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Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area
ENGLISH ABSTRACT: Droughts are significant climate events, that can have severe consequences and impacts on the planning and operation of water supplies, but the lack of consistent quantification and monitoring of droughts means that water resource managers often do not have information readily a...
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Stellenbosch : Stellenbosch University
2021
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| _version_ | 1867613799207403520 |
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| access_status_str | Open Access |
| author | Rhode, Paul William |
| author2 | Du Plessis, Jakobus A. |
| author_browse | Du Plessis, Jakobus A. Rhode, Paul William |
| author_facet | Du Plessis, Jakobus A. Rhode, Paul William |
| author_sort | Rhode, Paul William |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | ENGLISH ABSTRACT: Droughts are significant climate events, that can have severe consequences and impacts on
the planning and operation of water supplies, but the lack of consistent quantification and
monitoring of droughts means that water resource managers often do not have information
readily available to inform and assist with decision making in responding to droughts.
The drought experienced in Cape Town, South Africa, during 2015 – 2020 saw storages
rapidly depleting and severe restrictions needing to be implemented to ensure that water
supplies did not fail. Additional information to understand the onset and severity of the drought
as early as possible could have assisted with water resource management.
This research focused on adapting quantification methods and indices to assess and analyse
droughts in the Western Cape Water Supply System (WCWSS) area.
Suitable rainfall station data from the South African Weather Services, City of Cape Town and
Water Research Commission was selected to form a dataset for use in the research. Rainfall
records were corrected, and missing data filled in where required.
Precipitation data for each rainfall station was converted into a Standardised Precipitation
Index (SPI), which allows for the occurrence, duration, and magnitude of historical droughts
to be assessed for each rainfall record.
Threshold precipitation values were determined, from the SPI values developed, for each
rainfall record, which allows the onset and end of droughts to be determined from rainfall
measurements only. Threshold precipitation isohyets were mapped for the WCWSS area.
Severity, duration, and frequency (SDF) curves were produced, after deriving SDF
relationships for each rainfall station from the SPI values. The SDF curves allow for the
probability of a drought with a certain severity and duration to be determined. SDF
relationships were also mapped as isohyets across the WCWSS area.
As a case study, these methods were applied to the rainfall stations indicative of the
precipitation in the catchments of the major dams of the WCWSS, to study the droughts
experienced during 2015 – 2020. The case study showed that the return periods for droughts
during this period varied between 1:5 to 1:100 years, with the most significant impacts seen
in the catchments of the Theewaterskloof, Steenbras and Voëlvlei Dams. It also showed that
four of the five catchments were still in drought by the end of the analysis period in 2020. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/110552 |
| institution | Stellenbosch University (South Africa) |
| language | en_ZA |
| last_indexed | 2026-06-10T12:41:52.972Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2021 |
| publishDateRange | 2021 |
| publishDateSort | 2021 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/110552 Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area Rhode, Paul William Du Plessis, Jakobus A. Stellenbosch University. Faculty of Engineering. Dept. of Civil engineering. Droughts UCTD Climatic extremes Water supply -- Western Cape, South Africa Water-supply engineering Precipitation effectiveness index ENGLISH ABSTRACT: Droughts are significant climate events, that can have severe consequences and impacts on the planning and operation of water supplies, but the lack of consistent quantification and monitoring of droughts means that water resource managers often do not have information readily available to inform and assist with decision making in responding to droughts. The drought experienced in Cape Town, South Africa, during 2015 – 2020 saw storages rapidly depleting and severe restrictions needing to be implemented to ensure that water supplies did not fail. Additional information to understand the onset and severity of the drought as early as possible could have assisted with water resource management. This research focused on adapting quantification methods and indices to assess and analyse droughts in the Western Cape Water Supply System (WCWSS) area. Suitable rainfall station data from the South African Weather Services, City of Cape Town and Water Research Commission was selected to form a dataset for use in the research. Rainfall records were corrected, and missing data filled in where