Hydrological Drought Assessment Based on the Standardized Streamflow Index: A Case Study of the Three Cape Provinces of South Africa

Abstract

Global impacts of drought conditions pose a major challenge towards the achievement of the 2030 Sustainable Development Goals. As a result, a clarion call for nations to take actions aimed at mitigating the adverse negative effects, managing key natural resources and strengthening socioeconomic development can never be overemphasized. The present study evaluated hydrological drought conditions in three Cape provinces (Eastern, Western and Northern Cape) of South Africa, based on the Standardized Streamflow Index (SSI) calculated at 3- and 6-month accumulation periods from streamflow data spanning over the 3.5 decades. The SSI features were quantified by assessing the corresponding annual trends computed by using the Modified Mann–Kendall test. Drought conditions were also characterized in terms of the duration and severity across the three Cape provinces. The return levels of drought duration (DD) and drought severity (DS) associated with 2-, 5-, 10-, 20- and 50-year periods were estimated based on the generalized extreme value (GEV) distribution. The results indicate that hydrological drought conditions have become more frequent and yet exhibit spatial contrasts throughout the study region during the analyzed period. To this end, there is compelling evidence that DD and DS have increased over time in the three Cape provinces. Return levels analysis across the studied periods also indicate that DD and DS are expected to be predominant across the three Cape provinces, becoming more prolonged and severe during the extended periods (e.g., 20- and 50-year). The results of the present study (a) contribute to the scientific discourse of drought monitoring, forecasting and prediction and (b) provide practical insights on the nature of drought occurrences in the region. Consequently, the study provides the basis for policy- and decision-making in support of preparedness for and adaptation to the drought risks in the water-linked sectors and robust water resource management. Based on the results reported in this study, it is recommended that water agencies and the government should be more proactive in searching for better strategies to improve water resources management and drought mitigation in the region.

Review of In-Situ and Remote Sensing-Based Indices and Their Applicability for Integrated Drought Monitoring in South Africa

“…(2021) study results further affirm the need for more research to determine best fitting PDFs to calculate the SSI in SA catchments [67,72,73]. The recent local studies such as Botai et al (2021) have laid an important foundation for streamflow drought monitoring in SA, demonstrating the potential of the SSI to become a preferred hydrological drought index for use to develop integrated drought monitoring systems.…”

“…(2021) study results further affirm the need for more research to determine best fitting PDFs to calculate the SSI in SA catchments [67,72,73]. The recent local studies such as Botai et al (2021) have laid an important foundation for streamflow drought monitoring in SA, demonstrating the potential of the SSI to become a preferred hydrological drought index for use to develop integrated drought monitoring systems. Thus, based on the study review criteria, the SSI is recommended for integrated drought monitoring in SA because it is capable of characterising hydrological droughts according to their duration, magnitude, intensity, severity, spatial extent, and frequency, using streamflow data.…”

“…In the meantime, this paper recommends the SSI as the preferred method because it is the only recommended drought index for streamflow drought monitoring and early warning in the 2016 version of the 'Handbook of Drought Indicators and Indices' [31,66]. Furthermore, the SSI has been commonly used and has produced promising results in some SA catchments [67,68].…”

“…However, according to Svensson et al (2017), due to recent advances in parameter estimation methods, and implementation of these methods in the 'R software package', the Tweedie PDF is now a viable option [71]. Botai et al (2021) conducted a study to evaluate hydrological drought conditions in the Eastern Cape (EC), Northern Cape (NC), and the WC provinces in SA. The authors used the SSI calculated using a gamma PDF and 35 years (1985-2020) of streamflow records from 39 stations across the EC, NC, and WC provinces to characterise drought conditions in terms of its duration and severity.…”

Hydrological drought forecasting and monitoring system development using artificial neural network (ANN) in Ethiopia

“…Drought results from a prolonged period of precipitation deficiency and shortage of surface and subsurface water availability [ [1] , [2] , [3] , [4] ]. The definition of drought still depended on different perceptions but structurally it is categorized as meteorological, agricultural, hydrological, and socio-economic drought based on deficiency of precipitation, lack of surface and sub-surface water availability, and unfair water distribution between supply and demand [ 5 ].…”