Search

The study aimed to evaluate the SWAT and HBV model’s responses to estimate streamflow in three typical selected watersheds in the upper Blue Nile Basin, Ethiopia. The evaluation was carried out through simulation of runoff in three watersheds, namely: Gilgel Abay, Anger, and Maybar located in the upper Blue Nile Basin, Ethiopia. The same calibration and validation period were applied for each watershed model with their respective historical records. The objective function indicates the optimized range of model parameters during the calibration and validation procedure, and it gives a good model response result, which is determined by the recommended performance indicators and hydrograph plots. The HBV and SWAT model’s calibration responds with (NSE = 0.81, NSE = 0.48 and NSE = 0.46) and (NSE = 0.81, NSE = 0.63, NSE = 0.61), respectively for Gilgel Abay, Anger, and Maybar watersheds. The optimized parameters were applied to validate the model using independent historical data. The SWAT and HBV model responses for NSE during the validation period were 0.8, 0.55, and 0.54 and 0.63, 0.45, and 0.34, respectively, for Gilgel Abay, Anger, and Maybe watersheds. An alternative was observed over and under estimation runoff during stimulation periods in extreme wet and dry periods. Generally, the calibrated and validated hydrograph output result indicates that the model’s responses to too wet and dry conditions were not satisfactory. Therefore, further model refinements for extreme events, including season-based model evaluations that could improve the process representation of rainfall-runoff events, are recommended.

Soil loss is a continuous ecological problem. The loss of top fertile soil by erosion creates severe limitations to sustainable agricultural land use, which lead to reduced soil productivity and food insecurity. Soil loss is not affect only agriculture but also causes streams, lakes, dams, and reservoir sedimentation. Efforts should be focused to decrease risks of high erosion rates. This requires erosion prone area identification for urgent conservation plan. Techniques to know the spatial variability of erosion severity is essential for improving landuse management. The Revised Universal Soil Loss Equation (RUSLE) model with ArcGIS and Multi Criteria Evaluation (MCE) technique were used to estimate the annual soil loss rate and to identify high erosion risk areas in Gilegel beles watershed with a size of 770 km2, to prioritize area for conservation planning. land use land cover, soil map, digital elevation model (30m resolution), and twenty five years rainfall data at nine rain gage stations across the watershed were used to estimate soil loss in RUSLE model. The result showed that annual soil loss of the watershed ranges from 0 to 511.2ton/ha/yr with average of 28.68 ton /ha/ yr. Spatially 10.4% and 13.2% of the watershed was extremely sever and high soil erosion respectively. However, the remaining 46.4% and 30.03% of the area lies within low to moderate level. Our study revealed the most extensive soil loss rates were estimated in steep slopes, bare lands, crop land and biological degraded areas were aggravated soil loss problem. Finally, the combined models were effective for soil erosion estimation in small watersheds, so the outcome verified that integrated land management practices have needed to apply effective soil erosion control techniques like agroforestry, grass strips reduce runoff velocity and allow for infiltration, provide sediment trapping, erosion control blankets, dust control and percolation ditches for erosion prone areas and managing the inter catchment can be the best solution.

Study region: Robit-Bata watershed, Upper Blue Nile basin, Ethiopia. Study focus: Stable isotopes of water (Oxygen-18 and Deuterium) were used as tracers to estimate the contribution of groundwater in shallow hillslope aquifers to streamflow in the Robit-Bata watershed. To assess the spatiotemporal variability of shallow groundwater and develop a hydrograph separation technique, we collected rainfall, shallow groundwater, and streamflow samples and analyzed their δ18O and δ2H isotopic compositions. The local meteoric water line (LMWL) and local evaporative line (LEL) of the study area were determined and compared with the global meteoric water line (GMWL). A standard unweighted two-component isotope-based hydrograph separation model was used to determine the percentage contribution of shallow groundwater to streamflow. New hydrological insights for the region: The LMWL (δ2H = 8.63·δ18O + 18.2) mostly showed heavy isotopic enrichment relative to GMWL, and the LEL (δ2H = 5.45·δ18O + 6.96) indicated isotopic enrichment compared to Ethiopian lakes. Shallow groundwater responded rapidly to rainfall, with good spatial correlation depending on topographic positions of wells. Pre-event water contributed 90% when the watershed reached maximum storage. This finding gives insight towards the predominant runoff generation process and has significant implications for sustainable dry season irrigation expansion in the area as the sub-surface flow drains out of the watershed from October onwards reducing water tables in the shallow wells.

