Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Gully erosion is a major environmental problem not only having direct influences on site but also
indirect influences felt further down the catchment. Combating gully erosion has proven to be elusive
due to the difficulty in finding the causal factors and developing mechanisms involved. Soil is the
medium in which gully erosion occurs yet few research have investigated it as a driving factor behind
gully development and those that have has done it in a very elementary way.
The first aim of this project was to physically and chemically characterise and classify the
discontinuous gully system at Malansdam to establish the relationship between landscape hydrology
and geomorphologic gully development with a focus on control factors. This was done by field
observations, physical measurements and spatial and hydrological analyses with a Geographical
Information System (GIS). The Malansdam gully system was the first ever recorded Strahler stream
order (SSO) 5 classical gully system with the most active region being in the upper reaches where a
steeper slope is experienced. Although piping was observed the V-shape channels and SAR data from
traditional wet analysis indicated runoff to be the dominant formation process. A duo of factors,
consisting of one anthropogenic and one natural factor respectively, was found that the major control
factors behind the gully formation. A unique anthropogenic factor that has never been published
beforehand was found to be the anthropogenic driving factor namely the ploughed contour cultivation
technique employed by the farmers in the Sandspruit catchment. The ploughed contours act as
channels firstly collecting and secondly moving water that would have drained naturally downwards
in the valley to one exit point in the gully system. This allows increased erosive energy because of the
larger volumes of water entering one single point in the upper reaches of the gully system where a
steeper slope is experienced. The driving factor in the natural group was determined to be weak soil
structure due to an abundant amount of exchangeable Mg2+ cations occupying the exchange sites on
the clay fraction. This would cause soil to disperse in the presence of water even with a low amount of
exchangeable Na+. Combat methods would accordingly exist in the form of rectifying the soil
structure and finding an alternative to the ploughed contour system currently employed, but also
planting vegetation especially grass or wheat in the gully channels. The second aim of this project was to determine the capability of Near Infrared (NIR) spectrometry,
with wavenumbers 12 500 – 4 000 cm-1, to predict indicators used in soil science to establish the
dispersive nature of a soil. These indices included the Exchangeable Sodium Percentage (ESP),
Sodium Absorption Ratio (SAR), Magnesium Saturation Percentage (MS%), Electrical Conductivity
(EC), Potential Hydrogen (pH) as well as the four main exchangeable cations namely calcium (Ca2+),
potassium (K+), sodium (Na+) and magnesium (Mg2+). Surface and subsurface soil samples were
collected from active gully heads. These samples were minimally pre-processed thus only dried,
milled and sieved. Thereafter it was subject to NIR analysis making use of the Bruker multi-purpose FT-NIR Analyser (MPA; Bruker Optik GmbH, Germany) with a spectral range of 12 500cm-1 to
4000cm-1 which is. Partial Least Square Regression (PLSR) models were built for each index and the
exchangeable cations making use of QUANT 2 utility of OPUS 6.5 (MPA; Bruker Optik GmbH,
Germany) software. Five different regrssion statistics namely the coefficient of determination (r2),
Root Mean Square Error of Cross Validation (RMSECV), Ratio of Performance to Deviation (RPD),
Bias and the Ratio of Performance of Quartiles (RPIQ) were used to assess the legitimacy of each
PLSR model. Upon validation all the PLSR models performed in line with previously published work
and in certain cases better. The only exception was MS% which would require further investigation.
NIR thus possess the capability to predict a soil’s dispersive nature in a fast, reliable, inexpensive and
non- destructive way, thus implying whether or not it contributes to gully erosion at a significant level
or only minimally. / AFRIKAANSE OPSOMMING: Donga erosie is 'n groot omgewingsprobleem. Dit het nie net ‘n direkte invloed op die area waar dit
geleë is nie, maar het ook ‘n indirekte invloed wat elders in die opvangsgebied ervaar word.
Bekamping van donga erosie is moeisaam aangesien die faktore wat aanleiding gee tot die vorming en
dryf daarvan moeilik is om te bepaal. Grond is die medium waarin erosie plaasvind, maar daar is
nogtans steeds min navorsing wat grond ondersoek het as ‘n moontlike faktor aanleiding gee tot
donga erosie. Die wat dit al wel ingesluit het, het dit slegs op n baie elemntêre manier ondersoek.
Die eerste doel van hierdie projek was om die diskontinue donga stelsel fisies en chemise te
karakteriseer en klassifiseer om soedoende die verhouding tussen die landskap hidrologie en
geomorfologiese donga ontwikkeling te bepaal met n fokus op die faktore wat dit dryf. Dit was
gedoen deur middel van observasies gedoen terwyl veldwerk uitgerig was, fisiese metings asook
ruimtelike en hidrologiese analises deur gebruik te maak van n Geografiese Inligting Stelsel (GIS).
