Understanding runoff processes in a semi-arid environment through isotope and hydrochemical hydrograph separations

摘要

The understanding of runoff generation mechanisms is crucial for thesustainable management of river basins such as the allocation of waterresources or the prediction of floods and droughts. However, identifying themechanisms of runoff generation has been a challenging task, even more so inarid and semi-arid areas where high rainfall and streamflow variability, highevaporation rates, and deep groundwater reservoirs may increase thecomplexity of hydrological process dynamics. Isotope and hydrochemicaltracers have proven to be useful in identifying runoff components and theircharacteristics. Moreover, although widely used in humid temperate regions,isotope hydrograph separations have not been studied in detail in arid andsemi-arid areas. Thus the purpose of this study is to determine whetherisotope hydrograph separations are suitable for the quantification andcharacterization of runoff components in a semi-arid catchment consideringthe hydrological complexities of these regions. Through a hydrochemicalcharacterization of the surface water and groundwater sources of thecatchment and two- and three-component hydrograph separations, runoffcomponents of the Kaap catchment in South Africa were quantified using bothisotope and hydrochemical tracers. No major disadvantages while using isotopetracers over hydrochemical tracers were found. Hydrograph separation resultsshowed that runoff in the Kaap catchment is mainly generated by groundwatersources. Two-component hydrograph separations revealed groundwatercontributions of between 64 and 98 % of total runoff. By means ofthree-component hydrograph separations, runoff components were furtherseparated into direct runoff, shallow and deep groundwater components. Directrunoff, defined as the direct precipitation on the stream channel andoverland flow, contributed up to 41 % of total runoff during wet catchmentconditions. Shallow groundwater defined as the soil water and near-surfacewater component (and potentially surface runoff) contributed up to 45 % oftotal runoff, and deep groundwater contributed up to 84 % of total runoff.A strong correlation for the four studied events was found between theantecedent precipitation conditions and direct runoff. These findings suggestthat direct runoff is enhanced by wetter conditions in the catchment thattrigger saturation excess overland flow as observed in the hydrographseparations.