Spatial and temporal variations of water and nutrient fluxes within a steep-sloped agricultural catchment.
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Date
2012
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Abstract
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<r<-0.77). Water fluxes were found to increase
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from the microplot scale (208 l/m2) to the runoff plot scale (350 l/m2, delivery ratio of 1.68).
The scale ratios calculated for the period of 2010-2011 show that there was a steady decrease
in the delivery of water from the hillslope scale to the catchment scale. Cumulative water
fluxes were found to be 316 l/m2 at the 23 ha catchment and 284 l/m2 at the 100 ha catchment
(delivery ratios of 0.90 and 0.89 respectively). Water fluxes decreased sharply to 198 l/m2 at
the 1000 ha catchment outlets (delivery ratio of 0.70). Runoff at the 23 ha catchment outlet
was sourced from the mixing of GW (average of 63%), OF (22%) and SW (15%.) At the 100
ha outlet, GW contributions decreased to 50%, while OF contributions remained constant at
22% and SW contributions increased to 28%. The main contributor at the 1000 ha catchment
was GW (55%) followed by SW (37%) and OF (8%). During the strongest rainfall event of
the study period, OF contributed 97% to total runoff at the 23 ha catchment outlet, whilst at
the 100 ha catchment, OF and SW both contributed 50% each. Groundwater in all cases was
the major contributor to runoff at the 1000 ha catchment outlet. Both dissolved organic
Carbon (DOC) and particulate organic Carbon (POC) increased from the microplot (8.44 and
25.51 g/m2 for DOC and POC) to the plot scale (14.92 and 26.91 g/m2). Lower yields
occurred at the 23 ha catchment than on the hillslope (5.03 g/m2 and 8.18 g/m2). From the 23
and 100 ha catchment outlets, POC sharply decreased to 0.06 g/m2, while DOC increased
considerably to 9.58 g/m2. This pointed to the decomposition of POC, which not only releases
CO2 to the atmosphere but also adds DOC to runoff. At the 1000 ha catchment, POC yields
were minimal due to a lack of eroded sediments whilst DOC decreased slightly (6.42 g/m2).
These results yield a better understanding of the processes of water, nutrient and Carbon
movements within landscapes.
A further understanding of the processes leading to changes of nutrient and carbon fluxes
needs to be performed in order to link this study with the overall ecosystem functioning of a
landscape.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.
Keywords
Water hydrology--KwaZulu-Natal--Bergville., Sedimentation and deposition., Watersheds--KwaZulu-Natal--Bergville., Runoff--KwaZulu-Natal--Bergville., Theses--Bioresources engineering and environmental hydrology.