Dust from drought-ravaged Free State threatens SA food security

Large tracts of agricultural land, decimated by drought in the northern Free State, have been identified as a major source of wind-borne dust – and a threat to food security – in SA.

Researchers used satellite imagery, captured over a decade, to study and track the origin of dust plumes.

“The western Free State is clearly the most emissive dust-producing area in South Africa,” said associate professor Frank Eckardt from the University of Cape Town’s Department of Environmental and Geographical Science.

Eckardt is co-author of a new paper in the Aeolian Research journal, titled “South Africa’s agricultural dust sources and events from MSG SEVIRI”, which found that most of the dust in the country originated from exposed agricultural areas which was a red flag for food security since SA relied on maize fields in the Free State for staple foods.

“While in neighbouring dry countries of Botswana and Namibia, dust originates mostly from dry rivers and lakes. South Africa is unusual because most of its dust originates from exposed agricultural land, especially during drought,” said Eckardt.

It was essential to understand dust emissions originating from farming in dry lands.

Once lost, soil is hard to replace; its formation takes aeons, with erosion by wind and water usually outstripping soil production.

—  Frank Eckardt, UCT

“The fine soil fraction that’s dispersed by windstorms adds quality to soil. It holds the nutrients and the moisture. You do not want to lose the fine [silt] fraction. In time, ‘deflated soils’ become sandy and harder to farm. Sandy soils can absorb water but may not be good at holding moisture. They may also hold less carbon and fewer nutrients.”

Eckardt said farmers endeavour to keep some cover crop on the exposed farmlands, but the efforts are insufficient. The province’s large dairy herds are compounding the problem because livestock trampling and grazing “in a degraded field can cause more damage”.

“It’s not only about responding to the impacts mentioned; studying the impact of dust emissions helps to secure food production in the best possible conditions using these marginal lands, a resource that’s becoming increasingly scarce,” Eckardt said.

Eckardt used satellite imagery to track the dust plumes over hundreds of kilometres. For this research he used the “Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager image record from 2006 to 2016, examining 334 497 images”. These images enabled him to observe “dust plumes being initiated, growing and travelling, sometimes for many hours”.

“The resolution or detail is quite coarse, but we can spot the sources with an accuracy of a few kilometres. Our main interest though is on the source areas, which appear to be persistent and relatively small – on a national scale at least,” said Eckardt.

The province’s dust emission season, from June to January, straddles the dry season and the maize harvest period. Eckardt said “2015 and 2016 saw almost half of all event days in the 11-year record, which was matched by a severe drought index and stronger winds”.

He said this period was accompanied by a below-average vegetation coverage of the cropland areas.

“The then department of agriculture, forestry and fisheries’ crop data reported a notable decline in Free State maize cover from 1.2 to 0.6 million hectares, and an increase in fallow land from 140,000 to 790,000 hectares over the same time span,” he said.

The research is also part of a four-year, four-partner South African–Swiss project with UCT, the University of Pretoria, the Agricultural Research Council and the University of Basel.

“This will look at the environmental thresholds for the generation of dust, wind, soil moisture, soil crust, in relation to farmland management. It will also study the extent to which farmland dust sources affect ecosystem services, public health and, potentially, climate,” said Eckardt.