Overview on Droughts
Drought Module
This document provides a thematical background for the topic of droughts, including examples from Africa and the context of Earth Observation.
Introduction
Drought is considered to be the most destructive natural hazard due to its severity, duration, spatial extent, and severe consequences [1]. Drought affects millions of people around the globe every year and imposes substantial challenges to the environment, economy and society [2]. In general, drought can be categorized into four types: Meteorological, hydrological, and agricultural (impact on the environment), and socio-economic drought (impact on human population and society) [3]. The types are distinguished by the duration of the drought as well as its institutional and social impacts - see Figure 1. A prolonged meteorological drought can eventually lead to hydrological drought as well as agricultural drought, which in turn can cause socio-economic disruption depending on a country’s resilience and vulnerability. Actively monitoring drought conditions can improve preparedness and reduce potential impacts [4].
The overall impacts of drought include land degradation, deforestation, reduced crop, and rangeland productivity, decreased water levels, increased fire hazards, reduced energy production, raised livestock and wildlife death rates, and damage to marine and fish habitats [1], [5]. The social impacts of drought are remarkable in food-scarce developing countries with high dependency on agriculture; generally, African countries [2]. Besides the impacts of climate change, population growth also affects drought risk; both directly, i.e. through increasing the exposure component of the risk, and indirectly, i.e. by aggravating drought vulnerability. The increasing population will demand additional food, energy, and water resources for sustainable growth [2].
In the United States, droughts cause $6–8 billion in damage to the economy per year. Estimates by the European Commission in 2007 indicated that the damages of droughts in Europe over the last 30 years were at least €100 billion [6]. Africa is exceptionally vulnerable to climate variability and change compared with many other regions [7]. As presented in Figure 2, drought-related reported disasters between 1970 and 2019 covered 16% of the total with 271 events, and 26% of total economic losses with $10 billion. Drought-related deaths covered 95% of the total – with 695 150 fatalities [7]. In addition, EM-DAT data revealed more than 869 000 drought-related fatalities and approximately 514 million affected people in 353 events between 1910 and 2022 [8].
Based on six temperature datasets, Africa warmed faster than the global average. At sub-regional scales, the temperature analysis revealed that the warming trend in the 1991–2020 period was the highest in all African sub-regions – see Figure 3 [7].
Annual average temperatures in 2020 across the continent were above the 1981–2010 average in most areas. The largest temperature anomalies were recorded in the northwest of the continent, in western equatorial areas, and in parts of the Greater Horn of Africa. However, near-average or slightly below-average temperatures were recorded in Southern Africa, the north of Lake Victoria, and the Sahel region [7].
Drought in Africa – An Overview
According to the EM-DAT data, 48 countries in Africa experienced droughts between 1910 and 2022. Ethiopia ranks first place with over 402 000 deaths and 18 events, followed by Sudan with around 150 000 deaths and 9 events, and Mozambique with over 100 000 deaths and 15 events [9].
In the 1981 – 1985 period, more than half a million people lost their lives in Ethiopia (around 300 000), Sudan (around 150 000), and Mozambique (around 100 000) due to the famine caused by a combination of drought and civil wars in these countries [9]. Detailed statistics of the most affected ten countries in Africa are presented in Table 1, sorted by most fatalities.
![Table 1 Number of events, total deaths, and affected people for each country in Africa – 1910 to 2022 [9]..jpg](https://www.zfl.uni-bonn.de/sentinels-4-african-drr/drought-module/table-1-number-of-events-total-deaths-and-affected-people-for-each-country-in-africa-2013-1910-to-2022-9.jpg/@@images/image)
The Role of Earth Observation
Drought early warning facilities are important to improve drought resilience and coping strategies [1]. Timely assessment and monitoring of drought will increase drought preparedness, relief and mitigation and reduce the damage of drought impacts to the environment, economy and society [10]. Historically, traditional ground-based approaches (in-situ, station-based) have been used for drought monitoring, primarily from meteorological and agricultural perspectives. Since many agricultural areas are not well-instrumented with the ground-based instruments, it is not possible to obtain consistent data on precipitation, near-surface air temperature, wind speed, atmospheric water vapor, and relative humidity. Drought analysis using ground-based observations is challenging as inconsistency between different meteorological stations in terms of record lengths and variable data quality can be a problematic factor [3], [10].
Towards the end of the 20th century, a paradigm shift in drought monitoring approaches occurred, concurrent with advances in remote sensing and earth observation technologies [3]. With the growing number of remote sensing satellites in orbit and freely available near real-time data provided by earth observation (EO) missions offering high spatial and temporal resolutions; it is possible to monitor some of the drought conditions globally and consistently. EO data acquired by space-borne sensors (both optical and microwave) can be complementary and efficient combined with in-situ data – or without, in some cases - considering the fact that no physical intervention is required for remote sensing-based approaches [10]. EO data helps to estimate precipitation, evapotranspiration, soil moisture and vegetation conditions, such data can be used for the assessment and monitoring of drought characteristics: its intensity, duration and spatial extent [11].
Over the past decade, a rapid evolution has been seen in remote sensing technologies that can be applied in drought monitoring, such as the launch of the Sentinel satellites and the development of new indicators and analytical platforms [3]. Specifically for monitoring agricultural conditions, high to moderate resolution EO datasets, for instance the ones provided free of charge by the National Aeronautics and Space Administration - NASA and the European Space Agency - ESA (Landsat series, Sentinel-1/2, MODIS, etc.) are broadly used. Other satellite missions such as Gravity Recovery and Climate Experiment (GRACE) and Soil Moisture Active Passive (SMAP) for hydrological monitoring, and Global Precipitation Measurement Mission (GPM) for meteorological monitoring are also widely used for monitoring drought conditions.
