Impacts of climate change on wind erosion in southern Africa between 1991 and 2015

Chaonan Zhaoa, Hanbing Zhanga, Man Wanga, Hong Jiangb, Jian Penga, Yanglin Wanga11*Corresponding author E-mail address: ylwang@urban.pku.edu.cn (Y. Wang). Laboratory for Earth Surface Processes, Ministry of Education, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China
a Laboratory for Earth Surface Processes, Ministry of Education, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China
b Key Laboratory for Environmental and Urban Sciences, School of Urban Planning & Design, Shenzhen Graduate School, Peking University, Shenzhen 518055, China

Acknowledgments

This study was financially supported by the National Key Research and Development Program of China (No. 2017YFA0604704).

Abstract

Wind erosion is the main form of soil erosion in arid and semi-arid areas. It leads to soil loss and land degradation, which aggravates ecosystem vulnerability and threatens regional sustainable development. The assessment of wind erosion and the study of its driving factors can reduce soil wind erosion and provide decision-making assistance to solve environmental problems. Southern Africa is affected by severe soil erosion, which has brought a series of development problems, such as food crises and poverty. This study used meteorological and remote sensing data, and the revised wind erosion equation model to explore the temporal and spatial dynamics of soil erosion in southern Africa from 1991 to 2015. The impact of climate dynamics on soil wind erosion was also analyzed. The results showed that wind erosion fluctuated during the study period, and it first showed a downward trend and then stabilized at a relatively low level after 2010. Soil wind erosion across 66.65% of the study area significantly decreased (p < 0.05) and near-surface wind speed was the most important factor. The change in wind speed had a positive impact on soil wind erosion across 68.18% of the area. Temperature and precipitation were significantly related to soil wind erosion over 18.96% and 24.63% of the area, respectively. Both can also indirectly affect soil wind erosion through their effects on vegetation cover. This study will help decision-makers to evaluate areas that are at high-risk from soil erosion in southern Africa and enable them to effectively protect fragile ecosystems.
Keywords : wind erosion; RWEQ; climate change; Southern Africa

