dibatie only after implementation of the national resettlement program in the 1980s. Moreover, although gully density tended to increase over time in all six watersheds, in the three watersheds (one in each paired watershed) where soil and water conservation measures had been introduced, the rate of increase was lower than in those where no such measures were implemented. In addition, gully distribution was linked to land use and landscape position; gully density was higher in cultivated areas and where slope gradients were gentle. The results of this study indicate that careful site-specific identification of factors controlling gully initiation and development is crucial so that appropriate management strategies can be developed for these three sites and for other areas with similar agro-ecologies in the Upper Blue Nile basin. • Little is known about the distribution and rates of gully erosion in Ethiopia. • High-resolution imageries help to detect long-term gully erosion dynamics. • Cultivation history is a key factor for gully erosion dynamics across three sites. • Cultivated lands found in gentle slope are the most-prone to gully erosion. • Gully erosion is being reversed following land management interventions. [ABSTRACT FROM AUTHOR]

Improved knowledge of watershed-scale spatial and temporal variability of sediment yields ( SY ) is needed to design erosion control strategies, particularly in the most severely eroded areas. The present study was conducted to provide this knowledge for the humid tropical highlands of Ethiopia using the Akusity and Kasiry paired watersheds in the Guder portion of the Upper Blue Nile basin. Discharge and suspended sediment concentration data were monitored during the rainy season of 2014 and 2015 using automatic flow stage sensors, manual staff gauges and a depth-integrated sediment sampler. The SY was calculated using empirical discharge–sediment curves for different parts of each rainy season. The measured mean daily sediment concentration differed greatly between years and watersheds (0.51 g L − 1 in 2014 and 0.92 g L − 1 in 2015 for Kasiry, and 1.04 g L − 1 in 2014 and 2.20 g L − 1 in 2015 for Akusity). Sediment concentrations at both sites decreased as the rainy season progressed, regardless of the rainfall pattern, owing to depletion of the sediment supply and limited transport capacity of the flows caused by increased vegetation cover. Rainy season SYs for Kasiry were 7.6 t ha − 1 in 2014 and 27.2 t ha − 1 in 2015, while in Akusity SYs were 25.7 t ha − 1 in 2014 and 71.2 t ha − 1 in 2015. The much larger values in 2015 can be partly explained by increased rainfall and larger peak flow events. The magnitude and timing of peak flow events are major determinants of the amount and variability of SYs . Thus, site-specific assessment of such events is crucial to reveal SY dynamics of small watersheds in tropical highland environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Floodplain sediment storage is estimated by combining information from field-based and remote sensing approaches. • Average sediment storage of 284 and 227 Mg m−1 river length is deposited in the Gumara and Rib Rivers, respectively. • Local sediment storage is controlled by floodplain properties, morphological characteristics, and catchment-wide variables. • Sediment storage in temperate Europe is an order of magnitude higher than that of tropical catchments in Ethiopia. Floodplain sediment storage is an important component of a catchment's sediment budget. Here, we present the first quantification of floodplain sediment storage for tropical river catchments draining to Lake Tana, NW Ethiopia. The catchments are characterized by relatively gently sloping to flat lowlands towards the lake (1788 m a.s.l.) and steeper river reaches in the uplands towards Mount Guna (4120 m a.s.l.). Sediment storage for 65 homogeneous floodplain segments was estimated by combining information on floodplain spatial extent obtained through the field- and remote sensing-based approaches, with information on sediment thickness obtained through sediment coring and the analysis of river cut-banks. Extrapolation to the entire catchment was done making use of floodplain typology and following the classification scheme of Nanson and Croke (1992) and other floodplain properties and catchment-wide variables. The results showed average sediment storage of 284 Mg m−1 river length or 21,760 Mg ha−1 floodplain area for the Gumara and approx. 227 Mg m−1 river length or 16,909 Mg ha−1 floodplain area for the Rib River catchments. Total floodplain sediment storage at the catchments scale, above the upper gauging stations, amounts to 92.9 Mt and 35.5 Mt for the Gumara and Rib River catchments, respectively. At the scale of homogeneous floodplain segments, sediment storage is related to floodplain width, floodplain slope, upstream catchment area, the average slope of the upstream catchment area, sinuosity, and floodplain type. Overall, floodplain geomorphology largely controls sediment storage. Approx. 70 % of the sediment is stored in low-energy cohesive floodplains which only take up 11 % of the total river length in both catchments. For most of the total river and valley extent, where rivers are incised and have more energy, storage is limited. Comparing our findings with similar studies in temperate catchments of Europe and N -America showed that sediment storage in the Gumara and Rib Rivers is lower by approx. 1 order of magnitude, for similar-sized catchments. [ABSTRACT FROM AUTHOR]

