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

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 semiarid 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 threecomponent 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 nearsurface 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.

Hydrograph separation using tracers and digital filters to quantify runoff components in a semi‐arid mesoscale catchment

“…In the Kaap River, however, the volumetric baseflow contribution appears to be lower than in other subcatchments. Camacho Suarez et al () conducted an intense tracer study in the Kaap catchment during the wet season of 2013/2014. They used both hydrochemical and environmental tracers and reported groundwater contribution of 64% to 98% in four events studied, based on daily and shorter timescale data.…”

“…The concentration for subsurface runoff was assumed to be the concentration of streamflow when the flow is lowest (assuming these are baseflow conditions) and the concentration of the surface runoff was assumed to be similar to concentrations observed during rainfall events (Camacho Suarez, Saraiva Okello, Wenninger, & Uhlenbrook, ; van Wyk, van Tonder, & Vermeulen, ). The assumptions of this method are further discussed by Buttle (), Uhlenbrook et al (), and Uhlenbrook and Hoeg ().…”

“…The end members of surface and baseflow were estimated from the EC data. The surface end member is fixed at 20 μS/cm, which corresponds to the average rainfall EC as reported by Camacho Suarez et al (2015) and lowest observed EC from the stream. The baseflow end member was estimated from the highest observed EC in the streamflow.…”

Global Isotope Hydrogeology―Review

“…Storm hydrographs in temperate headwaters remain overrepresented in the primary literature (Table ), relative to the limited land areas and time intervals they encompass (Figure ). Groundwater contributions to streamflow have been quantified in some understudied settings, including cultivated, built, and disturbed landscapes (e.g., Buttle & Sami, ; Gremillion et al, ; Huang et al, ; Sandstrom, ; Tekleab et al, ), polar latitudes (Bishop et al, ; Dahlke et al, ; McNamara et al, ; St. Amour et al, ), glaciated basins (Kong & Pang, ; Williams et al, ), alpine deserts (Sun et al, ), semiarid areas (Camacho Suarez et al, ; Zhou et al, ), tropical high elevations (Correa et al, ; Mosquera et al, ), and the humid (sub)tropics (Burns et al, ; Calderon & Uhlenbrook, ; Litt et al, ; Mortatti et al, ; Muñoz‐Villers & McDonnell, ; Scholl et al, ; Wenjie et al, ). Other works have sampled conditions going beyond just storm hydrographs, studying time intervals encompassing spring freshet (Shanley et al, ; Wang et al, ), extreme rainfall and flood events (Lyon et al, ; Winston & Criss, ), and ecological disturbances (Bearup et al, ).…”

The Changing Relationship Between Rainfall and Surface Runoff on the Loess Plateau, China

“…In general, the base flow is more likely to be high in the basins with more groundwater (Rossi et al, 2016). Research shows that in the arid and semiarid regions, precipitation contributes much less to total runoff than base flow does (Camacho Suarez et al, 2015). Therefore, the relationship between rainfall and surface runoff is stronger in the southeastern region.…”