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High resolution measurements of soil surface roughness response to wind erosion and rainfall

Joanna Bullard 1, Helene Aubault1, Annie Ockelford2, Craig Strong3
1Loughborough University, Loughborough, UK, 2University of Brighton, Brighton, UK, 3Australian National University, Canberra, Australia

Soil surface roughness (SSR) is a crucial parameter controlling wind erosion through impacting the threshold wind velocity and the surface storage of loose erodible material. SSR in the vertical (z) axis is typically measured in centimetres, but many soil surfaces in arid areas comprise fine sediments <2 mm in diameter (sands, silts, clays) and consequently have limited vertical expression at small scales. These soils are often characterised by physical and biological crusts which further smooth the surface and consequently SSR is more appropriately measured at micron to millimetre scales. In addition to soil texture and in situ soil organism growth, processes such as saltation bombardment, raindrop impact and, particle aggregation, can also influence soil surface roughness. Understanding how SSR changes in response to wind and water erosion processes is crucial to better prediction of erosion. This study presents high resolution measurements of soil surface roughness (z = 15 μm) for two soil textures (loamy sand and sandy loam) with three surface types (physical structural crust, light biological crust, multi-species biological crust). All crusts received 8 mm simulated rainfall followed by simulated wind erosion (with abrasion). Soil surface roughness was measured using a laser profiler (spatial resolution xy = 80 μm) across a 100 x 100 mm quadrat of treated crust. Laser measurements were made on the same quadrat before and after both rainfall and wind erosion simulation. A detrended measure of SSR - random roughness - was calculated for the quadrat at each simulation stage. Multi-species biological crusts were the most stable presenting the least change in SSR after the erosion simulations. SSR of both the physical and light biological crusts increased following saltation bombardment and were also susceptible to raindrop impact. The results highlight the importance of biological crusts in stabilising dryland soils against erosion and the high resolution measurements make it possible to identify small scale spatial patterns of soil erodibility.