Summary In response to anthropogenic disturbances, alluvial rivers adjust their geometry. The alluvial river channels in the upper Blue Nile basin have been disturbed by human-induced factors since a longtime. This paper examines channel adjustment along a 38-km stretch of the Gumara River which drains towards Lake Tana and then to the Blue Nile. Over a 50 years period, agriculture developed rapidly in the catchment and flooding of the alluvial plain has become more frequent in recent times. The objectives of this study were to document the changes in channel planform and cross-section of the Gumara River and to investigate whether the changes could have contributed to the frequent flooding or vice versa. Two sets of aerial photographs (1957 and 1980) were scanned, and then orthorectified. Recent channel planform information was extracted from SPOT images of 2006 and Google Earth. Channel planform and bed morphology (vertical changes) were determined for these nearly 50 years period. The vertical changes were determined based on aggradation along a permanent structure, historic information on river cross-sections at a hydrological gauging station, and field observations. The results indicate that the lower reach of Gumara near its mouth has undergone major planform changes. A delta with approx. 1.12 km 2 of emerged land was created between 1957 and 1980 and an additional 1 km 2 of land has been added between 1980 and 2006. The sinuosity of the river changed only slightly: negatively (−1.1% i.e. meandering decreased) for the period from 1957 to 1980 and positively (+3.0%) for the period 1980–2006. Comparison of cross-sections at the hydrological gauging station showed that the deepest point in the river bed aggraded by 2.91 m for the period 1963–2009. The importance of sediment deposition in the stream and on its banks is related to land degradation in the upper catchment, and to artificial rising of Lake Tana level that creates a backwater effect and sediment deposition in Gumara River. Direct anthropogenic impacts (irrigation activities and building of dykes along the river banks) have contributed to the huge deposition in the river bed. Where the abstraction of water for irrigation is intensive, seepage water through the banks has contributed to river bank failure. In general, this study showed that changes to the planform at the mouth of the river and to the riverbed level are substantial. Moreover, the study indicated that the flood carrying capacity of the Gumara River channel has diminished in recent times. [ABSTRACT FROM AUTHOR]

Summary This paper presents a study on an extensive floodplain adjacent to Lake Tana (Ethiopia) and its lowland tributaries, to enhance our understanding of the water of the lake and to better manage the natural resources in the floodplain. Discharge measurements were made at 12 stations. The effects of the floodplain on river discharges were investigated using the upstream and downstream discharge observations of the Gumara, Rib and Megech Rivers. The total annual runoff coefficients ranged between 0.23 and 0.81 in 2012 in the basin. Discharge varied depending on drainage density ( r = 0.75), lithology ( r = −0.72 for percentage of Tertiary igneous rocks) and land use/land cover conditions ( r = 0.61 for dominantly cultivated land with no significant other land use types). Analyses revealed that the floodplain abstracted 809 mm of water with a corresponding increase in floodplain storage of 992 mm during the beginning of the rainy season (June to July) and released stored water starting from August until the middle of September. However, the annual water balance indicated that the runoff contribution from the Rib and Megech floodplains is negligible. But the floodplain downstream of the Gumara River showed a considerable runoff contribution to the river, also in relation to the presence of springs. The floodplain acts as storage of flood water, and consequently the magnitude of peak floods was on average 71% smaller downstream than upstream in the floodplain. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image [ABSTRACT FROM AUTHOR]

• Bedload to total load ratio varies from 0.1 to 5.4%. • Specific sediment yields are 0.06 Mg ha−1 y−1 as bedload and 22 Mg ha−1 y−1 as suspended load. • Elongated planform delta growth shows predominance of suspended load. • Stream-power based bedload equations performed better. Little is known about the contribution of bedload flux to the total sediment load exported from tropical sand-bed rivers. Yet, predicting bedload transport rates contributes to better river management, more appropriate river engineering works design, accurate knowledge of sediment delivery rates, which greatly benefits sediment-related water resources management. Therefore, bedload, suspended load and runoff discharge were measured at the lower Gilgel Abay River in the Blue Nile catchment of Ethiopia. Catchment area is 3887 km2, average stream discharge in the rainy season 315 (±164) m3 s−1, and the stream surface gradient at the sampling station 0.0007 m m−1. A standard Helley-Smith (U.S. BL-84) cable-suspended sampler was used to take 315 samples to measure bedload fluxes for the 2017 rainy season. The bedload predominantly comprises well to moderately well sorted, medium-sized sand (250–500 µm) with a symmetric and leptokurtic distribution. The mean daily observed bedload discharge varies from 0.7 to 427.5 Mg d−1 with an average rate of 175.1 Mg d−1. Field observations also show that the bedload dynamics become larger near the river banks and decrease towards the middle of the river section due to the presence of a central bridge pier. The average suspended sediment load ranges from 247 to 220,028 Mg d−1 and dominates the total sediment yield (>94.6%). The bedload accounts for only 0.1 to 5.4% of the total sediment load. The area-specific bedload and suspended load are also 0.6 and 22 Mg ha−1 yr−1 respectively. The elongated planform growth of the Gilgel Abay River delta in Lake Tana is also in line with the predominance of very fine, suspended load in the sediment transport of this river, in addition to limited alongshore drifting given the low waves of Tana lake. Among the tested bedload prediction criteria, the stream-power based equations of Martin and modified Bagnold performed better. Yet, the bedload prediction performance of simple power regression equations based on runoff discharge and excess unit stream power parameters provided the best agreement with the observed bedload data (Adj. R2 0.82–0.87). Finally, this study provides plausible estimates of bedload fluxes that are within predicted ranges reported for sand-bed rivers in Africa and other tropical regions. [ABSTRACT FROM AUTHOR]

