Fig. 5 Relationships
between wind erosion and mean annual maximum wind speed in southern
Africa from 1991 to 2015. (a) Temporal changes in mean annual maximum
wind speed and (b) spatial patterns of the partial correlations between
wind erosion and mean annual maximum wind speed.
4. Discussion
4.1 Climate dynamics in southern
Africa
Since the 1870s, the increase in atmospheric water demand and changes to
atmospheric circulation patterns have led to more frequent droughts in
Africa (Dai, 2011). In particular, since 1980, the area suffering from
severe drought has continued to increase (Rouault & Richard, 2005), and
the irrational utilization of limited natural resources and population
growth will further aggravate the risk of drought in the future
(Ahmadalipour, Moradkhani, Castelletti, & Magliocca, 2019). Southern
Africa was subjected to severe droughts in 2003 and 2007 (Rouault &
Richard, 2005; Winkler, Gessner, & Hochschild, 2017); and serious
drought events have occurred in local areas (Masih, Maskey, Mussá, &
Trambauer, 2014), such as Namibia in 1995, South Africa in 1995 and
2003, and Lesotho and Eswatini in 2007. These events may have led to
greater soil erosion across southern Africa in these years. Southern
Africa has a noticeable rising temperature trend with the minimum
temperature rising higher than the maximum temperature, especially in
Namibia (Collins, 2011; Hulme et al., 2001). There are great
uncertainties surrounding the multi-year rainfall intensity in southern
Africa, but the average precipitation in the region has significantly
decreased, the rainy season has shortened, and the rainfall intensity
has weakened (Kusangaya et al., 2014; Nicholson, 2001a, 2001b). Over the
last 30 years, the near-surface wind speed in southern Africa has shown
a downward trend (Horton, Skinner, Singh, & Diffenbaugh, 2014;
Torralba, Doblas-Reyes, & Gonzalez-Reviriego, 2017; Wu, Zha, Zhao, &
Yang, 2018), and aerosol emissions have decreased the near-surface wind
speed in sub-Saharan Africa by 0.05 m/s (Bichet, Wild, Folini, & Schär,
2012). During 1993–2010, the data derived from meteorological stations
in the South Africa showed an average decrease in near-surface wind
speed of 0.21 m/s/decade (Kruger, Goliger, Retief, & Sekele, 2010),
which may be one of the reasons for the decrease in annual average soil
erosion in southern Africa. In addition, vegetation productivity and
desertification in sub-Saharan Africa may be influenced by global
climate change, which is partly caused by the North Atlantic Oscillation
and the El Niño Southern Oscillation (ENSO) (Oba, Post, & Stenseth,
2001). For example, in 2002, 2003, and 2015, the ENSO led to drought in
southern Africa (Gizaw & Gan, 2017; Winkler et al., 2017), which was
probably further linked to soil wind erosion.
4.2 Relationship between vegetation and wind
erosion
Temperature and precipitation can have a direct impact on soil wind
erosion. However, vegetation growth, which is dominated by the temporal
and spatial water and heat patterns, can also indirectly affect soil
wind erosion (Miao, Yang, Chen, & Gao, 2012). A correlation analysis
between the average annual fractional vegetation cover (FVC), annual
temperature, and annual precipitation showed that in southern Africa,
temperature and FVC were negatively correlated across 91.8% of the
total area (Fig. 6a). However, in most areas (approximately 88.3% of
the total area), FVC had a positive correlation with precipitation (Fig.
6b). The correlations between different FVC values, and temperature and
precipitation were also different (Figs. 6c, 6d). When the FVC is below
0.4: the negative correlation between FVC and temperature increases. In
contrast, when FVC is greater than 0.4: the influence of temperature on
FVC decreases as FVC increases. For example, the Kalahari Basin, which
is in the middle of the study area and sparsely covered by shrubs and
herbs, has a vegetation coverage of between 0.2 and 0.4 (Fig. 7a). The
negative correlation between FVC and temperature and the positive
correlation with precipitation were clearly more robust than in other
regions (Figs. 6a, 6b). The high temperatures and reduced rainfall make
this area hotter and drier, which reduces vegetation growth. In
contrast, an increase in precipitation promotes the growth of
vegetation. However, in the southern coastal area with a marine climate,
the rise in temperature under warm and humid climate conditions
increases FVC, but the increase in precipitation actually inhibits
vegetation growth.