Historical evolution of gullies: Impact of climate

Abstract

Gully erosion is regarded as one of the worst land degradation processes in the world. It is a complex geomorphic process resulting in on-site land degradation due to the removal of soil, and off-site land degradation, due to mobilised soil, pollutants, and pesticides adversely affecting soil and water resources. Climate change predictions show that the frequency of high-magnitude rainfall events will increase, thereby exacerbating the degradation impacts of gullying. By assessing long-term datasets (>30 years), the relationship between gully evolution and climate variability can be observed, providing potential insight into gully evolution under climate change. We aim to isolate climate as a driving factor by investigating three sites under similar land-use, environments transformed into conservation areas but exhibiting contrasting climates. The sites are in South Africa (SA), located in the arid Karoo, a Lowveld area with Savanna land cover and a humid Grassland region. A triangulation of methods was implemented, including i) digitising gully change from remotely sensed imagery spanning up to 78 years, ii) conducting field measurements, and iii) interviewing land managers. In the arid Karoo, gullies were discontinuous, forming successive chains waning into deposition zones, only to be reactivated again with an abrupt headcut downstream of the depositional area. Continuous gullies were found in the Grassland and Savanna regions. Remotely sensed image analysis shows a mean annual linear expansion along the main drainage line was the largest in the Savanna, 2.8 m, compared to 0.5 m and 0.4 m recorded in the Grassland and Karoo. However, the mean planimetric area changes of gullied extent were 6 and 8.4 times larger in the Grasslands compared to the Karoo and Savanna. Fieldwork confirmed active gully processes at all three sites, identifying active sub-surface processes, only found in the Grassland, as the leading cause for the morphological difference between the extensive Grassland gullies and the narrower linear gully features in the Savanna and Karoo. However, certain gully headcuts in the Karoo present as bulbous transforming into narrower downstream channels, most likely a result of being artificially enlarged from failed gabion installations at the headcut. During interviews with land managers, concerns about contemporary gully erosion were noted at all three sites, with the implementation of mitigation measures ongoing. There is consensus among the three methodologies, identifying that gully erosion remains stubbornly active, even after transforming to a conservation-orientated land-use. In the Karoo and Savanna, human influence and rainfall variability were attributed as causal factors of gullying, inherited from ill-placed farm roads and overgrazing, respectively. In the Grasslands, soil type is deemed the dominant driver of gullying, although animal tracks (specifically those from antelope) forming pathways for water after rainfall was contentious. Our analysis shows that gully erosion severity follows the climate gradient of SA. However, it remains difficult to completely isolate climate as a driver due to different inherent soil properties, geology, and slope. We expect an increase in gully erosion for all three sites under climate change predictions, although more pronounced in the Grassland region.