The Lake Tana Basin suffers from soil erosion and sediment fluxes to the Lake threatens its ecology and storage capacity by siltation. Soil erosion control measures are often not successful as upland erosion processes and sediment sources are not properly understood. In this study, we aimed at understanding sediment sources and fluxes by integrating fingerprinting and catchment sediment yield measurements. As a proof of concept, we selected two small catchments (60 ha and 76 ha) within a 10.4 km² study area near the town of Arb Gebeya. At the outlets, sharp-crested rectangular weirs were installed to quantify runoff and suspended sediment fluxes. Therefore, the water stage was measured at the weirs at five-minute interval using a TD-Diver and suspended sediment was sampled in the morning and during peak runoff events. Phillips tube sediment samplers were also installed at the catchment outlets to collect time-integrated sediment samples. Within the two catchments, 75 soil samples were collected from five land use classes and gully banks. A compound specific stable isotope (CSSI) finger printing technique was used to apportion sediment sources using δ13C of plant derived fatty acids (δ13C–FAs). Gas chromatograph-combustion-isotope ratio mass spectrometry (GC-C-IRMS) was used to measure this isotopic signature and MixSIAR, a FA-content dependent, Bayesian-stable isotope mixing models was used to determine the proportional contribution of the different sediment sources to the sediment mixture. Finally, source-specific catchment sediment budget was determined and sediment fluxes were analyzed in relation to catchment connectivity. Primary results revealed that percent vegetation cover of a catchment strongly influences runoff and sediment yield processes. Keywords: Catchment, land uses, connectivity, fingerprinting, sediment yield [ABSTRACT FROM AUTHOR]

Little is known about the contribution of bedload flux to the total sediment load exported from tropical sand-bed rivers. Yet, predicting bedload transport rates contributes to accurate knowledge of sediment delivery rates, what greatly benefits sediment-related water resources management. Bedload, suspended load and runoff discharge were measured at the lower Gilgel Abay River in the Blue Nile catchment of Ethiopia. Catchment area is 3887 km², average stream discharge in the rainy season 315 (± 164) m³ s-1, and the stream surface gradient at the sampling station 0.0004 m m-1. A standard Helley-Smith (U.S. BL-84) cable-suspended sampler was used to take 315 samples to measure bedload fluxes for the 2017 rainy season. The bedload predominantly comprises well to moderately well sorted, medium sized sand (250–500 μm) with a symmetric and leptokurtic distribution. The mean daily observed bedload discharge varies from 0.7 to 427.5 Mg d-1 with an average rate of 175.1 Mg d-1. Field observations also show that the bedload dynamics become larger near the river banks and decrease towards the middle of the river section due to presence of a central bridge pier. The average suspended load ranges from 247 to 220,028 Mg d-1 and dominates the total sediment yield (99.3%). Thus, the bedload accounts for only 0.1 to 5.4% of the total sediment load. The elongated planform growth of the Gilgel Abay River delta in Lake Tana is also in line with the predominance of very fine, suspended load in the sediment dynamics, in addition to limited along-shore drifting given the low Tana lake waves. Among the tested bedload prediction criteria, the stream power based equations of Martin and modified Bagnold perform very well. Alternatively, the bedload prediction performance of simple power regression equations based on runoff discharge and stream power parameters provided satisfactory agreement with the observed bedload data. Finally, this study provides plausible estimates of bedload fluxes that are within predicted ranges reported for alluvial rivers in African and other tropical regions. [ABSTRACT FROM AUTHOR]