Blue Nile Catchment Research Articles

User-friendly workflows for catchment modelling: Towards reproducible SWAT+ model studies

A Graphical User Interface (GUI) is regularly used to support model applications in catchment hydrological modelling software. A GUI is generally user-friendly for novice users but opens sources of irreproducible research. We illustrate that none of the 10 Soil and Water Assessment Tool (SWAT) models over the Upper Blue Nile can easily be reproduced. Scripted workflows provide the ability to reproduce model set-ups, but they may be less user-friendly especially to novice users. We present a software (SWAT + AW) that promotes reproducible SWAT + model studies while remaining user-friendly for both novice and expert users. SWAT + AW uses a configuration file to create models that are compatible with GUI. We applied the workflow to the Blue Nile catchment and show that it yields the same results the SWAT + GUI. We conclude that such user-friendly scripted workflows enhance reproducibility, transparency and reusability of hydrological models. The software is publicly available at https://github.com/VUB-HYDR/SWATPlus-AW.

A Long–Term Response-Based Rainfall-Runoff Hydrologic Model: Case Study of The Upper Blue Nile

This study develops a response-based hydrologic model for long-term (continuous) rainfall-runoff simulations over the catchment areas of big rivers. The model overcomes the typical difficulties in estimating infiltration and evapotranspiration parameters using a modified version of the Soil Conservation Service curve number SCS-CN method. In addition, the model simulates the surface and groundwater hydrograph components using the response unit-hydrograph approach instead of using a linear reservoir routing approach for routing surface and groundwater to the basin outlet. The unit-responses are Geographic Information Systems (GIS)-pre-calculated on a semi-distributed short-term basis and applied in the simulation in every time step. The unit responses are based on the time-area technique that can better simulate the real routing behavior of the basin. The model is less sensitive to groundwater infiltration parameters since groundwater is actually controlled by the surface component and not the opposite. For that reason, the model is called the SCHydro model (Surface Controlled Hydrologic model). The model is tested on the upper Blue Nile catchment area using 28 years daily river flow data set for calibration and validation. The results show that SCHydro model can simulate the long-term transforming behavior of the upper Blue Nile basin. Our initial assessment of the model indicates that the model is a promising tool for long-term river flow simulations, especially for long-term forecasting purposes due to its stability in performing the water balance.

Bedload transport measurements in the Gilgel Abay River, Lake Tana Basin, Ethiopia

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.

Effects of River Capture and Sediment Flux on the Evolution of Plateaus: Insights From Numerical Modeling and River Profile Analysis in the Upper Blue Nile Catchment

AbstractThe Ethiopian Highlands, with up to 1,500‐m‐deep canyons surrounded by low relief plateau surfaces, are one of the most spectacular examples of transient fluvial landscapes on Earth. We analyze river profiles extracted from a 90 m digital elevation model of the upper Blue Nile catchment and identify 116 major knickpoints on 137 river profiles. We use 1‐D river profile models to simulate three potential mechanisms for knickpoint formation: plateau uplift, capture of large lakes or internal drainage on the plateau surface, and sediment‐flux‐dependent river incision with a low sediment flux from the plateau surface. We define a normalized upstream knickpoint propagation distance, χkpj, and demonstrate that the erosion models predict different distributions of this metric in transient profiles following plateau uplift. Model knickpoints resulting from the scenario of plateau uplift or common base‐level fall using the stream‐power model display similar upstream propagation distance in χ space. The results of the same scenario modeled with the sediment‐flux‐dependent incision model show upstream knickpoint propagation distance proportional to catchment area. Perturbations to these trends result from drainage capture. Comparing the model results with observed χkpj values of knickpoints and field observations, we recognize effects characteristic of the sediment‐flux‐dependent incision model. However, most profiles are best explained by the systematic transfer of drainage area from the plateau to the surrounding rivers. We propose a new model of landscape evolution for the upper Blue Nile catchment dominated by discrete events of capture of drainage area from the plateau.

Are we using the right fuel to drive hydrological models? A climate impact study in the Upper Blue Nile

Abstract. Climate simulations are the fuel to drive hydrological models that are used to assess the impacts of climate change and variability on hydrological parameters, such as river discharges, soil moisture, and evapotranspiration. Unlike with cars, where we know which fuel the engine requires, we never know in advance what unexpected side effects might be caused by the fuel we feed our models with. Sometimes we increase the fuel's octane number (bias correction) to achieve better performance and find out that the model behaves differently but not always as was expected or desired. This study investigates the impacts of projected climate change on the hydrology of the Upper Blue Nile catchment using two model ensembles consisting of five global CMIP5 Earth system models and 10 regional climate models (CORDEX Africa). WATCH forcing data were used to calibrate an eco-hydrological model and to bias-correct both model ensembles using slightly differing approaches. On the one hand it was found that the bias correction methods considerably improved the performance of average rainfall characteristics in the reference period (1970–1999) in most of the cases. This also holds true for non-extreme discharge conditions between Q20 and Q80. On the other hand, bias-corrected simulations tend to overemphasize magnitudes of projected change signals and extremes. A general weakness of both uncorrected and bias-corrected simulations is the rather poor representation of high and low flows and their extremes, which were often deteriorated by bias correction. This inaccuracy is a crucial deficiency for regional impact studies dealing with water management issues and it is therefore important to analyse model performance and characteristics and the effect of bias correction, and eventually to exclude some climate models from the ensemble. However, the multi-model means of all ensembles project increasing average annual discharges in the Upper Blue Nile catchment and a shift in seasonal patterns, with decreasing discharges in June and July and increasing discharges from August to November.

