Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The study of climate change and its effect on the eutrophication of surface waters is a current and critically important study for the well-being of the entire planet. Within the same emission scenario various probable climate change models outcomes are possible that affect the water quality of a body of water. Voëlvlei is an off-channel dam that supplies water to the city of Cape Town in the Western Cape Province of South Africa. Historically, it is a eutrophic dam and with climate change, its water quality is expected to worsen. Four statistically downscaled climate models are used to produce meteorological outputs that drive the hydrodynamic and water quality model. The times simulated were the present day (1971-1990), the intermediate future (2046-2065) and the distant future (2081-2100). The operating procedure was not expected to change for the dam and inflows and withdrawals were kept the same for each of the simulation periods. The water quality model CE-QUAL-W2 version 3.6 was used. The bathymetry was validated with measured data. The model was calibrated for temperature, phosphorus loading, ammonium, nitrite-nitrates and chlorophyll-a concentration. The model was used to predict a present day situation in the dam, which was the basis from which future changes would be assessed. The main driver for algal growth other than nutrients and light was water temperature, which was linked to air temperature. With climate change, the air temperature will raise and enhance algal growth. The limiting nutrient was phosphorus during the winter and the rest of the year nitrogen limits algal growth. In the present day, the dominant algal group was the green algae. With climate change an increase in the surface water temperature will increase evaporation and cause a decrease in the yield of the dam and further concentrates the algal nutrients. The surface phosphates concentration show increases in all months but especially in autumn. The total algal growth was increased annually and especially during autumn, signalling a seasonal shift and lengthening of the bloom season. The dominant algae however are still the green algae. There will be an increase in the annual concentration of diatoms. The green algae are present in the highest concentrations when compared to diatoms and cyanobacteria. The increase in its nutrients throughout the year as well as the increased water temperature allowed for unabated growth the entire year with peaks earlier in the year during autumn. Cyanobacteria are present at the surface for the entire year at significant concentrations but with intermediate and future climate change their concentrations does not change significantly. The result for cyanobacteria was inconclusive as the inter-variability between the climate models has the greatest variability for cyanobacteria, with 2 models showing an increased concentration and 2 a decreased concentration for intermediate and future time-period. For climate change, the water quality worsens especially during winter. With climate change water quality will worsen earlier in the year confirming a seasonal shift. The modelling of dissolved oxygen proved daunting as the results indicated supersaturation. The concentration of dissolved oxygen does not vary much as would be expected due to the warmer waters. / AFRIKAANSE OPSOMMING: Die studie van klimaatsverandering en die uitwerking daarvan op die eutrofiseering van die oppervlaktewater is 'n huidige en krities belangrike studie vir die welsyn van die hele planeet. Binne dieselfde emissie scenario, is verskeie moontlike uitkomste van klimaat modelle moontlik en die invloed op die kwaliteit van die oppervlakwater. Voëlvlei is 'n buite-bedding dam wat water verskaf aan die stad van Kaapstad in die Westelike Provinsie van Suid-Afrika. Histories is dit is 'n eutrofiese dam en met die verandering van die klimaat sal die kwaliteit van die water na verwagting verswak. Vier statisties afgeskaal klimaat modelle word gebruik om meterologiese toesttande te skep en hiedie word dan gebruik as invoer vir die hidrologiese and water kwaliteits model vir die huidige situasie (1971-1990), die intermediêre toekoms (2046-2065) en die verre toekoms (2081-2100). Die bedruifs-proses vir die dam was nie verwag om te verander nie en die invloei en onttrekkings was dieselfde gehou vir elk van die simulasie periodes. Die watergehalte model CE-QUAL-W2 3.6 was gebruik. Die bathymetrie was bevestig met gemete data. Die model was gekalibreer vir temperatuur, fosfor, ammonium, nitriet-nitrate en chlorofil-a konsentrasie. Die model was gebruik om 'n huidige situasie in die dam te simuleer, wat die basis vir klimaatsveranderinge sou wees. Die vernaamste aandrywer vir die alge groei anders as voedingstowwe en lig, was water temperatuur, wat met lugtemperatuur gekoppel was. Met klimaatsverandering word die lugtemperatuur verhoog en alge groei. Die beperkende voedingstof was fosfor gedurende die winter en die res van die jaar was die dam stikstof beperk. Die dominante alge-groep in die huidige situasie was die groen alge. Met klimaatsverandering stuig die temperatuur van die oppervlakwater, verhoog verdamping, veroorsaak afname in die vlak van die dam en verhoog die konsentrasie van die alge voedingstowwe. Die oppervlak fosfate konsentrasie verhoog in al die maande veral in die herfs. Die totale alge groei jaarliks en veral gedurende die herfs, 'n teken van 'n seisoenale verskuiwing en verlenging van die blom seisoen. Die dominante alge was nog steeds groen alge. Daar sal 'n toename in die jaarlikse konsentrasie van diatome wees. Die groen alge is in die hoogste konsentrasies vergelyk met diatome en sianobakterieë. Die toename in die voedingstowwe deur die loop van die jaar, sowel as die verhoogde watertemperatuur kan vir 'n onverpoos groei vir die hele jaar, veral in die herfs. Sianobakterieë is teenwoordig vir die hele jaar op beduidende konsentrasies, maar met intermediêre en toekomstige klimaat verander die konsentrasies nie veel nie. Die resultaat vir sianobakterieë was onoortuigend as gevolg van die inter-veranderlikheid tussen die klimaats modelle, met 2 modelle wat 'n toename in konsentrasie voorspel en 2 'n afname in konsentrasie voorspel. Vir klimaatsverandering, die kwaliteit van die water vererger veral in die winter. Met klimaatsverandering skyf hierdie verswakking van water kwaliteit na vroeër in die jaar, wat bevestig 'n seisoenale skui vir verergering. Die modellering van opgeloste suurstof was uitdagende en die resultate was super-versadig. Die konsentrasie van opgeloste suurstof wissel nie veel as wat verwag sou word as gevolg van die warmer water.