There is more to donga erosion than meets the eye

“My parting message to South Africa is a rather grim warning – a prophecy of threatening calamity. Everywhere I saw the scars of erosion, festering and spreading in the soil of your land,” visiting American soil scientist Dr Hugh Bennett remarked after a two-month tour of this country in 1944.

Bennett, the founding head of the US Soil Conservation Service, continued: “Some of the areas through which I passed can never be restored, for the soil and the sub-soil have gone. I do not think that the estimate that South Africa has already lost 25% of the fertility of the soil is any too big … A quarter has gone, completely ruined or is badly damaged. It is only a matter of generations before it is all gone.”

His words, along with similar grave warnings by National Veld Trust director TC Robertson, clearly hit home.

Two years later, the Union government passed the Soil Conservation Act of 1946, the first soil-specific law designed to curb decades of neglect, overgrazing and degradation of one of South Africa’s most valuable natural resources.

Eight decades on, however, debates continue around the varied causes of soil erosion and the techniques needed to arrest the problem and rehabilitate the landscape.

In a keynote speech to a global soil erosion symposium in Italy, the acclaimed Belgian researcher Prof Jean Poesen reiterated the necessity of protecting the world’s soils in an epoch of ever-increasing human impacts and unprecedented climate change.

“Given the large number of research papers on this topic, one might think that we know now almost everything about soil erosion and its control so that little new knowledge can be added.”

That would be a mistake, Poesen said, noting that there are still some major research gaps – most notably in understanding both the human and the natural processes responsible for global soil erosion.

“In the Anthropocene, soil losses by human activities have become very significant: eg tillage erosion, soil erosion by land levelling, soil quarrying, crop harvesting (mainly root and tuber crops), explosion cratering and trench digging.”

But it can be tricky to disentangle the relative impacts of erosion caused by human influence on one hand, compared with those caused by climatic and other environmental factors, including severe rainstorms or soil types more susceptible to washing away.

Last year, Richard Lyons and fellow researchers from Aberystwyth University and the University of the Witwatersrand, published an article that challenged the “often default assumption” that soil erosion is necessarily attributable to human factors.

Based on their studies of three separate areas of severe gully erosion in different provinces of South Africa, they suggest that these particular erosion events began several centuries ago – mainly due to climate change and massive flooding, possibly as far back as AD 900.

Lyons and his colleagues note that it is often assumed that dongas are the result of land mismanagement, particularly since European settlement or by previous grazing pressure by domestic cattle and sheep owned by Iron Age farmers.

“Overall, based on current evidence, and despite the initiation of erosion at our study sites broadly coinciding with the arrival of Iron Age settlers in some parts of present‐day South Africa, it seems unlikely that land use changes at this time would have been sufficiently intense and widespread to have caused unprecedented erosion across such a broad geographical area …”

The three studies were done around Blood River in KwaZulu-Natal, Modder River in the Free State and Moopetsi River in Limpopo.

Rather than being driven by human impacts, they propose that erosion at these three study areas was triggered by abrupt hydroclimatic oscillations during the Medieval Climatic Anomaly (~AD 900-1300) possibly involving heavy rainfall and floods after drought, with further erosion occurring during the generally cool and dry Little Ice Age (~AD 1300-1800) in response to climate‐driven, large floods.

“In other areas of South Africa, clear links between land mismanagement and soil erosion have been demonstrated, but for sites where detailed investigations have yet to be undertaken, these findings challenge an often default assumption that soil erosion is necessarily attributable to human factors. Our findings have significant implications for soil erosion control strategies and assessment of South African dryland landscape response to future climate changes.”

Significantly, their conclusions were partly shaped by the use of optically stimulated luminescence (OSL) dating, a technology that helps to determine the length of time that quartz sediments have not been exposed to sunlight.

In this way, they were able to more accurately date sediments washed further downstream by gully erosion.

Elaborating in a more recent commentary article, Prof Stephen Tooth of Aberystwyth University says it is important to avoid an oversimplification of the previous study by Lyons and to also outline some of the more nuanced aspects of the commonly polarised “human versus natural causes” debate.

“In reality, ‘human’ and ‘natural’ causes are catch-all terms for various activities and factors that can interact in complex ways. In some areas of the interior, different human activities may be the dominant cause of erosion, while various natural factors may be the dominant cause elsewhere, but rarely do these two sets of factors operate independently.”

He further emphasises that while gully and other forms of soil erosion have many negative impacts, some other human activities (such as mining) are moving earth materials in much greater volumes and at greater rates.

Also, a clearer understanding of the causes of erosion (especially the relative importance of human and natural factors) was essential to prioritise the limited funds available for erosion control and sustainable land management initiatives.

“Once gullies have been initiated and are actively eroding, they are very difficult to stabilise. Where human land use (eg ill-designed ploughing regimes, overgrazing, excessive burning) is indisputably causing new gullies or worsening existing gully erosion, the cessation of activities and erosion control efforts (eg revised land-use strategies and revegetation efforts) may have some chance of success.

“But where gullies initially formed in response to natural factors occurring many decades or hundreds of years ago and are not now rapidly expanding, then erosion control efforts may be futile or worse.”

For example, there was evidence that some gabion weirs installed in the Blood River dongas may have encouraged water retention on soils prone to erosion, leading to erosive water flow around the sides of such weirs and localised donga widening.

“With the approach of the 20th anniversary of the United Nations’ International Year of Deserts and Desertification (2006), there is an opportunity to reflect critically on the targeting and effectiveness of erosion control and sustainable land management efforts, particularly in view of projected increases in climate variability and growing land-use pressures.” Tooth concludes. DM