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).