The largest freshwater lake in Ethiopia, Lake Tana, has faced ecological disaster due to water hyacinth (Eichhornia crassipes) infestation. The water hyacinth is a threat not only to the ecology but also to the socioeconomic development of the region and cultural value of the lake, which is registered as a UNESCO reserve. This study aims to map the spatiotemporal dynamics of the water hyacinth using high-resolution PlanetScope satellite images and assesses the major environmental variables that relate to the weed spatial coverage dynamics for the period August 2017 to July 2018. The plausible environmental factors studied affecting the weed dynamics include lake level, water and air temperature, and turbidity. Water temperature and turbidity were estimated from the moderate resolution imaging spectroradiometer (MODIS) satellite image and the water level was estimated using Jason-1 altimetry data while the air temperature was obtained from the nearby meteorological station at Bahir Dar station. The results indicated that water hyacinth coverage was increasing at a rate of 14 ha/day from August to November of 2017. On the other hand, the coverage reduced at a rate of 6 ha/day from December 2017 to June 2018. However, the length of shoreline infestation increased significantly from 4.3 km in August 2017 to 23.4 km in April 2018. Lake level and night-time water temperatures were strongly correlated with water hyacinth spatial coverage (p < 0.05). A drop in the lake water level resulted in a considerable reduction of the infested area, which is also related to decreasing nutrient levels in the water. The water hyacinth expansion dynamics could be altered by treating the nutrient-rich runoff with best management practices along the wetland and in the lake watershed landscape.

Low flow analysis provides crucial information for the planning and design water resource development, risk assessment and environmental flow management. Understanding the low flow regimes and evaluating the magnitudes for incorporating in water resources management is vital for the countries like Ethiopia where demand for water is increasing. However, there were hardly enough studies in understanding the trends of low flow and frequency analysis. Therefore, this study focuses on evaluation of the trends in low flows and regional low flow analysis in the Blue Nile Basin, Ethiopia. In order to carry out the study, 15 river sub-basins in the Blue Nile Basin were selected based on the long term data availability and presence of quality of data. The 3-day sustained low flow (3d-slf), the 7-day sustained low flow (7d-slf) and the 14-day sustained low flow (14d-slf) models were used to extract the data from the daily time series stream data obtained from MoWIE. Trends in low flow were analyzed separately by using Mann-Kendall (MK) trend test. Low flow frequency analysis was used to estimate the long term low flow quantiles. In addition, regional analysis for estimating the quantiles for ungaged catchments was also developed based on the regional growth curve and catchment characteristic of drainage basins. The results indicated that 3d-slf, 7d-slf and 14d-slf models of low flow series indicated no significant difference for each station at 95% CI. Out of the 15 selected stations, 12 of stations have indicated decreasing; two stations indicated increasing and remaining one station with no trend. Mainly decreasing trend was associated with the land cover and climate change which results in increasing runoff and evapotranspiration respectively. Weibull distribution—GEV and LGN was found best fit based on the L-Moment Ratio Diagram (L-MRD). Hence quantile estimations have indicated diminishing magnitudes of low flow quintiles for 2 - 500 years return periods. Regional low frequency analysis has provided a very good relationship between discharge and catchment characteristics with an R2 of 0.72. Where area (A) and rainfall (R) followed by slope were found sensitive to compute in developing the regional region equations between mean low flows and the physiographic data. This study indicated that there needs to be a new water management scenario and adaptation mechanism of climate change and land use land cover dynamics for utilizing water resource in the Blue Nile Basin.

