5 research outputs found
Understanding runoff processes in a semi-arid environment through isotope and hydrochemical hydrograph separations
The understanding of runoff generation mechanisms is crucial for the sustainable management of river basins such as the allocation ofwater 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 ofrunoffcomponents 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, runoffcomponents 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
Understanding runoff processes in a semi-arid environment through isotope and hydrochemical hydrograph separations
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
Drivers of spatial and temporal variability of streamflow in the Incomati River basin
The Incomati is a semi-arid trans-boundary river basin in southern Africa, with a high variability of streamflow and competing water demands from irrigated agriculture, energy, forestry and industries. These sectors compete with environmental flows and basic human water needs, resulting in a "stressed" water resource system. The impacts of these demands, relative to the natural flow regime, appear significant. However, despite being a relatively well-gauged basin in South Africa, the natural flow regime and its spatial and temporal variability are poorly understood and remain poorly described, resulting in a limited knowledge base for water resource planning and management decisions. Thus, there is an opportunity to improve water management, if it can be underpinned by a better scientific understanding of the drivers of streamflow availability and variability in the catchment.
In this study, long-term rainfall and streamflow records were analysed. Statistical analysis, using annual anomalies, was conducted on 20 rainfall stations, for the period 1950–2011. The Spearman test was used to identify trends in the records on annual and monthly timescales. The variability of rainfall across the basin was confirmed to be high, both intra- and inter-annually. The statistical analysis of rainfall data revealed no significant trend of increase or decrease. Observed flow data from 33 gauges were screened and analysed, using the Indicators of Hydrologic Alteration (IHA) approach. Temporal variability was high, with the coefficient of variation of annual flows in the range of 1 to 3.6. Significant declining trends in October flows, and low flow indicators, were also identified at most gauging stations of the Komati and Crocodile sub-catchments; however, no trends were evident in the other parameters, including high flows. The trends were mapped using GIS and were compared with historical and current land use. These results suggest that land use and flow regulation are larger drivers of temporal changes in streamflow than climatic forces. Indeed, over the past 40 years, the areas under commercial forestry and irrigated agriculture have increased over 4 times