ALBERT

Description: In the present study, we developed a catchment hydrological model which can be used to inform water resources planning and decision making for better management of the Migina Catchment (257.4 km2). The semi-distributed hydrological model HEC-HMS (Hydrologic Engineering Center – the Hydrologic Modelling System) (version 3.5) was used with its soil moisture accounting, unit hydrograph, liner reservoir (for baseflow) and Muskingum–Cunge (river routing) methods. We used rainfall data from 12 stations and streamflow data from 5 stations, which were collected as part of this study over a period of 2 years (May 2009 and June 2011). The catchment was divided into five sub-catchments. The model parameters were calibrated separately for each sub-catchment using the observed streamflow data. Calibration results obtained were found acceptable at four stations with a Nash–Sutcliffe model efficiency index (NS) of 0.65 on daily runoff at the catchment outlet. Due to the lack of sufficient and reliable data for longer periods, a model validation was not undertaken. However, we used results from tracer-based hydrograph separation from a previous study to compare our model results in terms of the runoff components. The model performed reasonably well in simulating the total flow volume, peak flow and timing as well as the portion of direct runoff and baseflow. We observed considerable disparities in the parameters (e.g. groundwater storage) and runoff components across the five sub-catchments, which provided insights into the different hydrological processes on a sub-catchment scale. We conclude that such disparities justify the need to consider catchment subdivisions if such parameters and components of the water cycle are to form the base for decision making in water resources planning in the catchment.

Description: Understanding of dominant runoff generation processes in the meso-scale Migina catchment (257.4 km2) in southern Rwanda was improved using analysis of hydrometric data and tracer methods. The paper examines the use of hydrochemical and isotope parameters for separating streamflow into different runoff components by investigating two flood events which occurred during the rainy season "Itumba" (March–May) over a period of 2 yr at two gauging stations. Dissolved silica (SiO2), electrical conductivity (EC), deuterium (2H), oxygen-18 (18O), major anions (Cl− and SO2−4) and major cations (Na+, K+, Mg2+ and Ca2+) were analyzed during the events. 2H, 18O, Cl− and SiO2 were finally selected to assess the different contributing sources using mass balance equations and end member mixing analysis for two- and three-component hydrograph separation models. The results obtained by applying two-component hydrograph separations using dissolved silica and chloride as tracers are generally in line with the results of three-component separations using dissolved silica and deuterium. Subsurface runoff is dominating the total discharge during flood events. More than 80% of the discharge was generated by subsurface runoff for both events. This is supported by observations of shallow groundwater responses in the catchment (depth 0.2–2 m), which show fast infiltration of rainfall water during events. Consequently, shallow groundwater contributes to subsurface stormflow and baseflow generation. This dominance of subsurface contributions is also in line with the observed low runoff coefficient values (16.7 and 44.5%) for both events. Groundwater recharge during the wet seasons leads to a perennial river system. These results are essential for better water resources planning and management in the region, which is characterized by very highly competing demands (domestic vs. agricultural vs. industrial uses).

Description: Understanding of dominant runoff generation processes in the meso-scale Migina catchment (257.4 km2) in Southern Rwanda was improved using analysis of hydrometric data and tracer methods. The paper examines the use of hydrochemical and isotope parameters for separating streamflow into different runoff components by investigating two flood events occurred during the rainy season "Itumba" (March–May) over the period of 2 yr at two gauging stations. Dissolved silica (SiO2), electrical conductivity (EC), deuterium (2H), oxygen-18 (18O), major anions (Cl− and SO42−) and major cations (Na+, K+, Mg2+ and Ca2+) were analyzed during the events. 2H, 18O, Cl− and SiO2 were finally selected to assess the different contributing sources using mass balance equations and end member mixing analysis for two- and three-component hydrograph separation models. The results obtained applying two-component hydrograph separations using dissolved silica and chloride as tracers are generally in line with the results of three-component separations using dissolved silica and deuterium. Subsurface runoff is dominating the total discharge during flood events, More than 80% of the discharge was generated by subsurface runoff for both events. This is supported by observations of shallow groundwater responses in the catchment (depth 0.2–2 m), which show fast infiltration of rainfall water during events. Consequently, shallow groundwater and contributes to subsurface stormflow and baseflow generation. This dominance of subsurface contributions is also in line with the observed low runoff coefficient values (16.7–44.5%) for both events. Groundwater recharge during the wet seasons leads to a perennial river system, and wet season recharge is isotopically characterising all discharge components.

Description: In the last couple of years, different hydrological research projects were undertaken in the Migina catchment (243.2 km2), a tributary of the Kagera river in Southern Rwanda. These projects were aimed to understand hydrological processes of the catchment using analytical and experimental approaches and to build a pilot case whose experience can be extended to other catchments in Rwanda. In the present study, we developed a hydrological model of the catchment, which can be used to inform water resources planning and decision making. The semi-distributed hydrological model HEC-HMS (version 3.5) was used with its soil moisture accounting, unit hydrograph, liner reservoir (for base flow) and Muskingum-Cunge (river routing) methods. We used rainfall data from 12 stations and streamflow data from 5 stations, which were collected as part of this study over a period of two years (May 2009 and June 2011). The catchment was divided into five sub-catchments each represented by one of the five observed streamflow gauges. The model parameters were calibrated separately for each sub-catchment using the observed streamflow data. Calibration results obtained were found acceptable at four stations with a Nash–Sutcliffe Model Efficiency of 0.65 on daily runoff at the catchment outlet. Due to the lack of sufficient and reliable data for longer periods, a model validation (split sample test) was not undertaken. However, we used results from tracer based hydrograph separation from a previous study to compare our model results in terms of the runoff components. It was shown that the model performed well in simulating the total flow volume, peak flow and timing as well as the portion of direct runoff and base flow. We observed considerable disparities in the parameters (e.g. groundwater storage) and runoff components across the five sub-catchments, that provided insights into the different hydrological processes at sub-catchment scale. We conclude that such disparities justify the need to consider catchment subdivisions, if such parameters and components of the water cycle are to form the base for decision making in water resources planning in the Migina catchment.