Gully cut‐and‐fill cycles as related to agro‐management: a historical curve number simulation in the Tigray Highlands

Abstract

Gully cut‐and‐fill dynamics are often thought to be driven by climate and/or deforestation related to population pressure. However, in this case‐study of nine representative catchments in the Northern Ethiopian Highlands, we find that neither climate changes nor deforestation can explain gully morphology changes over the twentieth century. Firstly, by using a Monte Carlo simulation to estimate historical catchment‐wide curve numbers, we show that the landscape was already heavily degraded in the nineteenth and early twentieth century – a period with low population density. The mean catchment‐wide curve number (> 80) one century ago was, under the regional climatic conditions, already resulting in considerable simulated historical runoff responses. Secondly, twentieth century land‐cover and runoff coefficient changes were confronted with twentieth century changing gully morphologies. As the results show, large‐scale land‐cover changes and deforestation cannot explain the observed processes. The study therefore invokes interactions between authigenic factors, small‐scale plot boundary changes, cropland management and sociopolitical forces to explain the gully cut processes. Finally, semi‐structured interviews and sedistratigraphic analysis of three filled gullies confirm the dominant impact of (crop)land management (tillage, check dams in gullies and channel diversions) on gully cut‐and‐fill processes. Since agricultural land management – including land tenure and land distribution – has been commonly neglected in earlier related research, we argue therefore that it can be a very strong driver of twentieth century gully morphodynamics. Copyright © 2014 John Wiley & Sons, Ltd.

Gully prevention and control: Techniques, failures and effectiveness

“…permanent gullies) usually develop according to a ‘cut‐and‐fill’ cycle. Such cycles have been shown to be a recurrent phenomenon in many regions, taking typically 50–100 years to complete (Nachtergaele et al ., 2002; Porter and An, 2005; Vanwalleghem et al ., 2005; Larsen et al ., 2013; Lanckriet et al ., 2015). Similar mechanisms have been reported for ‘erosion cells’ in central Australia (Bourke and Pickup, 1999).…”

Morphological dynamics of gully systems in the subhumid Ethiopian Highlands: the Debre Mawi watershed

“…The mechanical actions of the flowing water can result in a rapid mass movement in the gullies by undercutting of the banks (see Fig. 1; Lanckriet et al, 2015). When these mechanical actions at the gully head exceed the cohesive strength of soil, erosion proceeds upslope through a headward cutting gully (Muñoz-Robels et al, 2010).…”

“…1d). Soil saturation by a rising water table decreases the soil shear strength (Poesen, 1993;Langendoen and Simon, 2008) and therefore destabilizes banks (Simon et al, 2000;Langendoen et al, 2013). Active gully networks are therefore predominantly found in the saturated valley bottomlands (Tebebu et al, 2010;Steenhuis et al, 2014), and the deepest and the most spectacular gullies occur in the bottom of the watershed, where, in subhumid monsoonal and wetter climates, the soil becomes saturated starting around the middle of the rain phase and then remain saturated until the end of the rain phase .…”

Measuring, modelling and managing gully erosion at large scales: A state of the art

“…Likewise, gully channels can become significant sediment traps and fill-up over time, especially when they are well vegetated (e.g. Vanwalleghem et al, 2005c;Lanckriet et al, 2015;Molina et al, 2009). Furthermore, they can be of significant geo-archeological value, providing important insight into (pre-)historic land use and human occupation (e.g.…”