90% of the remote sensing-based drought indices are used for agricultural drought monitoring, and the remaining 10% of the indices are used for hydrological and meteorological monitoring [13]. In this context, the Precipitation Condition Index (PCI), the Palmer Drought Severity Index (PDSI), and the Standardized Precipitation Index (SPI) for precipitation monitoring, the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Water Index (NDWI), the Enhanced Vegetation Index (EVI), the Soil-Adjusted Vegetation Index (SAVI), the Normalized Vegetation Moisture Index (NDMI), and the Vegetation Health Index (VHI) for vegetation and soil moisture monitoring are widely used. Additionally, the Gross Primary Productivity (GPP) and anomalies such as the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) are other significant indicators of drought. Among the vegetation indices, NDVI is by far the most commonly applied method in agricultural drought monitoring [3] , and is calculated using the near-infrared (NIR) and red bands – see Equation 1.
Additionally, EVI is also a widely used index for drought monitoring which is quite similar to NDVI and can be used to quantify vegetation greenness. The EVI corrects for some atmospheric conditions and canopy background noise, and is more sensitive in areas with dense vegetation where Leaf Area Index (LAI) is high [14]. The EVI uses the NIR, red and blue bands and is calculated as presented in Equation 2.
C1 and C2 are aerosol resistance coefficients; G is a gain factor, and L is the canopy background adjustment [15] :
Due to the limitations that came across in NDVI-based approaches, the Vegetation Health Index (VHI) was proposed by Kogan (1997), offering notable improvements over standalone NDVI-based monitoring as it provides a representation of vegetation condition relative to long term change. The VHI is the weighted average of two sub-indices: the Vegetation Condition Index (VCI), and the Temperature Condition Index (TCI) [3], [16]–[18] –. The indices are calculated as presented in Equations 3-5. The VCI, therewithal VHI, can also be used with other indices, such as EVI and NDWI [3], [4].
Where max - min represent the maximum and minimum values of that variable over the study period, and BT is Brightness Temperature recorded from a thermal sensor [3].
Additionally, several integrated drought indices were developed to exploit the strengths of both remote sensing and climate-based drought monitoring techniques, such as the Vegetation Drought Response Index (VegDRI) [3].
Finally, using freely available remote sensing data for drought monitoring processes can help stakeholders, scientists and individuals to better assess drought conditions, prepare and manage possible challenges that might occur; in a wide spatial extent.
2010 - 2011 East Africa Drought
The 2010 - 2011 drought was one of the extreme events that led to severe food crises and famine [10]. The worst drought in 60 years [19] , as described, hit the entire East African region and affected more than 9 million people in Somalia, Djibouti, Ethiopia, Kenya, Uganda, and neighboring areas [10]. Millions of people were internally displaced within the continent, and the number of fatalities in the hazard were estimated as more than 260 000, mostly in Somalia alone [20]. These countries were affected economically as well, since a significant increase in inflation had been seen in some of them [21].
The drought was attributed to a strong La Niña event, which originated from the steady Indian Ocean warming in the summer of 2010, and caused poor precipitation in East Africa. The European Centre for Medium-Range Weather Forecasts (ECMWF) seasonal forecasts predicted the event from June 2010 onwards, and also a dry precipitation anomaly for the region from July 2010 onwards [22], [23]. Thus, reduced precipitation in both October – December 2010 and March-May 2011 led to severe drought in the region [1].
On 20 July 2011, the United Nations officially declared famine in some regions of Somalia; however, thousands of people lost their lives even before the declaration. During this famine, around 3.1 million people were affected and half a million children were malnourished [25]. According to the report by the United Nations High Commissioner for Refugees (UNHCR), around 1.46 million Somalis were already displaced in late 2010 [21].
Several factors triggered the famine in Somalia. These include the drought, the rapid increase in the price of food, and the conflict [25]. However, the drought was the major trigger of the famine in the whole Greater Horn region, due to insufficient rainfall during the 2010 short rains (October–November) and 2011 long rains (March-May) [1], [25], [26].
2015 – 2016 Southern Africa Drought
Southern Africa experienced extreme drought in 2015-2016, induced by one of the strongest El Niño events on record, with severe impacts on local food security, livelihoods and economy. Countries such as Angola, Botswana, Lesotho, Malawi, Madagascar, Mozambique, Namibia, South Africa, Swaziland, Zambia and Zimbabwe were terribly affected by the drought. The drought was most intense during the planting season, October - December, which hampered planting and significantly reduced planted area [27], [28].
In South Africa, extreme drought conditions prevented farmers from planting nearly one million hectares of maize, and the total maize output was reduced by nearly 50% from the five-year average [28]. Maize production change in 2014-2015 and 2015-2016 cropping seasons in South Africa and other countries, with a comparison to pre-drought seasons, are presented in Figure 6 [29].
The overall impact of the 2015 - 2016 droughts in Southern Africa showed that at least 18 million people were worst-hit by drought. About 7 million of these were targeted for assistance by World Food Programme (WFP), and by the end of September 2016, only 1.7 million people had received food assistance. By the end of September 2016, more than half of the national population (58%) in Swaziland was experiencing food insecurity resulting from drought impacts, as well as 38% in Malawi, 34% in Lesotho and 29% in Zimbabwe [29]. The damage caused by the events to the economy of Southern African countries was estimated to be more than $1.75 Billion [9].
References
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