A stochastic, daily time-step model for the conjunctive use of surface water, groundwater, desalination and water reclamation for municipalities

Braune, Erika Gertrud (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: South Africa has a broadly-developed water infrastructure based mainly on surface water, localised groundwater and occasional desalination as resources. However, the most suitable sites for surface water storage have been utilised and with population growth and economic factors driving the increase in demand, as well as changing climate conditions, it is projected by the Department of Water and Sanitation that South Africa’s water demand will exceed the available fresh-water by 2025. To mitigate water scarcity, more conjunctive water use solutions need to be investigated. To implement conjunctive management of scarce water resources at a local authority level, a Microsoft Excel model was developed to assess the combined yields of surface water, groundwater and desalinated and reclaimed water, using a daily time step. The model is stochastically driven by synthetically generated streamflow sequences. Monthly streamflow is disaggregated into daily streamflow and a streamflow-rainfall relationship is established to generate corresponding synthetic rainfall sequences. The conventional dam balancing equation, with daily streamflow, is used in the modelling. Groundwater is modelled using a similar approach as the Aquifer Firm Yield Model with the saturated volume fluctuation equation as the stochastic link between rainfall, recharge and water levels. This model is paired with the Cooper-Jacob model and data from Groundwater Resource Assessment Phase 2 project. Desalination and water reclamation are modelled as one source which provides water at 100% assurance of supply at different operational capacity levels over fixed three-monthly time steps. The model evaluates the available yield of the system with water from desalinated sources (including reclaimed) and groundwater used first, according to minimum operational procedures, after which surface water is utilised. A control is built into the model which shuts down the desalination plant if the dam capacity reaches user-defined levels. The model allows for multiple alternative water resources, based on consumer defined input. Additionally, the short-term and long-term assurance of supply is graphically presented which can aid management decisions. An analysis of the historical water supply system is produced while information towards improvement of water management is also provided.

AFRIKAANSE OPSOMMING: Suid-Afrika beskik oor ‘n watervoorsieningsinfrastruktuur wat oor ‘n breë spektrum ontwikkeld is, maar hoofsaaklik gebasseer is op oppervlak waterbronne, plaaslike grondwater en, in sommige gevalle, ontsouting. Die mees geskikte liggings vir die berging van oppervlak waterbronne is reeds in gebruik, terwyl bevolkingsgroei, ekonomiese faktore en veranderende klimaatsomstandighede die vraag na water steeds laat styg. Daar word beraam volgens die Departement van Water en Sanitasie dat Suid-Afrika se verskillende water bronne teen 2025 gesamentlik nie meer aan die vraag sal kan voorsien nie. Om water tekorte te bestuur, moet volledige ondersoeke na die gebruik van verskillende bronne ingestel word. Om meer geintegreerde oplossings van hierdie skaars bron te implementeer, is ‘n model in Microsoft Excel ontwikkel om plaaslike waterverskaffers, byvoorbeeld munisipaliteite, te help om die gekombineerde opbrengs van verskeie waterbronne op ‘n daaglikse basis te bepaal. Die model is stogasties (ewekansig) van aard, deurdat dit gebruik maak van tydsreekse wat sinteties gegenereer is. Tydsreekse is gebasseer op die agtereenvolgende veranderinge in die vloei van ‘n stroom (rivier) oor ‘n bepaalde tydperk. Maandelikse vloei word verdeel in daaglikse vloei met behulp van ooreenstemmende sinteties gegenereerde reënval tydsreekse. Die verhouding tussen die vloei van die stroom en reënval word bepaal om die bogenoemde te bewerkstellig. Die konvensionele dam balanseringsvergelyking, met behulp van die daaglikse tydreekse, word in the modelleering gebruik. Grondwater bronne word gemodelleer deur ‘n metode soortgelyk aan die “Aquifer Firm Yield Model” met die “saturated volume fluctuation”-vergelyking as die stogastiese (ewekansige) skakel tussen reënval, hervulling van die waterdraer en veranderinge in watervlakke. Die model maak ook gebruik van die “Cooper-Jacob”-vergelyking en inligting wat ingesamel is tydens die “Groundwater Resource Assessment Phase 2”-projek van die destydse Departement van Waterwese en Bosbou. Ontsoute waterbronne word gemodelleer as ‘n bron met 100% sekerheid van lewering gegee teen spesifieke bedryfsvlakke, oor ‘n drie-maandelikse typerk. Die model gebruik die balanseringsvergelyking om die lewering van die stelsel, waar ontsouting en grondwater eerste benut word, in ooreenstemming met minimum bedryfsvlakke, waarna oppervlak waterbronne benut word. ‘n Kontrole is in die model ingebou om die ontsouting af te skakel indien ‘n dam ‘n gespesifiseerde kapasiteit bereik. Die model maak voorsiening vir ‘n reeks alternatiewe waterbronne, gebasseer op die gebruiker se gedefinieerde invoer data. ‘n Ontleding van ‘n watervoorsieningstelsel word gedoen en die korttermyn en langtermyn versekerde lewering word grafies voorgestel, as hulpmiddels wat aangebied word vir die verbetering van bestuur van waterbronne.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107961
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