Runoff and precipitation dynamics in the Blue and White Nile catchments during the mid-Holocene: A data-model comparison

The Blue Nile is the major contributor of freshwater and sediments to the modern-day main Nile River and exerts a key control on seasonal flooding in the Nile valley. Recent studies have postulated that the relative contribution from the Blue Nile to the main Nile runoff might have been reduced during the mid-Holocene, at a time when higher boreal summer insolation stimulated enhanced precipitation in North Africa. Whether the decrease in the relative contribution from the Blue Nile resulted from a decrease in precipitation over the catchment, from an increase in White Nile runoff or from a combination of both is still a matter of debate. By comparing regional proxy-records with the output from a global atmospheric model zoomed on Africa, we propose that the reduced contribution from the Blue Nile at 6 ka originated from both a higher White Nile runoff and a lower Blue Nile runoff. Enhanced African and Indian monsoons at 6 ka induced a northern shift of the Intertropical Convergence Zone and an eastward shift of the Congo Air Boundary. Such an atmospheric configuration led to a negative anomaly of summer precipitation over the Blue Nile catchment that likely resulted in a reduction in the Blue Nile runoff. By contrast, a sustained positive anomaly of precipitation over the White Nile catchment during both summer and autumn most likely induced a higher main Nile runoff during the mid-Holocene. Using the model output, we propose a first synoptic view on regional rainfall dynamics that permits to reconcile contrasting proxy records.

Sensitivity of Horn of Africa Rainfall to Regional Sea Surface Temperature Forcing

The Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate Model (RegCM4) is used to investigate the rainfall response to cooler/warmer sea surface temperature anomaly (SSTA) forcing in the Indian and Atlantic Oceans. The effect of SSTA forcing in a specific ocean basin is identified by ensemble, averaging 10 individual simulations in which a constant or linearly zonally varying SSTA is prescribed in individual basins while specifying the 1971–2000 monthly varying climatological sea surface temperature (SST) across the remaining model domain. The nonlinear rainfall response to SSTA amplitude also is investigated by separately specifying +1K, +2K, and +4K SSTA forcing in the Atlantic and Indian Oceans. The simulation results show that warm SSTs over the entire Indian Ocean produce drier conditions across the larger Blue Nile catchment, whereas warming ≥ +2K generates large positive rainfall anomalies exceeding 10 mm·day−1 over drought prone regions of Northeastern Ethiopia. However, the June–September rainy season tends to be wetter (drier) when the SST warming (cooling) is limited to either the Northern or Southern Indian Ocean. Wet rainy seasons generally are characterized by deepening of the monsoon trough, east of 40°E, intensification of the Mascarene high, strengthening of the Somali low level jet and the tropical easterly jet, enhanced zonal and meridional vertically integrated moisture fluxes, and steeply vertically decreasing moist static energy. The opposite conditions hold for dry monsoon seasons.

Simulating the connections of ENSO and the rainfall regime of East Africa and the upper Blue Nile region using a climate model of the Tropics

Abstract. We simulate the observed statistical relationship between ENSO and the rainfall regime of the upper Blue Nile using the tropical-band version of the regional climate model RegCM4 (or Reg-TB). An ensemble of nine simulations for the 28-year period 1982–2009 is completed to investigate the role of ENSO in modulating rainfall over the upper Blue Nile catchment. Reg-TB shows a good skill in simulating the climatology of temperature, outgoing long-wave radiation patterns as well as related atmospheric circulation features during the summer season (i.e. the rainy season over the Blue Nile catchment). The model also succeeds in reproducing the observed negative correlation between Pacific SST and rainfall anomalies over the Blue Nile catchment, and in particular the association of droughts over the Blue Nile with El Niño events that start in April–June. We thus propose that observations and model forecasts of Pacific SST during this season could be used in seasonal forecasting of summer rainfall over the upper Blue Nile region.