Agricultural intensification to meet the food needs of the rapidly growing population in developing countries affects water quality. In regions such as the Lake Tana basin, knowledge is lacking on measures to reduce non-point source pollutants in humid tropical monsoon climates. The aim of this paper was, therefore, to develop a non-point model that can predict the placement of practices to reduce the transport of sediment and phosphorus (P) in a (sub) humid watershed. In order to achieve the objective, hydrometeorological, sediment, and P data were collected in the watershed since 2014. The parameter efficient semi-distributed watershed model (PED-WM) was calibrated and validated in the Ethiopian highlands to simulate runoff and associated sediments generated through saturation excess. The P module added to PED-WM was used to predict dissolved (DP) and particulate P (PP) loads aside from discharge and sediment loads of the 700 ha of the Awramba watershed of Lake Tana basin. The PED-WM modules were evaluated using the statistical model performance measuring techniques. The model parameter based prediction of source areas for the non-point source sediment and P was also evaluated spatially and compared with the Topographic Wetness Index (TWI) of the watershed. The water balance component of the non-point source model performed well in predicting discharge, sediment, DP, and PP with NSE of 0.7, 0.65, 0.65, and 0.63, respectively. In addition, the predicted discharge followed the hydrograph with insignificant deviation from its pattern due to seasonality. The model predicted a sediment yield of 28.2 t ha−1 year−1 and P yield of 9.2 kg ha−1 year−1 from Awrmaba. Furthermore, non-point source areas contributed to 2.7 kg ha−1 year−1 (29%) of DP at the outlet. The main runoff and sediment source areas identified using PED-WM were the periodically saturated runoff areas. These saturated areas were also the main source for DP and PP transport in the catchment. Using the PED-WM with the P module enables the identification of the source areas as well as the prediction of P and sediment loading which yields valuable information for watershed management and placement of best management practices.

Understanding soil moisture at a small scale is beneficial for predicting productivity and management of both rained and irrigated agriculture in mostly smallholder communities. This study aims to accurately represent micro-watershed scale soil moisture using the optimization capability of SWAT (SUFI2) model and soil information derived from Sentinel 2 A level 1 C satellite images with OPtical TRApezoid Model (OPTRAM) and MNDWI. The study was carried in the 700 ha Awramba watershed in the Upper Blue Nile, Ethiopia. Calibration and validation of SWAT were performed using in-situ stream flow data to enable the accurate simulation of water balance components such as soil moisture. The spectral water index was evaluated using MNDWI from the green band (560 nm) and short wave infrared band (2190 nm). The Results were evaluated based on the runoff response n and soil moisture fit to measured values. The runoff fit against the measured data using Nash Sutcliffe Efficiency (NSE) and R2 criteria is 0.7 is and 0.75, respectively. The simulated daily soil moisture against the in-situ constant soil moisture provided NSE = 0.51, R2 = 0.77, RMSE = 0.19 and PBIAS = −0.242. The simulation results indicate that validation of SWAT, OPTRA M and MNDWI models with in situ soil moisture data leads to acceptable accuracy with 0.0027 cm3 cm−3, 0.0022 cm3 cm−3 and 0.034 cm3 cm−3 standard errors, respectively. Furthermore, Sentinel 2A imagery is found to have a higher potential to simulate soil moisture compared to TDR data. The overall study indicates satellite-based soil moisture provides an encouraging pathway to setting up soil moisture-based prediction for smallholder agriculture in Ethiopia.

This study aims to evaluate stream flow predication capability of three hydrological models including Parameter Efficient Semi-Distributed Watershed Model (PED-WM) model, Hydrologiska Byrans Vattenbalansavdelning (HBV) and Hydraulic Engineering Center-Hydrologic Modeling System (HEC-HMS) in range of sizes of watersheds, Upper Blue Nile Basin, Ethiopia. The model efficiency on daily time scale during calibration period for PED-W (NSE = 0.76, 0.81 and 0.57), HBV-IHMS (NSE = 0.68, 0.79 and 0.59) and HEC-HMS (NSE = 0.63, 0.68 and 0.48) were obtained for Anjeni, Gumara and Main Belles watersheds respectively. Similarly, for validation period PED-W (NSE = 0.6, 0.73 and 0.37), HBV-IHMS (NSE = 0.56, 0.79 and 0.55) and HECHMS (NSE = 0.52, 0.74 and 0.37) were obtained for Anjeni, Gumara and Main Belles watersheds respectively. Similarly, the model performances on monthly time steps were also varied among three hydrological models and the results better than the daily time scale. In PED-W, saturation excess is the main direct runoff process. The overall model performance indicated that PED-W model was better than the other two models. The result indicates that the models in the highlands of Ethiopia are dominantly dependent on the runoff mechanism dominantly on saturation excess runoff mechanism. Hence, there should be an approach to integrate climate region specific model in our water resource development system for predicting stream flow for ungagged catchments.

