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University of Basel (2021)

Dust emission from croplands in the Free State, South Africa

Vos, Heleen C..

Titre : Dust emission from croplands in the Free State, South Africa

Auteur : Vos, Heleen C..

Université de soutenance : University of Basel

Grade : Doctor of Philosophy (PhD) 2021

The global dust load showed a large increase during the last century due to climate change and the expansion of vulnerable land, both of which are caused by human modifications. The increase in vulnerable land, both in size as in intensity, is mainly attributed to the increase in agricultural areas and agricultural intensification. Dust emission has both an onsite effect due to the degradation of the emitting area, and an offsite effect on human health and climate. The degradation of land is especially relevant for agricultural areas where crop yield can strongly diminish due to the depletion of clay, silt, and nutrients from soils. The semi-arid Free State province has been identified as the largest emitter of dust in South Africa, which is caused by the large-scale agriculture, climate, and soil type. Dust storms have the potential to reach the densely populated Gauteng province causing negative consequences on human health and well-being. Dust events in the Free State show a strong seasonality that is attributed to the agricultural practices that leave soils bare and vulnerable to erosion after harvesting. However, the large differences in dust events per year indicate that additional surface characteristics control the emissions from these harvested croplands. One of the primary potential controls that farmers have on the emissivity of the land is the management of soil crusts, but, the role of soil crusts on sandy soils is often not considered. Therefore, this thesis will address the possible role of soil crusts on the dust emission from the Free State croplands. This thesis examines the formation of crusts by rainfall, the dust emissions from cropland soils, and the surface characteristics that control this erosion. The main instrument used to measure the erodibility of a surface is the Portable In-Situ Wind Erosion Laboratory (PI-SWERL). The comparability of this portable instrument was assessed by a cross-comparison with a traditional straight-line wind tunnel. The threshold friction velocity of sandy surfaces was similar for the two instruments, whereas the threshold friction velocity of loamy sand indicated that the PI-SWERL is a more precise instrument that is capable of detecting the initial, small PM10 emissions from a surface. To determine the potential for crusts to form on the sandy cropland soils, rainfall experiments combined with shear strength measurements were performed on Free State soils. The results showed that significant crusts develop within 15 mm of rainfall, and shear strengths similar to those measured in the laboratory were observed in the field. PI-SWERL measurements showed that these experimental crusts can limit the PM10 emission flux from 10.53 and 3.87 mg m-2 s-1 Luvisol and Arenosol soils, respectively, to below 0.03 mg m-2 s-1 for both. The addition of abraders increased the emission from a crust to 0.43 and 0.31 mg m-2 s-1 for Luvisol and Arenosol, respectively.


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