Food production in semi-arid areas principally depends on the availability of water. Consequently,
improving rainwater productivity and modifying the available energy for unproductive water
losses is an important and necessary step towards promoting rainfed agriculture in dryland
farming. It has been convincingly argued that water management strategies on rainfed semi-arid
areas, including in-field rainwater harvesting (IRWH) deserve considerable attention. However,
integrated studies of water and energy balance on the IRWH technique in particular in optimizing
runoff to basin area ratio and mulching levels (ML) was not comprehensively appraised.
Therefore, in this thesis, the two main research questions concern: (i) what is the optimal runoff
to basin area ratio to sustain maize crop yield? and (ii) how do the microclimatic conditions
change under wide and narrow runoff strip length (RSL)?
Field experiments were conducted (2007/08 and 2008/9) on the Kenilworth Bainsvlei ecotope,
associated with high evaporative demand of 2294 mm per annum and relatively low and erratic
rainfall (528 155.6 mm). Topographically the area had a gentle slope (< 1%) with reddish
brown in colour (Amalia family) a fine sandy loam texture soil, thus was classified as a Bainsvlei
form. The soil is regarded as very suitable for dryland agriculture, because it is deep (2000 mm)
and drains freely in the top and the upper sub-soil. So the study was performed by quantifying and
evaluating the soil-crop-atmosphere parameters. In the first part of the thesis, the soil water
balance components and different efficiency parameters were assessed. In the second part of the
thesis, the micrometeorological variable profiles within and above the maize canopy for the heat
and water vapour exchange processes were characterized. Furthermore, comparison of available energy for evapotranspiration (ET) was evaluated for both wide and narrow runoff strips through
the quantification of energy balance components.
A multiple regression model was developed to predict in-field runoff by combining the effects of
rainfall event characteristics and surface treatments. From the results of runoff-rainfall (RR) ratio
a lower efficiency was observed from full mulch covered wide runoff strip length (RSL-3) i.e.
only about 4% of the rainfall, while the highest mean RR was about 27% from bare, narrow RSL-
1. From the estimation of rainfall canopy interception (RCI) it was revealed that the highest
interception was in the range of 4.5% to 9.0% of the precipitation. The RCI capacity of a maize
field under IRWH reached a plateau at about 0.5 â 0.6 mm for narrow RSL and 1.0 â 1.1 mm for
wide that would be evaporated eventually from the canopy. Furthermore the cumulative Es
(ΣEs) was evaluated as influenced by both mulch (âdry-mulchâ) and shading (âgreen-mulchâ)
effects. Thus, the proportion of water loss by Es from seasonal rainfall is about 62%, 64% and
66% in the bare treatments and as low as 28%, 30% and 32% for full mulch cover treatments
under full shade, (FC), partial canopy shaded (PC) and unshaded (UC) respectively. This implies
that, reduction of runoff and evaporation losses through surface treatments can promote improved
water use efficiency, of the stored available water in the root zone and thus, enhance yield. The
final grain yield decreased slightly as an order of increasing the length of the runoff strip. The
performance of the harvest index (HI) was slightly variable among the treatments due to more
water for yield being collected from bare plots than mulch covered plots. The higher mulch
conserves much water by suppressing the soil evaporation. In expressing grain yield per unit ET
(WUEET) and transpiration, Ev (WPEv) the RSL-2 m and RSL-1.5 m at lower mulch cover showed
significant higher values than RSL-1 and RSL-3 treatments. However, the transpiring water for
yield and unproductive evaporation losses more under IRWH should be evaluated in terms of
micrometeorological profile characterization and available energy.
With regard to micrometeorological variables, the growth stage had a strong effect on the vertical
profiles of climatic variables. In wide runoff strips lapse conditions extended from lowest
measurement level (LP) to the upper middle section (MU) of the canopy and inversion was
apparent at the top layer (UP) of the canopy. The reason for the extension of temperature
inversion into the upper part of the wide RSL canopy was as a result of higher air movements compared to narrow strips. From this result it was confirmed that the effect of wind on water
vapour removal decreased downward from wind flow within the canopy. This had an influence on
the resistance of the boundary layer and canopy and soil surface resistance. This is a clear
indication that wide strips supply more drying power to respond to evaporative demand of the
atmosphere compared to narrow strips. From the measurement of profiles within and above the
canopy, it was suggested that, the presence of local advection in the wide runoff strips of IRWH
could be a common phenomenon causing variations in water vapour removal under the
heterogeneous nature of IRWH tillage system. Thus, profile characteristics within and above a
plant canopy are playing a great role in determining the vapour pressure deficit and consequently,
can explain the ET rate. Therefore based on micrometeorological measurements, results indicated
that the latent heat (LE) was dominant and higher in wide compared to narrow runoff strips (RSL)
under both dry and wet conditions. However, sensible heat (Hs) showed lower values on wide
runoff strips during wet conditions due to the advective effect of the runoff area. Thus, the wide
runoff strip with a higher basin leaf area ratio (BLAR) of 2.43 had higher ET and used more
energy in evaporating water than the narrow runoff with a lower BLAR of 1.42. Wide runoff
strips converted the higher available energy more efficiently into a higher biomass production.
During wet days, the wide RSL used more than 70% of the available energy for
evapotranspiration, while the narrow RSL response to the available energy (63%) was stronger
during dry compared to wet days. In general the wide and narrow RSL used the available water
and energy differently during dry and wet conditions under IRWH system.
From this experiment finding, important implications were described such as better yield obtained
from narrow RSL-1, however RSL-1.5 and 2 m with minimum mulch cover gave higher water
productivity compared to narrow RSL-1 and wide RSL-3. On the other hand when quantifying
and evaluating the cause behind the effect of available energy, the wide RSL converted available
energy more efficiently into higher biomass production than the narrow RSL. Therefore, this
challenge should be addressed on the basis of an integrated approach to water and energy
resources in order to develop comprehensive management strategies. Furthermore, for improved
rainwater use management strategies, it is recommended to link an integrated approach of water
and energy resources with crop growth simulation models. The application of the crop models could be important by incorporating a range of planting dates and densities along with the
selection of surface treatment management strategies
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-09172013-092518 |
| Date | 17 September 2013 |
| Creators | Tesfuhney, Weldemichael Abraha |
| Contributors | Prof LD van Rensburg, Prof S Walker |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-09172013-092518/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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