where \(r_{xy\bullet z}\) means that when variable \(z\) is fixed, the
correlation coefficient is between \(x\) and \(y\); and\(r_{\text{xy}}\), \(r_{\text{xz}}\), and \(r_{\text{yz}}\) represent
the correlation coefficients between variables \(x\) and \(y\), \(x\)and \(z\), and \(y\) and \(z\), respectively. A t-test was used to test
the partial correlation coefficient.
3. Results
3.1 Wind erosion
distribution
The spatial distribution of soil wind erosion in southern Africa is
shown in Fig. 2. The average wind erosion modulus was 0–169.87 t/ha/a
from 1991 to 2015 and the spatial distribution was heterogeneous. The
soil wind erosion in most parts of South Africa is relatively light,
with a modulus of less than 10 t/ha/a. However, the areas with severe
soil erosion are in the Namib desert, which is a narrow strip along the
western coastal area, and the Kahalari Basin, which is the border area
between Namibia, southern Botswana, and South Africa, has an average
wind erosion modulus of more than 90 t/ha/a. The strong wind, erodible
soil texture, and sparse vegetation mean that soil erosion in these
areas is very serious. The soil wind erosion had roughly the same
spatial distribution at certain times (Figs. 2b–2f). The proportion of
the area with a wind erosion modulus greater than 60 t/ha/a (very high
erosion hazard) gradually decreased from 20.36% in 1991–1995 to
15.92% in 2001–2005 and 8.78% in 2011–2015. The wind erosion modulus
in the western and central regions was always higher than that in the
other regions, but severe soil erosion greatly improved after 2000 in
the eastern coastal area, that is, the northeastern part of South Africa
and Eswatini.