Abstract
Abstract. The understanding of runoff generation mechanisms is crucial for the sustainable management of river basins such as the allocation of water resources or the prediction of floods and droughts. However, identifying the mechanisms of runoff generation has been a challenging task, even more so in arid and semi-arid areas where high rainfall and streamflow variability, high evaporation rates, and deep groundwater reservoirs may increase the complexity of hydrological process dynamics. Isotope and hydrochemical tracers have proven to be useful in identifying runoff components and their characteristics. Moreover, although widely used in humid temperate regions, isotope hydrograph separations have not been studied in detail in arid and semi-arid areas. Thus the purpose of this study is to determine whether isotope hydrograph separations are suitable for the quantification and characterization of runoff components in a semi-arid catchment considering the hydrological complexities of these regions. Through a hydrochemical characterization of the surface water and groundwater sources of the catchment and two- and three-component hydrograph separations, runoff components of the Kaap catchment in South Africa were quantified using both isotope and hydrochemical tracers. No major disadvantages while using isotope tracers over hydrochemical tracers were found. Hydrograph separation results showed that runoff in the Kaap catchment is mainly generated by groundwater sources. Two-component hydrograph separations revealed groundwater contributions of between 64 and 98 % of total runoff. By means of three-component hydrograph separations, runoff components were further separated into direct runoff, shallow and deep groundwater components. Direct runoff, defined as the direct precipitation on the stream channel and overland flow, contributed up to 41 % of total runoff during wet catchment conditions. Shallow groundwater defined as the soil water and near-surface water component (and potentially surface runoff) contributed up to 45 % of total runoff, and deep groundwater contributed up to 84 % of total runoff. A strong correlation for the four studied events was found between the antecedent precipitation conditions and direct runoff. These findings suggest that direct runoff is enhanced by wetter conditions in the catchment that trigger saturation excess overland flow as observed in the hydrograph separations.
Highlights
Understanding runoff processes facilitates the evaluation of surface water and groundwater risks with respect to quality and quantity (Uhlenbrook et al, 2002)
Runoff processes were poorly understood in the Kaap catchment, limiting rainfall–runoff models to lead to better informed water management decisions
Three component hydrograph separations suggested a third component that we addressed as the shallow groundwater component
Summary
Understanding runoff processes facilitates the evaluation of surface water and groundwater risks with respect to quality and quantity (Uhlenbrook et al, 2002). It assists in quantifying water resources for water allocation, hydropower production, design of hydraulic structures, environmental flows, drought and flood management, and water quality purposes (Blöschl et al, 2013). Several hydrograph separation studies using environmental isotopes and geochemical tracers have been carried out in forested, semi-humid environments, which have led to new insights into runoff processes in these areas Camacho Suarez et al.: Understanding runoff processes in a semi-arid environment
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