Abstract
Abstract. 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).
Highlights
Understanding of runoff components separation processes is essential for the proper assessment of water resources availability within catchments
This indicates that surface discharge is dominated by subsurface runoff components during flood events in the Migina catchment
The stability of end members was tested for the application of the threecomponent hydrograph separation technique
Summary
Understanding of runoff components separation processes is essential for the proper assessment of water resources availability within catchments. The use of environmental isotopes in combination with hydrochemical tracers and hydrometric measurements can help to gain further insights into hydrological processes because the methods separate and quantify different runoff components during rainfall events. Most hydrograph separations involve the standard twocomponent mixing models of Sklash and Farvolden (1979), in which the stream water is separated into old (pre-event) and new (event) water components. This approach identifies the age of streamflow components, but cannot be used to assess the spatial origin (Ladouche et al, 2001). To obtain both temporal and spatial origins, some investigations using stable
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