Investigating the suitability of land type information for hydrological modelling in the mountain regions of Hessequa, South Africa

Malan, Gert Jacobus (2016-12)

Thesis (MSc)--Stellenbosch University, 2016.


ENGLISH ABSTRACT: The Land Type database of South Africa combines soil associations with various terrain positions within a larger Land Type polygon. The Land Type structure provides the opportunity to unlock the terrain unit information through segmenting the larger Land Type polygon into terrain units. Geographical information systems have the capability to dissect the landscape into terrain morphological units, using remote sensing technology. There is a range of methods and software available that can be used to dissect the landscape, the challenge is to identify a method that would be compatible with Land Type terrain units. The study area is the catchment of the Korentepoort dam, north of Riversdale in the Hessequa district of the Western Cape. The Hessequa region is regularly struck with drought which leads to an investigation into the water security of the region. The investigation includes the development of a hydrological model for the Korentepoort Dam and bordering catchments. Physically based hydrological models require detailed soil distribution maps with soil physical data. The physical characteristics are used to calculate the amount of surface runoff, drainage and streamflow. Hydrologists use the Land Type information to supply soil character for modelling purposes. The most common soil type from the Land Type memoir is selected to represent the whole Land Type polygon. This representation varies depending on the homogeneity of soils within the landscape, but can be as little as 20%. The segmentation method is evaluated within the Korentepoort catchment by field observations of the terrain at 190 points in the landscape. This point data is compared to the segmentation map with a different range of acceptable error. The segmentation method is constructed on a 90-meter digital elevation model, which was refined to a 30 meter. The highest acceptable error was selected as 30 meters. At this error, the terrain map was able to predict 77% of the field observation points. Transects were created from the terrain map, which also indicates a good fit with terrain units. The Land Type information in the catchment was found to be conflicting with field observations and thus updated. The updated Land Type information was used to populate the segmented terrain map. The high resolution of the terrain map was found to be too complex for the hydrological model. A well-used method of soil type aggregation on the basis of hydrology was applied to the updated Land Types. The method divides the soil types into three hydrological response units and was found to be accurate on 10 out of 13 selected profiles. These profiles are selected as modal profiles and represent the soil types of their respective terrain units. This research made it possible to dissect the landscape into units comparable with those in the Land Type database. This increases the resolution of the Land Type information and could possibly be applied to the whole of South Africa. Methods are suggested in which these terrain maps can be aggregated in a meaningful manner which would enhance its applicability for hydrological modelling.

AFRIKAANS OPSOMMING: Die Land Tipe databasis van Suid Afrika groepeer grond tipes in assosiasies op verskillende terrein eenhede binne ʼn groter Land Tipe blok. Die Land Tipe inligting bied die geleentheid om hierdie terrein eenheid inligting te ontsluit deur die groter Land Tipe blok op te breek in verskillende terrein eenhede. Geografiese inligting stelsels het die potensiaal om deur middel van afstandswaarnemings tegnologie, ʼn landskap te verdeel in terrein morfologiese eenhede. Daar is wel ʼn verskeidenheid sagteware en metodes wat gebruik kan word om ʼn landskap te segmenteer, die uitdaging is om ʼn metode te identifiseer wat die landskap verdeel in eenhede wat ooreenstem met die in die Land Tipes. Die studie area is die Korentepoort Dam opvangsgebied, noord van Riversdal in die Hessequa distrik van die Weskaap. Die Hessequa distrik word gereeld deur droogtes geraak wat daartoe gelei het dat ʼn ondersoek geloots is om die water sekuriteit van die gebied te ondersoek. Die ondersoek sluit in die ontwikkeling van ʼn hidrologiese model vir die Korentepoort Dam en nabye opvangsgebiede. Fisies gebaseerde hidrologiese modelle benodig gedetailleerde grond distribusie kaarte waaraan grond fisiese eienskappe gekoppel is. Hierdie fisiese eienskappe word gebruik deur die model om oppervlak afloop, dreinering en stroom vloei te bereken. Hidroloë maak gebruik van die Land Tipe databasis om grond inligting te bekom en dit in die model te gebruik. Die grond tipe wat die messte voorkom in ʼn Land Tipe blok word geselekteer om die hele blok te verteenwoordig. Die persentasie voorkoms kan varieer afhangende die homogeniteit van die gronde in die landskap, maar kan so laag as 20% wees. Die segmentasie metode is geëvalueer binne die Korentepoort opvangsgebied deur terrein observasies te maak en dit te koppel aan punt data. Die punt data is vergelyk met die segmentasie kaart met inagneming van sekere faktore wat variasie kan veroorsaak. Die segmentasie metode is gebaseer op ʼn 90 meter digitale terrein model, wat verfyn is tot ʼn 30 meter. ʼn Aanvaarbare variasie van 30 meter is daarom geselekteer, waar die terrein kaart 77% van die observasie punte verteenwoordig het. Terrein deursneë is vergelyk met die terrein eenhede van die morfologie kaart wat visueel aanvaarbaar pas. Die Land Tipe inligting in die Korentepoort opvangsgebied het afgewyk van die veld waarnemings en is opgedateer. Die opgedateerde Land Tipe inligting is gebruik om die terrein morfologie kaarte te vul met grond inligting. Hierdie hoë resolusie kaart was te besig vir die hidrologiese model wat gelei het na samevoeging van sekere grond tipes. Hierdie samevoegings metode kombineer grond tipes teen opsigte van modale profiele wat die gronde beste voorstel. Die metode het samevoeging van blokke bewerkstellig en nogtans 10 uit 13 profiele in die opvangsgebied korrek verteenwoordig. Die navorsing maak dit moontlik om die landskap in segmente in te deel wat vergelykbaar is met die Land Tipe terrein eenhede, wat die algehele resolusie van die Land Tipe inligting verbeter. Daarby is metodes voorgestel om hierdie inligting op ʼn sinvolle manier te groepeer wat dit ideaal maak vir hidrologiese modulering.

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