Browsing by Author "Orchard, C. M."

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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