Abstract
Abstract. Gullies lead to land degradation and desertification as well as increasing environmental and societal threats, especially in arid and semiarid regions. Despite this fact, there is a lack of related research initiatives. In an effort to better understand soil loss in these systems, we studied small permanent gullies, which are a recurrent problem in the Brazilian northeastern semiarid region. The increase in sediment connectivity and the reduction of soil moisture, among other deleterious consequences, endanger this desertification-prone region and reduce its capacity to support life and economic activities. Thus, we propose a model to simulate gully-erosion dynamics, which is derived from the existing physically based models of Foster and Lane (1983) and Sidorchuk (1999). The models were adapted so as to simulate long-term erosion. A threshold area shows the scale dependency of gully-erosion internal processes (bed scouring and wall erosion). To validate the model, we used three gullies that were over 6 decades old in an agricultural basin in the Brazilian state of Ceará. The geometry of the channels was assessed using an unmanned aerial vehicle and the structure from motion technique. Laboratory analyses were performed to obtain soil properties. Local and regional rainfall data were gauged to obtain sub-daily rainfall intensities. The threshold value (cross-section area of 2 m2) characterizes when erosion in the walls, due to loss of stability, becomes more significant than sediment detachment in the wet perimeter. The 30 min intensity can be used when no complete hydrographs from rainfall are available. Our model could satisfactorily simulate the gully-channel cross-section area growth over time, yielding a Nash–Sutcliffe efficiency of 0.85 and an R2 value of 0.94.
Highlights
With a view to sustainable development and environmental conservation, soil erosion by water has been emphasized as a key problem to be faced in the 21st century (Borrelli et al, 2017; Poesen, 2018)
When the parameter λ is introduced (FLM-λ), the output cross sections are modelled with piled rectangles, as in Fig. 8d, e and f
It is important to highlight that the model can produce triangular geometry, but this was observed in this study
Summary
With a view to sustainable development and environmental conservation, soil erosion by water has been emphasized as a key problem to be faced in the 21st century (Borrelli et al, 2017; Poesen, 2018). The impact of water-driven soil erosion on the economy and food supply alone represents an annual loss of between USD 8 and 40 billion, a cut in food production of 33.7 million tonnes and a 48 km increase in water usage. Estimates of the total investments to mitigate land degradation effects on-site (e.g. productivity losses) and their off-site effects (e.g. biodiversity losses and water body siltation) lead to more alarming values, averaging USD 400 billion annually (Nkonya et al, 2016) These values have been obtained in soil loss studies using the USLE (Universal Soil Loss Equation) or similar methods, with none considering gully erosion. The real economical and social impacts of soil erosion cannot be completely comprehended unless we can better understand gully erosion and how to model it
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