A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements of the degree of Doctor of Philosophy.
Johannesburg, February 2017 / Rainwater harvesting is used as a way of improving crop yields in rain fed agriculture by
capturing excess rainfall and storing it in-situ or in reservoirs for use during dry spells.
Contour ridges are one of the many rainwater harvesting technologies that are used
although little is known about their effectiveness. Contour ridges harvest runoff generated
in the cropped field upstream of the ridges.
The traditional contour ridge type in Zimbabwe was introduced by the government in the
1950s to control soil erosion through safely draining away runoff from cropped fields and
is commonly referred to as graded contour (GC) ridges. In the 1990s the country
experienced severe and more frequent droughts leading stakeholders to experiment on
contour ridges that retain the runoff instead of draining it away which are known as dead
level contour (DLC) ridges. There was therefore the need to find out if there are benefits
derived from this change and assess conditions under which benefits would be
experienced. Previous studies have shown that rainwater harvested by contour ridges
can improve water availability in downstream fields. However these studies did not
investigate the conditions under which such benefits are realised. In addition no attempt
to model water harvesting by contour ridges have been made in Zimbabwe while the
contour ridges are widely being used for soil and water conservation. This research
investigated the effect of contour ridges by comparing soil moisture between plots with
DLC and GC ridges using plots with no contours as a control.
Experimental work was carried out in Zhulube, in Matebeleland South Province of
Zimbabwe. Matebeleland South Province falls within the semi-arid area in which rainfall
is characterised by mid-season dry spells leading to frequent crop failure. In addition, the
area often receives high rainfall intensities leading to soil erosion and sedimentation of
rivers. DLC and GC ridges were constructed in farmers’ fields where maize crops were
planted. Soil moisture measurements were done using a micro gopher soil moisture
profiler while runoff plots were used to measure runoff generation. A fuzzy model was
developed using data from this experiment and a previous study in Masvingo Province of
Zimbabwe to simulate runoff generation at field scale while a process based water
balance model was also developed to simulate soil moisture changes within the root zone
of the cropped area.
The results from this study indicate that DLC are effective in clay and loamy soils where
runoff generation is significant and not in sandy soils due to insignificant generation of
runoff under the rainfall regimes of semi-arid areas. Fuzzy logic was found to be a useful
method of incorporating uncertainty in modelling runoff at field scale. A mass water
balance model developed on process based principles was able to model soil moisture
in the root zone reasonably well (NSE =0.55 to 0.66 and PBIAS=-1.3% to 6.1%) and could
help to predict the water dynamics in contour ridged areas as would be required in
determining the suitable dimensions and spacing of contour ridges. Further research is
required to improve the fuzzy component of the model for estimation of runoff when more
data becomes available. In addition experiments to validate methods of estimating macro
pore fluxes and lateral transfer of water from the contour ridge channel to the downslope
field are also recommended. The model structure can be improved by adopting the
representative elementary watershed approaches to include momentum and energy
balances in addition to mass balance that was used in this study. / MT2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/22996 |
| Date | January 2017 |
| Creators | Mhizha, Alexander |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (1 volume (various pagings)), application/pdf |
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