Anthropogenic landscape conversion from forest to agricultural land affects baseflow. Baseflow is a source of potable water and can be used for the irrigation of high value crops. Finding ways to increase base and inter flow (i.e., groundwater flow) is, therefore, essential for the improvement of the livelihood of rural inhabitants. Therefore, the objective is to investigate the effect of landscape interventions on stream discharge and, in particular, on groundwater flow. The Tikur-Wuha experimental watershed in the upper reaches of the Blue Nile was selected because discharge data were available before and after implementation of a suite of land management practices that, among others, enhanced the percolation of water to below the rootzone. The parameter efficient distributed (PED) model was used to separate overland flow from total flow. The groundwater flow index (GWFI), defined as the quotient of the annual groundwater flow to the total stream discharge at the outlet of the watershed, was calculated. Our analysis with the PED model showed that at similar annual rainfall amounts, more baseflow and less surface runoff was generated after the landscape intervention, which promoted deep infiltration of the rainwater. The decrease in surface runoff shortly after the implementation of the land management practices is similar to observations in other watersheds in the Ethiopian highlands.

The Soil Water Assessment Tool (SWAT) is employed throughout the world to simulate watershed processes. A limitation of this model is that locations of saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth cannot be simulated realistically. The objective of this research is to overcome this limitation with minor changes in the original SWAT code. The new approach is called SWAT-with-impervious-layers (SWAT-wil). Adaptations consisted of redefining the hillslope length, restricting downward percolation from the root zone, and redefining hydrologic response units (HRUs) such that they are associated with the landscape position. Finally, input parameters were chosen such that overland flow from variable saturated areas (VSAs) corresponds to the variable source interpretation of the Soil Conservation Service (SCS) curve number runoff equation. We tested the model for the Town Brook watershed in the Catskill Mountains. The results showed that the discharge calculated with SWAT-wil agreed with observed outflow and results simulated with the original SWAT and SWAT-hillslope (SWAT-HS) models that had a surface aquifer that transferred water between groups of HRUs. The locations of the periodically saturated runoff areas were predicted by SWAT-wil at the right locations. Current users can utilize the SWAT-wil approach for catchments where VSA hydrology predominates.

This study presents modeling runoff and sediment with management scenarios for watershed management and resource erosion in Koga watershed using AnnAGNPS model. Calibration of the model was carried from 1988–2001 and validation from 2002–2007. The result of sensitivity analysis indicated that the CN was the most sensitive parameter to runoff and peak runoff rate whereas LS and K-factor were for sediment yield following RF, and these parameters were subjected to calibration. For model calibration, R2 of 0.69, 0.35, 0.55; NSE of 0.69, −0.38, 0.55; RSR of 0.54, 1.14, 0.67; and PBIAS of 0.07%, −80.56% and 4.09% were obtained for surface runoff, peak runoff rate, and sediment load, respectively. Similarly validation results indicated an R2 of 0.76, 0.54, 0.62; NSE of 0.76, 0.38, 0.62; RSR of 0.43, 0.71, 0.56, and PBIAS of 2.31%, −36.58% and 5.68% for surface runoff, peak runoff rate, and sediment load, respectively. Where the model efficiency was rated at the range of fair to excellent for three of the outputs of the model for both calibration and validation period. Only 21.5% of the area was able to generate the 78.8% of total soil erosion, with higher than tolerable limit. Hence converting of 21.5% of highest eroding cropland cells either to forest or grassland would reduce soil erosion, sediment yield and load significantly. Ultimately it would help to reduce the sedimentation in Koga dam which could result in reduction of storage capacity.

Future river discharge predictions seldom take into account the degrading landscape. The objective of this study was to investigate the relationship of river discharge and sediment concentrations, and the effect of changing landscape and climate on discharge and sediment transport in the Ethiopian Blue Nile basin. This study used past precipitation records and the Parameter Efficient Distributed (PED) model to examine how the relationship between precipitation, discharge, and sediment concentration changed with time. All input data to the PED model were kept constant except for a conversion of permeable hillside to degraded soil in time. The results of this study show that with a gradual increase of the degraded areas from 10 % in the 1960s to 22 % in 2000s, the observed discharge pattern and sediment concentration can be simulated well. Simulated annual runoff increased by 10 % over the 40-year periods as a result of the increase in degraded soils. Sediment loads appeared to have increased many times more, but this needs to be further validated as data availability is limited. In general, the results indicate that rehabilitating the degraded and bare areas by planting permanent vegetation can be effective in decreasing the sediment concentration in the rivers. Research should be undertaken to evaluate the effectiveness of vegetation planting.