Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Bugan, R.D.H. Modelling and regulating hydrosalinity dynamics in the Sandspruit River
catchment (Western Cape). PhD dissertation, Stellenbosch University.
The presence and impacts of dryland salinity are increasingly become evident in the semi-arid
Western Cape. This may have serious consequences for a region which has already been
classified as water scarce. This dissertation is a first attempt at providing a methodology for
regulating the hydrosalinity dynamics in a catchment affected by dryland salinity, i.e. the
Sandspruit catchment, through the use of a distributed hydrological model. It documents the
entire hydrological modelling process, i.e. the progression from data collection to model
application. A review of previous work has revealed that salinisation is a result of land use
change from perennial indigenous deep rooted vegetation to annual shallow rooted cropping
systems. This has altered the water and salinity dynamics in the catchment resulting in the
mobilisation of stored salts and subsequently the salinisation of land and water resources. The
identification of dryland salinity mitigation measures requires thorough knowledge of the water
and salinity dynamics of the study area. A detailed water balance and conceptual flow model was
calculated and developed for the Sandspruit catchment. The annual streamflow and precipitation
ranged between 0.026 mm a-1 - 75.401 mm a-1 and 351 and 655 mm a-1 (averaging at 473 mm a-
1), respectively. Evapotranspiration was found to be the dominant component of the water
balance, as it comprises, on average, 94% of precipitation. Streamflow is interpreted to be driven
by quickflow, i.e. overland flow and interflow, with minimal contribution from groundwater.
Quantification of the catchment scale salinity fluxes indicated the Sandspruit catchment is in a
state of salt depletion, i.e. salt output exceeds salt input. The total salt input to and output from
the Sandspruit catchment ranged between 2 261 - 3 684 t Catchment-1 and 12 671 t a-1 - 21 409 t
a-1, respectively. Knowledge of the spatial distribution of salt storage is essential for identifying
target areas to implement mitigation measures. A correlation between the salinity of sediment
samples collected during borehole drilling and the groundwater EC (r2 = 0.75) allowed for the
point data of salt storage to be interpolated. Interpolated salt storage ranged between 3 t ha-1 and
674 t ha-1, exhibiting generally increasing storage with decreasing ground elevation. The
quantified water and salinity fluxes formed the basis for the application of the JAMS/J2000-NaCl
hydrological model in the Sandspruit catchment. The model was able to adequately simulate the
hydrology of the catchment, exhibiting a daily Nash-Sutcliffe Efficiency of 0.61. The simulated
and observed salt outputs exhibited discrepancies at daily scale but were comparable at an annual
scale. Recharge control, through the introduction of deep rooted perennial species, has been
identified as the dominant measure to mitigate the impacts of dryland salinity. The effect of various land use change scenarios on the catchment hydrosalinity balance was evaluated with the
JAMS/J2000-NaCl model. The simulated hydrosalinity balance exhibited sensitivity to land use
change, with rooting depth being the main factor, and the spatial distribution of vegetation. Revegetation
with Mixed forests, Evergreen forests and Range Brush were most effective in
reducing salt leaching, when the “salinity hotspots” were targeted for re-vegetation (Scenario 3).