Die klassieke Malansdam donga stelsel is ‘n Strahler stroomorde (SSO) van 5 toegeken en is die
eerste een ooit wat dit behaal het. Die mees aktiefste area was in die bolope waar die steilste helling
ervaar was. Alhoewel ondergrondse pyp formasie waargeneem was het die V-vormige donga kanale
en SAR data van die tradisionele nat analise aangedui dat afloop die dominante vorming proses was.
Daar was gevind dat 'n duo van faktore, wat bestaan uit een menslike en een natuurlike faktor
onderskeidelik, die faktore was wat donga ontwikkeling in die area dryf. 'n Unieke menslike faktor
wat nog nie vantevore gepubliseer is, was bevind as die menslike faktor wat aanleiding gee tot donga
erosie. Hierdie faktor is die bewerkiingsmetode wat in die Sandspruit opvangsgebied gebruik word
naamlik geploegde kontoerbewerking. Die geploegde kontoere tree op as kanale om eerstens water te
versamel en tweedens om die vloeirigting daarvan te wysig. Water wat onder natuurlike toestande
afwaarts sou dreineer tot in die vallei word vasgevang deur die kontoere en gekanaliseer na een
invloei punt in die donga. Hierdie proses verhoog die erosiekrag van die water aangesien groter
volumes by 'n enkele punt in die steiler bolope van die donga stelsel invloei. Die dryf faktor in die
natuurlike groep was swak grond struktuur. Die oorsaak hiervan was die besetting van ‘n grootmaat
uitruilbare Mg2+ katione op die uitruil plekke van die kleifraksie. Dit sou veroorsaak dat grond in die
teenwoordigheid van water maklik sou dispergeer, selfs in die teenwoordigheid van 'n lae hoeveelheid uitruilbare Na+ katione. Metodes om donga erosie te bekamp sal dienooreenkomstig bestaan uit die
herstel van die grondstruktuur en die toepassing van 'n alternatiewe gondbewerkings stelsel. Die
aanplanting van plantegroei, veral gras en koring binne die donga kanale sal verder help met die veg
tot bekamping
Die tweede doel van hierdie projek was om te bepaal indien naby infrarooi (NIR) spektrometrie (met
golfnommer van 12 500 – 4 000cm-1) oor die vermoë beskik om aanwysers wat traditioneel in
grondkunde gebruik word om die dispergering van grond te meet te voorspel. Hierdie aanwysers sluit vyf indekse in naamlik die Veranderlike Natrium Persentasie (ESP), Natrium Absorpsie Verhouding
(SAR), Magnesium Versadiging Persentasie (MS%), Elektriese Geleidingsvermoë (EC) en die
Potensiële Waterstof (pH) sowel as die vier hoof uitruilbare katione naamlik kalsium (Ca2+) , kalium
(K+), natrium (Na+) en magnesium (Mg2+). Oppervlak en ondergrondse grondmonsters is ingesamel
by die punt van oorsprong by aktiewe dongas. Hierdie monsters is minimaal voorberei, dus slegs
gedroog, gemaal en gesif. Daarna was dit onderworpe aan die NIR analise. Die Bruker meerdoelige
FT-NIR Analiseerder (MPA; Bruker Optik GmbH, Duitsland) met 'n spektrale omvang van 12 500cm
1 4000cm-1 is hiervoor gebruik. Parsiële kleinste kwadraat regressie (PLSR) modelle is gebou vir
elke indeks asook die uitruilbare katione deur gebruik te maak van die nutsprogram Quant 2 van die
OPUS 6.5 (MPA; Bruker Optik GmbH, Duitsland) sagteware. Vyf verskillende regressie statistieke
naamlik die bepalingskoëffisiënt (r2), vierkantswortel fout tydens kruis validasie (RMSECV),
verhouding van prestasie teenoor voorspellingsafwyking (RPD), sydigheid en die verhouding van
prestasie van kwartiele (RPIQ) was gebruik om die geldighied van elke PLSR model te asseseer. Alle
PLSR modelle het goed presteer, behalwe vir MS% wat verdere navorsing vereis. NIR beskik dus oor
die vermoë om die aard van dispergering van grond te bepaal op 'n vinnige, betroubare, goedkoop en
nie afbrekende manier. Dit kan dus effektief aangewend word as ‘n substitusie vir die traditionele
metodes om te bepaal as grond a beduidende faktor is of nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/85794 |
| Date | 12 1900 |
| Creators | Olivier, George |
| Contributors | De Clercq, W. P., Schloms, B. H. A., Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Format | xv, 135 p. : ill. |
| Rights | Stellenbosch University |
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