required. Precipitation data for each rainfall station was converted into a Standardised Precipitation Index (SPI), which allows for the occurrence, duration, and magnitude of historical droughts to be assessed for each rainfall record. Threshold precipitation values were determined, from the SPI values developed, for each rainfall record, which allows the onset and end of droughts to be determined from rainfall measurements only. Threshold precipitation isohyets were mapped for the WCWSS area. Severity, duration, and frequency (SDF) curves were produced, after deriving SDF relationships for each rainfall station from the SPI values. The SDF curves allow for the probability of a drought with a certain severity and duration to be determined. SDF relationships were also mapped as isohyets across the WCWSS area. As a case study, these methods were applied to the rainfall stations indicative of the precipitation in the catchments of the major dams of the WCWSS, to study the droughts experienced during 2015 – 2020. The case study showed that the return periods for droughts during this period varied between 1:5 to 1:100 years, with the most significant impacts seen in the catchments of the Theewaterskloof, Steenbras and Voëlvlei Dams. It also showed that four of the five catchments were still in drought by the end of the analysis period in 2020. AFRIKAANSE OPSOMMING: Droogtes is beduidende klimaatsgebeure, wat ernstige gevolge kan inhou en die beplanning en werking van die watervoorraad beïnvloed, maar die gebrek aan konsekwente kwantifisering en monitering van droogtes beteken dat waterhulpbronbestuurders dikwels nie geredelik oor die inligting beskik om besluitneming in reaksie op droogtes te staaf nie. Die droogte wat Kaapstad, Suid-Afrika, van 2015 tot 2020 getref het, het veroorsaak dat opgaarvoorraad vinnig verminder het en streng beperkings nodig was om te verseker dat die watervoorraad nie heeltemal uitgeput word nie. Bykomende inligting om die aanvang en felheid van die droogte, so vroeg as moontlik te verstaan, kon gehelp het met die bestuur van die waterhulpbronne. Hierdie navorsing het gefokus op die toepassing van kwantifiseringsmetodes en -indekse om droogtes in die gebied van die Wes-Kaapse Watertoevoerstelsel (WKWTS) te assesseer en te ontleed. Geskikte reënvalstasiedata van die Suid-Afrikaanse Weerdiens, Stad Kaapstad en die Waternavorsingskommissie is uitgesoek om die datastel vir die navorsing te vorm. Reënvalrekords is, waar nodig, reggestel en aangevul. Neerslagdata vir elke reënvalstasie is omgeskakel na ʼn gestandaardiseerde neerslagindeks (GNI), wat assessering van die voorkoms, duurte en omvang van historiese droogtes vir elke reënvalrekord moontlik maak. Uit die GNI-waardes wat ontwikkel is, is neerslagdrempelwaardes vir elke reënvalrekord vasgestel, wat dit moontlik maak om die aanvang en einde van droogtes uit slegs reënvalmetings te bepaal. Neerslagdrempel-isohiëte is vir die WKWTS-gebied gekarteer. Felheid-, duurte- en frekwensiekrommes (FDF-) is opgestel, nadat die FDF-verwantskappe vir elke reënvalstasie uit die GNI-waardes afgelei is. Uit die FDF-krommes kan die waarskynlikheid van ʼn droogte van ʼn sekere felheid en duurte bepaal word. FDF- verwantskappe is ook as isohiëte vir die hele WKWTS-gebied gekarteer. Hierdie metodes is op die reënvalstasies, wat die neerslag in die opvanggebiede van die hoofdamme van die WKWTS aantoon, toegepas om die droogtes wat van 2015 tot 2020 ervaar was, as ʼn gevallestudie te bestudeer. Die gevallestudie het getoon dat die herhaalperiodes vir droogtes tydens hierdie periode gewissel het van 1:5 tot 1:100 jaar, met die mees beduidende invloed wat in die opvangsgebiede van die Theewaterskloof-, Steenbras- en Voëlvleidam gesien kon word. Dit het ook getoon dat vier uit die vyf opvangsgebiede teen die einde van die ontledingstydperk in 2020 steeds n droogte ervaar het. Masters 2021-06-07T10:31:30Z 2021-06-07T10:31:30Z 2021-03 Thesis http://hdl.handle.net/10019.1/110552 en_ZA Stellenbosch University 147 pages application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Droughts UCTD Climatic extremes Water supply -- Western Cape, South Africa Water-supply engineering Precipitation effectiveness index Rhode, Paul William Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area |
| title | Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area |
| title_full | Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area |
| title_fullStr | Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area |
| title_full_unstemmed | Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area |
| title_short | Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area |
| title_sort | using the standardised precipitation index to determine the severity duration and frequency of drought events in the western cape water supply system area |
| topic | Droughts UCTD Climatic extremes Water supply -- Western Cape, South Africa Water-supply engineering Precipitation effectiveness index |
| url | http://hdl.handle.net/10019.1/110552 |
| work_keys_str_mv | AT rhodepaulwilliam usingthestandardisedprecipitationindextodeterminetheseveritydurationandfrequencyofdroughteventsinthewesterncapewatersupplysystemarea |