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Environmental Hydrology

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Browsing Environmental Hydrology by Author "Chaplot, Vincent A. M."

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    Erosion dynamics at the catchment level : spatial and temporal variations of sediment mobilization, storage and delivery.
    (2011) Oakes, Ernest Gene Martin.; Chaplot, Vincent A. M.; Lorentz, Simon Antony.; Titshall, Louis William.
    Soil material exported from river catchments by soil erosion is a key issue in environmental sustainability. Although soil erosion processes have been thoroughly investigated, their dynamics, specifically the continuity of erosion processes and sediment source locality, are less studied. The aim of this investigation was to evaluate the changes in the fluxes and characteristics of sediments during their downslope and downstream transport. The study was conducted in a 1000 ha catchment of the Drakensberg foothills, South Africa. Sediment fluxes were monitored at nested scales during the 2009-2011 rainy seasons using 1×1m and 2×5 m erosion plots and H-flumes coupled to automatic samplers from 23 ha, 100 ha catchments. In addition, soil texture, colour and total organic carbon and nitrogen contents in sediments exported from the nested scales and a 1000 ha catchment were compared to in-situ surface and sub-surface soil horizons in a 23 ha catchment river bank and hillslope soils and fluvial sediments. There was a sharp increase of sediment fluxes with increasing slope length (846±201 gm-1y-1 for 1 m2 vs 6820±1714 gm-1y-1 for 10 m2), revealing a limited contribution of splash erosion compared to rain-impacted flow erosion. Sediment fluxes decreased to 500±100 gm-1y-1 and 100±10 gm-1y-1 at the 23 ha and 100 ha catchments respectively, indicating the occurrence of sedimentation during sediment downslope and downstream transport. A principal component analysis (PCA) suggested that rain impacted flow erosion efficiency at the 10 m2 scale was significantly correlated with soil bulk density, clay content and antecedent rainfall (P<0.05). Moreover, strong correlations existed between runoff, sediment concentration and soil loss and selected soil surface and environmental variables at the plot scales. Correlations became weaker at the catchment scales due to increasing landscape heterogeneity and the complexity of soil erosion dynamics. An additional PCA suggested that stream bank erosion contributed to 63% of the soil loss from the 23 ha catchment. During their downstream transport, sediments were discriminated by the second PCA axis, which correlated with the clay and fine silt content, 100 ha sediments showed negative coordinates to this axis while 1000 ha catchment sediment had positive coordinates.
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    Spatial and temporal variations of water and nutrient fluxes within a steep-sloped agricultural catchment.
    (2012) Orchard, C. M.; Chaplot, Vincent A. M.; Lorentz, Simon Antony.
    A proper understanding of the spatial and temporal variations of runoff and nutrient fluxes are critical in understanding catchment hydrology. Runoff and nutrient fluxes may exhibit large variations both spatially and temporally, but this issue has largely been overlooked in the existing literature. The present study intends to respond to two main research objectives: (a) improve the understanding of the spatial and temporal variations (i.e. the dynamics) of overland flow (OF) and its factors of control and (b) quantify the evolution of runoff, nutrient and sediment fluxes from hillslope crest to catchment outlet. The research study was undertaken in a 1000 ha agricultural catchment of the Drakensberg foothills in the Bergville District, KwaZulu-Natal, South Africa under rangeland, small scale agriculture and commercial agriculture. The first objective was to evaluate the dynamics of OF during four rainfall seasons (2007 to 2011) by using 1×1m² microplots (n=15) located at five landscape positions within the rangeland upper part of the catchment. Automatic tipping buckets linked to a datalogger were used to estimate the delay between the start of the rain and the start of OF, which corresponded to the time of runoff initiation (TRI). Multivariate analysis was applied to the OF data and the information on selected environmental factors (rainfall characteristics, selected soil physical properties, soil water content and soil surface conditions). Nested scales of 1 and 10 m2 plots, and 23, 100 and 1000 ha catchments equipped with buckets for plots and conventional H-flumes for catchments, were used to quantify the downstream evolution of water and nutrient (C, NO3 - and P) fluxes. The fluxes were compared with data from the shallow and deep groundwater (GW) collected from piezometers and boreholes, respectively. This allowed for the determination of the mixing sources at the three catchment outlets, using stable isotopes of water (to differentiate between old and new water) and silica concentrations to identify soil water (SW) contributions. The average OF rate varied 2.3-fold across the Potshini Catchment (from 15% footslope to 35% backslope), while the average TRI varied by a 10.6-fold factor (between 0.6 minutes in the bottomland and 6.4 minutes at the footslope position). TRI temporal variations correlated the most with the duration of rainfall (Pearson r coefficient of 0.8) and the cumulative amount of rainfall after the onset of the rainy season (r=-0.47), while TRI spatial variations were significantly controlled by soil crusting (-0.97