The Impact of Climate Change on Drought and Rainfall Variability: Insights from SPI Data in Bloemfontein and Johannesburg

Climate change significantly affects global precipitation patterns, leading to increased rainfall variability and it the rising frequency of extreme droughts.

This study examines Standardized Precipitation Index (SPI) data for Bloemfontein and Johannesburg using monthly rainfall data from South African Weather Services (SAWS). The SPI analysis covers short-term (3-month), medium-term (6-month), and long-term (12-month) drought conditions, revealing key wet and dry periods over the past decades.

Findings highlight intensifying droughts (2019–2021) alongside extreme wet periods, reflecting the growing impact of climate change on regional precipitation patterns. The results underscored the urgent need for climate adaptation strategies, including improved water management, early warning systems, and sustainable agricultural practices.

Keywords: Climate Change, Drought, Rainfall Variability, Standardized Precipitation Index (SPI), Water Security, Agriculture, Hydrological Drought

1. Introduction

Climate change is a global crisis that is reshaping weather patterns, water availability, and agricultural productivity. One of its most critical effects is the increasing variability of rainfall and the frequency of extreme droughts. These changes significantly affect food security, economic stability, and water resources, especially in semi-arid regions like Southern Africa. Drought, a prolonged period of below-average precipitation, severely affects agriculture, ecosystems, and human livelihoods. The Standardized Precipitation Index (SPI) is widely used to assess precipitation anomalies and identify drought conditions over different timescales.

This study utilises monthly rainfall data from the South African Weather Service (SAWS) to analyse SPI data for Glen College and O.R. Tambo, providing insights into historical drought and wet periods. By examining short-term (3-month), medium-term (6-month), and long-term (12-month) SPI trends. This study aims to understand how climate change is influencing rainfall variability and its implications for water security and agricultural sustainability.

2. Understanding the Standardized Precipitation Index (SPI)

The Standardized Precipitation Index (SPI) is a widely used method for analysing rainfall anomalies over different timescales. It measures deviations from historical precipitation averages, allowing researchers to detect droughts and excessive rainfall events. Positive SPI values indicate above-normal precipitation, while negative SPI values signify below-normal precipitation, with values below -1.5 indicating severe drought and above 1.5 representing highly wet conditions.

Different SPI timescales provide insights into various types of droughts. The 3-month SPI reflects agricultural drought, affecting soil moisture and crop yields. The 6-month SPI indicates hydrological drought, which impacts rivers, reservoirs, and groundwater levels. The 12-month SPI measures socio-economic drought, leading to long-term water shortages and economic instability. By analysing historical SPI trends for Bloemfontein (Glen College weather station) and Johannesburg (O.R. Tambo weather station), this study identifies critical drought and wet periods, helping to assess the impact of climate change on precipitation patterns.

3. Trends in Drought and Rainfall Variability at Glen College and O.R. Tambo

3.1 Wettest Periods (Above-Normal Precipitation)

Analysis of SPI data reveals historically wet periods with above-average rainfall, often linked to regional climate phenomena such as El Niño and La Niña events. Glen College experienced significant wet periods in 1987, 1988, and 1991, where short-term excessive rainfall events were recorded. Extended wet seasons occurred in 2016, and multi-year periods of above-normal precipitation were observed in 1988 and 1989. Similarly, O.R. Tambo witnessed short-term heavy rainfall episodes in 1972, 1975, 1996, and 2000, with prolonged wet seasons in 1988 and 1997.

Several climate drivers contributed to these wet periods. La Niña conditions during 1988 and 1996–1997 significantly increased rainfall levels, while shifts in the Indian Ocean Dipole (IOD) and monsoonal patterns amplified precipitation variability. Although increased rainfall during these periods helped recharge water bodies, it also led to flooding and soil erosion. Graph 1 presents the 3-month SPI for Glen College and O.R. Tambo from 1960 to 2023, highlighting short-term excessive rainfall events. Figure 2 illustrates the 12-month SPI trends over the same period, showing long-term rainfall variability.

Figure 1: 3-month Standardized Precipitation Index (SPI) for Glen College and O.R. Tambo, showing variations in drought and wet conditions from 1960 to 2022.

Figure 2: 6-month Standardized Precipitation Index (SPI) for Glen College and O.R. Tambo, showing variations in drought and wet conditions from 1960 to 2022.

3.2 Prolonged Drought Periods (Below-Normal Precipitation)

Conversely, SPI data highlights severe and prolonged droughts, particularly in recent decades, reinforcing the concern that climate change is driving an increasing trend of water shortages. Glen College recorded significant drought events in 1991 and 1994/5, 2003, and 2012, with agricultural droughts affecting crop production.

Hydrological droughts, reducing water storage and availability, occurred in 2011 and 2019, while prolonged socio-economic droughts were evident in 2020 and 2021. At O.R. Tambo, seasonal droughts were observed in 1962, 1965, and 1975, with prolonged water shortages occurring in 1972, 2015, and 2020. Multi-year droughts affecting groundwater availability were prominent in 1965, 1992, and 1993.

Figure 1 & 2 presents the frequency of drought events (SPI ≤ -1.5) from 1980 to 2023, illustrating an increase in drought frequency over time.  The figures compare wet and dry periods using SPI values, highlighting significant regional climate variability. The increasing frequency of extreme dry events underscores the urgency of implementing effective water management strategies.

4. Conclusion and Recommendations

The analysis of monthly rainfall data from SAWS and SPI trends for Glen College and O.R. Tambo confirms that climate change drives increased rainfall variability and more frequent droughts. The 2019–2021 droughts highlight a troubling trend toward prolonged water shortages, while extreme wet periods suggest that climate-induced variability intensifies. The study finds that droughts have become more frequent and severe since the 1990s, while extreme rainfall events are increasing, leading to flood risks and soil erosion. These changes significantly threaten water security, necessitating urgent adaptation measures.

Early warning systems using SPI-based drought forecasting should be strengthened to mitigate these challenges and enable proactive planning and response. Sustainable water management practices, such as rainwater harvesting, and recharge should be prioritised to ensure water availability during dry periods. Climate-smart agricultural practices can enhance resilience against droughts ensuring food security and economic stability. Moreover, policy frameworks should be improved to support long-term climate adaptation planning, integrating scientific data into decision-making processes.

Climate change reshapes rainfall and drought patterns, seriously threatening regional water security and agricultural productivity. Urgent action is required to safeguard future water resources and food production. Implementing adaptive strategies will be crucial in mitigating the adverse effects of climate variability and ensuring sustainable development in Southern Africa.

Written by: Tshedza Matladi & Thakane Hoeane

Edited By: Muhammad Abdullahi Ibrahim