With climate change exacerbating rainfall variability, Botswana’s frequent droughts are likely to intensify. The Standardised Precipitation Index (SPI) provides a critical tool for not only identifying drought conditions but also preparing for their impacts
DOUGLAS RASBASH
Special to The Botswana Gazette
Worst drought in a century devastates Southern Africa, millions at risk. More than 27 million lives affected by worst drought in a century, with 21 million children malnourished, says World Food Programme. Lesotho, Malawi, Namibia, Zambia, and Zimbabwe have all declared a state of national disaster in 2024 as the drought has destroyed crops and livestock. Angola and Mozambique are also severely affected, the UN’s World Food Programme (WFP) said in a briefing recently, warning that the crisis is expected to deepen until the next harvests in March or April 2025.
Botswana declared 2023/24 drought years and will most likely extend it to 2025. Prolonged above-average temperatures and minimal precipitation have led to severe soil moisture deficits and vegetation stress across Southern Africa. The satellite-derived vegetation index indicates severe stress in several countries, reflecting the harsh impact of the drought on ecosystems and agriculture. Soil moisture deficit was already severe in March 2024 but it is much worse now in December.
Since gaining independence in 1966, Botswana has endured 19 droughts in 58 years, averaging one drought every three years. Despite their frequency and devastating impact, few people fully understand how droughts are identified or the science behind declaring one. This article explains how droughts are assessed using the Standardised Precipitation Index (SPI) and why this is critical for triggering emergency responses in a drought-prone country like Botswana.
What is a drought?
A drought occurs when there is an extended period of below-average precipitation, leading to water shortages that impact agriculture, ecosystems, and livelihoods. However, not all periods of low rainfall are immediately classified as droughts. Determining whether a drought exists requires a scientific and standardised approach, and this is where the SPI comes into play.
How the SPI Works
The SPI is a statistical tool used to measure precipitation deficits or surpluses over a specific period. It provides a standardised way to compare rainfall patterns across different regions and timeframes, making it particularly useful for countries like Botswana with highly variable rainfall.
- Data Collection: Historical rainfall data, ideally spanning 30 years or more, is gathered for a specific location. For Botswana, data from weather stations across the country is crucial for accuracy.
- Selection of Time Scale: The SPI can be calculated for different timeframes, such as 1 month, 3 months, 6 months, or more.
- Short-term droughts (1-3 months) affect agriculture directly.
- Long-term droughts (6-12 months or more) impact water resources, reservoirs, and ecosystems.
- Standardisation: The precipitation data is compared to the long-term average and fitted to a statistical model. This process accounts for natural rainfall variability and transforms the data into a standardised score.
- SPI Values and Interpretation: The SPI score indicates how much rainfall deviates from the average, measured in standard deviations:
- SPI > 0: Above-average rainfall (wet conditions).
- SPI = 0: Normal conditions.
- SPI < 0: Below-average rainfall (dry conditions).
Specific thresholds help classify drought severity:
- Mild drought: SPI between -1.0 and -1.5
- Moderate drought: SPI between -1.5 and -2.0
- Severe drought: SPI below -2.0
Calling a drought in Botswana
Botswana’s drought declaration process involves analysing SPI data, typically at national and district levels. If SPI scores consistently indicate moderate to severe drought across key regions, authorities may declare an official drought.
However, the decision also considers broader impacts such as crop failures, reduced livestock productivity and drying of water sources. The Botswana Gazette has provided SPI-derived drought information for drought years 2011 to 2017 to show how the SPI can be applied.
Why it matters: Linking SPI to emergency responses
Declaring a drought is not just a scientific exercise; it has tangible implications for government policy and relief efforts. In Botswana, a drought declaration can trigger:
- Emergency Food Relief: SPI data ensures that food aid reaches the most affected areas promptly.
- Water Tankering Services: Rural communities and livestock often rely on emergency water supplies during droughts.
- Livelihood Support: Subsidies for animal feed or programmes to assist small-scale farmers.
- International Assistance: Accurate SPI data lends credibility to appeals for external funding or aid.
By relying on objective SPI thresholds, the government can prioritise resources and avoid political or emotional biases in responding to droughts.
The Bigger Picture
With climate change exacerbating rainfall variability, Botswana’s frequent droughts are likely to intensify. The SPI provides a critical tool for not only identifying drought conditions but also preparing for their impacts. However, its effectiveness depends on accurate and timely data collection, robust weather monitoring infrastructure, and public awareness.
Droughts and the food insecurity that they bring should not be considered nationally but regionally. It is not of the slightest use for one country to ban food imports in times of drought in order to protect farmers’ incomes. Far better that droughts and food security be considered regionally because countries need to work in harmony and not compete with each other.
Droughts are not just a natural phenomenon – they are a recurring challenge that requires proactive management at the regional level. Understanding how droughts are determined, and adopting a regional approach, is the first step towards building resilience in one of the world’s most arid nations. By appreciating tools like the SPI, citizens can better grasp the scale of the problem and the importance of timely interventions.