This re-vegetation strategy resulted in an almost 50% reduction in catchment salt output. Overall,
the results of the scenario simulations provided evidence for the consideration of re-vegetation
strategies as a dryland salinity mitigation measure in the Sandspruit catchment. The importance
of a targeted approach was also highlighted, i.e. mitigation measures should be implemented in
areas which exhibit a high salt storage. / AFRIKAANSE OPSOMMING: Die teenwoordigheid en impak van droëland versouting word duideliker in die halfdor Wes-Kaap. Dit kan ernstige gevolge inhou vir die streek wat reeds as ‘n waterskaars area
geklassifiseer is. Hierdie verhandeling is ‘n poging om ‘n metode vir die regulering van
waterversoutingsdinamiek in ‘n opvangsgebied wat deur verbrakking van grond geaffekteer is,
i.e. die Sandspruit opvangsgebied, te bepaal deur gebruik te maak van ‘n verspreide hidrologiese
model. Dit dokumenteer die volledige hidrologiese modeleringsproses, i.e. vanaf die versameling
van data tot die aanwending van die model. ‘n Oorsig van vorige studies bevestig dat versouting
‘n gevolg is van die verandering vanaf meerjarige inheemse plantegroei met diep wortelstelsels
tot die verbouing van gewasse met vlak wortelstelsels. Dit het ‘n verandering in die water en
versoutingsdinamiek in die opvangsgebied tot gevolg gehad in soverre dat dit die mobilisering
van versamelde soute en gevolglike versouting van die grond en waterbronne tot gevolg gehad
het. Die identifikasie van maatreëls om droëland versouting te verminder, vereis ‘n deeglike
kennis van die water- en versoutingsdinamiek van die studie gebied. ‘n Gedetailleerde
waterbalans en konseptuele vloeimodel was bereken vir die Sandspruit opvangsgebied. Die
jaarlikse stroomvloei en neerslag varieer tussen 0.026 - 75.401 mm a-1 en 351 - 655 mm a-1
(gemiddeld 473 mm a-1), onderskeidelik. Dit is bevind dat evapotranspirasie die dominante
komponent is van die waterbalans, aangesien dit 94% uitmaak van die neerslag. Stroomvloei
word aangedryf deur snelvloei, i.e oppervlakvloei en deurvloei met minimale bydrae van
grondwater. Die omvang van die opvangsgebied se soutgehalte het aangedui dat die Sandspruit
opvangsgebied tans ‘n toestand van soutvermindering ondervind, i.e. sout invloei word
oorskrei deur sout uitvloei. Die totale sout in- en uitvloei in die Sandspruit opvangsgebied het
gewissel tussen 2 261 - 3 684 t Opvangsgebied-1 en 12 671 - 21 409 t a-1 onderskeidelik. Kennis
van die ruimtelike verspreiding van opbou van soute in die grond is belangrik om areas te
identifiseer vir die toepassing van voorsorgmaatreëls. ‘n Korrelasie tussen die soutinhoud van
sediment monsters wat versamel is tydens die boor van boorgate en die grondwater EC (r2 =
0.75) het die interpolasie van puntdata waar sout aansamel toegelaat. Hierdie interpolasie van
sout aansameling het gewissel tussen 3 t ha-1 and 674 t ha-1 en bewys ‘n algemeen verhoogde
opbou met vermindering in grond elevasie. Die hoeveelheidsbepaling van water en die
versoutings roetering vorm die basis vir die aanwending van die JAMS/J2000-NaCl hidrologiese
model in die Sandspruit opvangsgebied. Die model het ‘n geskikte simulasie van die hidrologie
van die opvangsgebied geimplimenteer, en het ‘n daaglikse Nash-Sutcliffe Efficiency van 0.61
getoon. Die gesimuleerde en waargenome sout afvoer het teenstrydighede getoon t.o.v daaglike
metings maar was verenigbaar op ‘n jaarlikse skaal. Aanvullingsbeheer deur die aanplanting
van meerjarige spesies met diep wortelstelsels is geidentifiseer as ‘n oorwegende maatreël om die
impak van verbrakking van grond teë te werk. Die effek van verskeie veranderde grondgebuike op die balans van die opvangsgebied se hidro-soutgehalte is geëvalueer met die JAMS/J2000-NaCl model. Die balans van gesimuleerde hidro-saliniteit het ‘n sensitiwiteit t.o.v veranderde
grondgebruik getoon, met die diepte van wortelstels as die hoof faktor, asook die ruimtelike
verspreiding van plantegroei. Hervestiging van verskeie tipes bome, meerjarige bome en “Range
Brush” was die effektiefste t.o.v die vermindering in sout uitloging waar die soutgraad
konsentrasie areas ge-oormerk was vir hervestiging van plantegroei (Scenario 3). Die strategie
van hervestinging het ‘n afname van 50% in versouting in die opvangsgebied getoon. In die
geheel het die resultate van die simulasies genoegsame bewys gelewer dat ‘n strategie van
hervestiging en groei as ‘n voorsorg maatreël kan dien om droëland versouting in die Sandspruit opvangsgebied teen te werk. Die belangrikeid daarvan om ‘n geteikende benadering te volg is
benadruk, i.e. voorsorg maatreëls kan toegepas word in areas met hoë soutgehalte.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86492 |
| Date | 04 1900 |
| Creators | Bugan, Richard D. H. |
| Contributors | De Clercq, W. P., Jovanovic, N., 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 |
| Type | Thesis |
| Format | 216 p. : ill. |
| Rights | Stellenbosch University |
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