Sediment balances in the Blue Nile River Basin

Rapid population growth in the upper Blue Nile basin has led to fast land-use changes from natural forest to agricultural land. This resulted in speeding up the soil erosion process in the highlands and increasing sedimentation further downstream in reservoirs and irrigation canals. At present, several dams are planned across the Blue Nile River in Ethiopia and the Grand Ethiopian Renaissance Dam is currently under construction near the border with Sudan. This will be the largest hydroelectric power plant in Africa. The objective of this paper is to quantify the river flows and sediment loads along the Blue Nile River network. The Soil and Water Assessment Tool was used to estimate the water flows from un-gauged sub-basins. To assess model performance, the estimated sediment loads were compared to the measured ones at selected locations. For the gauged sub-basins, water flows and sediment loads were derived from the available flow and sediment data. To fill in knowledge gaps, this study included a field survey in which new data on suspended solids and flow discharge were collected along the Blue Nile and on a number of tributaries. The comparison between the results of this study and previous estimates of the sediment load of the Blue Nile River at El Deim, near the Ethiopian Sudanese border, show that the sediment budgets have the right order of magnitude, although some uncertainties remain. This gives confidence in the results of this study providing the first sediment balance of the entire Blue Nile catchment at the sub-basin scale.

Rainfall-Runoff Model(s) Performance in the Blue Nile River

This paper presents the performance of some tested rainfall-runoff models to forecast river flows of the Blue Nile catchment up to Eddeim. Flow forecasting is an important step in river basin management in particular and water resources management in general. River flow models are used as components in actual flow forecasting schemes. They are also used in providing for efficient operation of storage reservoirs. Usually, flow forecasts are obtained in real time by transforming the input into a discharge using models. These forecasts may subsequently be modified or updated in accordance with the errors observed in the previous forecasts up to the time of making the new forecast. The system analysis or black box approach depends on a prior assumption of flexible linear and time invariant relationship the expression of which can be obtained by the application of systems analysis approach to records. The conceptual model provides an alternative approach in which the input-output transformation goes through a series of steps. In this study, systems and conceptual modelling techniques are applied to the Blue Nile catchment up to Eddeim of the Ethiopian high lands. The models were applied in non-parametric and parametric forms. Parameter optimisation is carried out by ordinary least squares, Rosenbrock, Simplex and genetic algorithm. The areal rainfall which is the main input to these models was estimated using arithmetic mean. However, attempts to estimate the areal rainfall by the Thiesen polygon method was made but the improvement in the model performance can not justify the amount of work involved in making Thiesen’s estimate. It is shown that the simple assumption of linearity is not adequate in modelling the rainfall runoff transformation. However, in the Blue Nile catchment, which exhibits a marked seasonal behaviour, good results were obtained with Linear Perturbation Model (LPM) which involves the assumption of linearity between the departures from seasonal expectations in input and output series. The application of the GFFS (collection of systems and conceptual models) software proved to be possible with variable efficiencies. The LPM in non-parametric or parametric form, the LVGF model, the ANN and the SMAR model can be used to forecast (reproduce) Ed-deim flows with high performance. Within the range of the tested models LPM was found to be the best candidate model that can forecast the flows under a wide range of conditions of marked seasonality by accounting for more than 90% of the initial variance.

Rainfall-runoff modelling using artificial neural networks technique: a Blue Nile catchment case study

A rainfall-runoff model based on an artificial neural network (ANN) is presented for the Blue Nile catchment. The best geometry of the ANN rainfall-runoff model in terms of number of hidden layers and nodes is identified through a sensitivity analysis. The Blue Nile catchment (about 300 000 km2) in the Nile basin is selected here as a case study. The catchment is classified into seven subcatchments, and the mean areal precipitation over those subcatchments is computed as a main input to the ANN model. The available daily data (1992–99) are divided into two sets for model calibration (1992–96) and for validation (1997–99). The results of the ANN model are compared with one of physical distributed rainfall-runoff models that apply hydraulic and hydrologic fundamental equations in a grid base. The results over the case study area and the comparative analysis with the physically based distributed model show that the ANN technique has great potential in simulating the rainfall-runoff process adequately. Because the available record used in the calibration of the ANN model is too short, the ANN model is biased compared with the distributed model, especially for high flows. Copyright © 2005 John Wiley & Sons, Ltd.

Real-Time Flood Forecasting on the Blue Nile River

The SMAR-AR Model is used for real-time riverflow forecasting on the Blue Nile catchment at Eldeim Station, which is located near the Sudanese-Ethiopian border. The SMAR-AR model is calibrated using data for the period 1992–1994 and verified by the data for the year 1995. The results of the SMAR-AR model are compared with those of the SAMFIL model that is currently used by the Sudanese Ministry of Irrigation and Water Resources (MOIWR) for riverflow forecasting at Eldeim Station. The SAMFIL was developed and brought into operation in Sudan by Delft Hydraulics of the Netherlands. The comparison of results is carried out during four flood seasons for the years 1992–1995. The discharge forecasts of the SAMFIL model are obtained from the archives of the MOIWR. The results of the comparison show that the discharge forecasts of the SMAR-AR model are far more reliable than those of the SAMFIL.