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UNESCO-IHE Institute for Water Education, Delft (2014)

Identification and quantification of runoff components in the Kaap catchment, South Africa

Camacho Suarez V.V.

Titre : Identification and quantification of runoff components in the Kaap catchment, South Africa

Auteur : Camacho Suarez V.V.

Etablissement de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2014

The understanding of runoff generation mechanisms is crucial for water management of river basins. From water allocation decisions to forecasting of floods or droughts, the insights of runoff processes provide foundations to build hydrological tools that can assist in the sustainable management of water resources. The Kaap River is a tributary of the Crocodile River which itself is part of the Inkomati River Basin. The trans-boundary basin shares waters between South Africa, Swaziland and Mozambique. Water tensions between these countries have triggered water sharing agreements emphasizing the need for water accountability. This thesis project aims to assist in the quantification process by understanding the mechanisms of runoff generation and determining their contributions to total runoff. Multiple approaches have been applied n this study including : (1) the analysis of climate, hydrology, geology, river and groundwater hydrochemistry, (2) sampling of stream and rain water for spatial characterization of the catchment and hydrograph separation studies (3) laboratory analysis of major ions, environmental isotopes and physical parameters and (4) end member mixing analysis and hydrograph separation. Results showed that runoff in the Kaap catchment is mainly generated by groundwater sources. Two-component hydrograph separations resulted in groundwater contributions between 71% and 95% of total runoff. Furthermore, through three-component hydrograph separations, runoff components from different geographic and groundwater sources were found. Electrical conductivity and silicon were used for the separation of runoff components from the upstream region, where granite occupies approximately 46% of the total catchment area, and the downstream area, which has a more diverse geology and land use influence. For the four studied events, water from the granite region contributed approximately one third of the total runoff while the downstream area contributed up to two thirds of total runoff leaving a minimal contribution of direct runoff. Separation of shallow and deep groundwater components demonstrated that the shallow groundwater component, rich in ionic content, alkalinity, silicates and nutrients, was the dominant runoff component in total runoff contributing up to 70% of total runoff. The deeper groundwater component, which was less mobile and had less ionic content, contributed up to 17% of total runoff. Finally, a strong correlation was found between the antecedent precipitation conditions and direct runoff showing the catchment’s functioning during rain events. This finding reveals that direct runoff is enhanced by wetter conditions in the catchment which can also trigger saturation overland flow. The knowledge in runoff processes gained in this study provides solid foundations forthe buildup of hydrological models that can assist in water management decisions. Moreover, by understanding the mechanisms of runoff generation in the Kaap River, the forecast of extreme events such as the recent devastating flood of March 2014 can be better predicted.

Sujets  : runoff processes ; river basins ; water management ; South Africa

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Page publiée le 29 décembre 2016, mise à jour le 12 novembre 2019