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Accueil du site → Master → Afrique du Sud → 2021 → Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area

Stellenbosch University (2021)

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

Titre : Using the standardised precipitation index to determine the severity, duration, and frequency of drought events in the western cape water supply system area

Auteur : Rhode, Paul William

Université de soutenance : Stellenbosch University

Grade : Master Degree (Civil Engineering) 2021

Résumé
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.

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