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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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| Format: | Thesis |
| Language: | en_ZA |
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Stellenbosch : Stellenbosch University
2021
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| Summary: | 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. |
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