1. Introduction

Land degradation caused by soil erosion is a major environmental threat across the globe and has a direct impact on global social-economic development and human wellbeing (Millennium Ecosystem Assessment, 2005; Pimentel, 2006; Montgomery, 2007). Most global land degradation occurs in arid and semi-arid areas, which account for about 40% of the total global land area (Batunacun, Nendel, Hu, & Lakes, 2018; Verón, Paruelo, & Oesterheld, 2006). Soil wind erosion is the most common form of land degradation in arid and semi-arid areas, and it continues to spread in many parts of the world, such as America, Africa, Australia, and Asia (Buschiazzo & Zobeck, 2008; Hoffmann, Funk, Reiche, & Li, 2011; Larney, Bullock, Janzen, Ellert, & Olson, 1998; Shi, Yan, Yuan, & Nearing, 2004; Webb, McGowan, Phinn, Leys, & McTainsh, 2009). It has been estimated that about 5.05 × 106km2 of land in the world has been degraded and desertified due to soil wind erosion. This land accounts for 46.4% of the global degraded area. Soil wind erosion has caused considerable damage to the ecosystem, resulting in the loss of soil nutrients and organic matter, and declines in land productivity (D’Odorico, Bhattachan, Davis, Ravi, & Runyan, 2013; Larney et al., 1998; Visser & Sterk, 2007). It has also produced a large number of aerosol particles, which are suspended in the atmosphere. These particles contribute to sandstorms and cause severe air pollution (Jiang, Gao, Dong, Liu, & Zhao, 2018; Kjelgaard, Chandler, & Saxton, 2004; Sharratt, Feng, & Wendling, 2007). Furthermore, land degradation has exacerbated ecological vulnerability (Lal, 2014) and has caused considerable damage to development and human health (Riksen & De Graaff, 2001).
Most studies have shown that developing countries are subject to acute soil erosion (Ananda & Herath, 2003; Pimentel et al., 1995; Rosas & Gutierrez, 2020), but the harm caused by soil erosion has not received enough attention (Borrelli et al., 2017). In sub-Saharan Africa, about 360 Mha of land, or 20%–25% of the total land area, is currently at serious risk of soil erosion (Vågen, Lal, & Singh, 2005). Severe soil erosion results in soil fertility decline and a decrease in the soil utilization rate. The cultivated land productivity loss is about 0.5–1% each year (Scherr, 1999; Sivakumar, 2007), which has caused a series of development problems, such as a decrease in food security and an increase in poverty (Cohen, Brown, & Shepherd, 2006). Around 70% of the land area in South Africa suffers from different types and degrees of degradation (Le Roux, Newby, & Sumner, 2007), and deforestation and unreasonable reclamation mean that Namibia faces moderate or even high soil erosion risks (Klintenberg & Seely, 2004). Increasing population and climate change mean that soil erosion in southern Africa will become more serious in the future (Tamene & Le, 2015).
Soil wind erosion is a process that blows, transports, and deposits fine surface soil particles and nutrients into the atmosphere (Shao, 2008). Soil wind erosion modeling is an important means of predicting and evaluating wind erosion. It can integrate the wind erosion process and influencing factors, and help evaluate the in-situ and off-site effects of wind erosion at different spatial and temporal scales (Blanco & Lal, 2008). In the 1960s, based on Chepil and Woodruff (1959), Woodruff and Siddoway (1965) developed an empirical wind erosion (WEQ) model using wind tunnel and field measurements. After continuous improvement, the revised wind erosion equation (RWEQ) was obtained, which fully considered meteorological, vegetation, soil, roughness, and other factors, and allowed short-term soil wind erosion assessments (Tatarko, Sporcic, & Skidmore, 2013; Van Pelt, Zobeck, Potter, Stout, & Popham, 2004). The model has been widely used in European Union countries (Borrelli et al., 2017), the United States (Pi, Sharratt, Feng, & Lei, 2017; Zobeck, Parker, Haskell, & Guoding, 2000), Pampas in Argentina (Buschiazzo & Zobeck, 2008), China (Du, Xue, Wang, & Deng, 2015; Guo, Zobeck, Zhang, & Li, 2013; Hanbing Zhang et al., 2019), and Syria (Youssef, Visser, Karssenberg, Bruggeman, & Erpul, 2012). However, there have been few studies on soil wind erosion in southern Africa. At the research scale, Mhangara, Kakembo and Lim (2012) assessed the soil erosion risk for the Keiskamma basin in South Africa based on GIS and remote sensing data. For example, Shikangalah, Paton, Jetlsch and Blaum (2017) used Windhoek, Namibia, to quantify the soil erosion severity of cities in arid areas based on field survey data and Sonneveld, Everson and Veldkamp (2005) explored the dynamics of soil erosion by undertaking a remote sensing project in KwaZulu Natal, South Africa. However, the previous studies in southern Africa focused more on short-term and smaller scales, such as specific fields, cities, and watersheds, which means that there is a lack of long-term wind erosion studies at the regional scale.
Wind speed, temperature, and precipitation are the main climatic factors affecting wind erosion (Chi, Zhao, Kuang, & He, 2019; Du, Wang, & Xue, 2017; Gao, Ci, & Yu, 2002). The wind is the major driving force influencing soil wind erosion, and the near-surface wind transportation of soil particles in arid and semi-arid regions can strongly influence the wind erosion process (Pi et al., 2017; Tegen, Lacis, & Fung, 1996; G. Zhang et al., 2019). Temperature and precipitation can determine the stability of soil surface particles and alter the balance between moisture and energy at the soil surface. Drought also makes the soil more prone to wind erosion (Mckenna Neuman, 2003; Haiyan Zhang et al., 2018).
Due to a lack of relevant data, studies on the influence factors affecting soil wind erosion in southern Africa have focused more on the qualitative assessment and estimation of single indicators, such as precipitation and vegetation. However, they have not previously considered the effects of multiple determinants (Kakembo & Rowntree, 2003; Meadows, 2003). Therefore, this study estimated soil wind erosion based on the RWEQ model in southern Africa and used trend analysis methods to reveal the changes in soil wind erosion. This study also used correlation analysis methods to identify the influences of climate factors on wind erosion. The aims of this study were to (1) estimate the distribution of soil wind erosion and its temporal and spatial changes in southern Africa from 1991 to 2015 and (2) explore the impacts of climate factor dynamics, such as temperature, precipitation, and wind speed, on soil wind erosion.

2. Materials and Methods

2.1 Study area and data sources

The study area was located in southern Africa and had a total area of 2.67 × 106 km². It contained five countries: Namibia, Botswana, Eswatini, South Africa, and Lesotho (Fig. 1), with a population of approximately 65.74 million people (FAO, 2019). Southern Africa is one of the most underdeveloped regions in the world. In 2015, the total regional Gross Domestic Product (GDP) was 349.759 billion (current US$), which accounted for 0.47% of global GDP. The proportion of people in poverty (the international standard poverty line is US $1.9 per day) is 13.4%–28.4%, which is higher than the global average poverty population of 9.6% (WDI, 2019). The altitude in the region ranges from −251 m to 3473 m. The central South African plateau is high and the surrounding coastal areas are relatively low. The Namib Desert and the Kalahari Desert are located on the western coast and central basin, respectively. According to the Koppen climate classification, southern Africa is dominated by arid steppe and arid desert. The average annual temperature is 13.8 °C, the average summer maximum temperature is 34.7 ± 0.05 °C (Conradie, Woodborne, Cunningham, & McKechnie, 2019), and the average annual precipitation ranges from 299.9 to 570.4 mm. The rainfall has distinct seasonal differences and the rainy season in most areas is mainly concentrated in the summer (November to February) (Pohl, Macron, & Monerie, 2017). It has been predicted that southern Africa will become drier in the future, annual rainfall will decrease, and drought frequency will increase (Arnell, Hudson, & Jones, 2003; Hulme, Doherty, Ngara, New, & Lister, 2001; Kusangaya, Warburton, Archer van Garderen, & Jewitt, 2014).