The biggest challenge in semi-arid areas is finding ways of reducing the major unproductive
water loss: evaporation from the soil surface. A large number of subsistence farmers east of
Bloemfontein, in and around ThabaâNchu in the Free State Province of South Africa occupy
about 11 000 ha of land. The economic potential of this communal land still needs to be
unlocked and the natural resource base is critical for this endeavour. However, the prevalence
of clay and duplex soils is a major constrain towards improving food security in this area. Poor
soil water regimes resulting from prolific runoff and evaporation losses is one of the reasons
especially when conventional tillage is used. It was therefore hypothesized that by quantifying
soil surfaces evaporation (Es); characterizing of the soil hydraulic properties and understanding
the effect of temperature on mulch type and coverage of the Bonheim (Bo) soil can contribute
to the improvement of the infield rainwater harvesting (IRWH) system and fill a gap in
knowledge under South African conditions that is in terms of promoting water storage capacity
and minimizing Es for better crop yields.
The ECH2O-TE probes used in this study were calibrated to measure soil water content () and
temperature (T). The evaporative desorption procedure (EDP) of Van der Westhuizen (2009)
for coir was modified to calibrate probes in undisturbed soils. The probes were evaluated
against measured volumetric soil water content (mm mm-1) on their accuracy, precision and
repeatability to measure soil water content in the 26oC treatment (Chapter 2). Most of the
laboratory derived equations had RMSE close to zero, on average at 0.003 mm mm-1 and
precision (R2) ranged between 93 and 99% and accuracies up to 96%. These probes were found
to be sensitive to soil temperature changes in the measurement of water content. Under wet to
dry soil conditions about 48, 62 and 34% errors were obtained for the A, B and C-horizons,
respectively and therefore temperature compensated equations had to be developed in Chapter 3. Temperature compensated equations predicted soil water content measurements with an
accuracy, precision and repeatability at 99, 99 and 95%, respectively. Manufacturerâs generic
equation tended to over predict soil water content measurements and lacked accuracy with
errors ±40% and repeatability.
Chapter 4 investigated how mulch type and percentage cover influenced temperature above and
below the soil surface. First: results indicated that mulch did not influence air temperature at an
elevation of 160 mm above the soil surface. Secondly: percentage coverage affected soil
temperature up to 450 mm, and thirdly: the 100% reed mulch cover treatment was
recommended for farmers in order to minimise evaporation especially under semi-arid
conditions where normally the evaporative demand exceeds supply.
Chapter 5 on the other hand profiled and characterized the hydraulic properties of the Bo soil
for the A, B and C-horizons. Soil pores were separated into structural and textural pore classes
for each of the horizons that were identified for the three master horizon of the Bonheim soil
using a method first used in this study known as the âin situ internal drainageâ (ISID) method.
The drained upper limit (DUL) for each horizon was determined using the ISID method and
were found to be associated with micro pore class. The structural pores of the three horizons
were found to be associated with low suctions and that they allowed water to flow at rates
between 1-20 mm hr-1. The transitional pore class (Meso pores) conducted water at rates
between 3-12 mm hr-1 and micro pores between 3-10 mm hr-1.
Five methods were used to estimate evaporation (Es) during three Es drying cycles (Chapter 6)
and these estimations were compared to a weighing lysimeter [Es(lys)] measurements in order
to evaluate their accuracy in the measurement of Es, using Willmot test statistics for paired values. The field hydraulic method had a good performance with an average D-index value of
0.60 in all the three drying cycles selected and thus estimated Es closer to Es(lys) hence it was
recommended for use in estimating Es for Bo soils.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-10172011-153301 |
| Date | 17 October 2011 |
| Creators | Nhalbatsi, Nhlonipho Nhlanhla |
| 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-10172011-153301/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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