Surface Roughness Evolution of Soils Containing Rock Fragments

Abstract

Soil surface roughness is a dynamic property which determines, to a large extent, erosion and infiltration rates. Although soils containing rock fragments are widespread in the Mediterranean region, the effect of the latter on surface roughness evolution is yet poorly understood. Therefore, laboratory experiments were conducted in order to investigate the effect of rock fragment content, rock fragment size and initial moisture content of the fine earth on the evolution of interrill surface roughness during simulated rainfall. Surface elevations of simulated plough layers along transects of 50 cm length were measured before and after simulated rainfall (totalling 192.5mm, Z = 70mm h-') with a laser microreliefmeter. The results were used to investigate whether systematic variations in interrill surface roughness along stony hillslopes in southeastern Spain could be attributed to rock fragment cover and rock fragment size. Soil surface elevations were measured along the contour lines (50 cm long transects) with a contact microreliefmeter. Roughness was expressed by two parameters related to the height and frequency of roughness elements, respectively: standard deviation of de-trended surface elevations (random roughness: RR), and correlation length (L) derived from exponential fits of the autocorrelation functions.The frequently used assumption that surface roughness (RR) of cultivated topsoils decreases exponentially with cumulative rain is not valid for soil surfaces covered by rock fragments. The RR of soils containing small rock fragments (1.7-2.7 cm) increased with cumulative rainfall after an initial decrease during the first 17.5 mm of rainfall. For soils containing large rock fragments (7.7 cm), RR increased with rainfall above a threshold rock fragment content by mass of 52 per cent. For a given rainfall application, RR increased non-linearly with rock fragment content. The correlation length for soils containing small rock fragments decreases with rock fragment content and is significantly lower than for soils with large rock fragments. Soils covered with small rock fragments (large RR and small L) are thus well protected against raindrop impact by a water film in the depressions between the rock fragments.On abandoned agricultural fields along hillslopes in southeastern Spain, rock fragments cover increases non-linearly with slope owing to selective erosion of finer particles on steep slopes. The increase of surface cover by large rock fragments (>25 mm) is even more pronounced. The simultaneous increase of rock fragment cover and rock fragment size with slope explains the non-linear increase of RR with slope. These relationships differ for soils covered by platy misaschists and those covered with cubic andesites. The variations in correlation length along the hillslopes are not clear, probably owing to a simultaneous increase in rock fragment cover and rock fragment size. These findings may provide a better prediction of soil surface roughness of interrill areas covered by rock fragments using s...

Soil loss and run-off in young forest stands as affected by site preparation technique: a study in NE Portugal

“…Plots responded according to disturbance intensity for either run-off or soil loss, although much more expressively in the latter case. Changes in erosion rates, or related soil properties, with time since tillage induced disturbance are reported in literature for many environments, soil use types and management systems, including forests on marginal land (Dissmeyer and Foster 1984;van Wesemael et al 1996;Bresson et al 2006). It seems that structural rearrangements following tillage, as affected by rains falling onto the ground had a much faster consequence to soil particles removal than to run-off generation and development.…”

“…Ground surface profiles obtained, two per microplot, allowed computing random roughness (RR) as the standard deviation of detrended elevations (Allmaras et al 1966;van Wesemael et al 1996), with trend determined by linear regression over the entire microplot length or over the two separate sections of the profile, in the case of treatments with contour bunds (Mo_D1, Mo_D2 and Hi_D1). Slopes of the linear fit, in %, are adopted as the ground local slope gradient and pooled to compute average slope gradient of experimental plots and blocks.…”

The effects of rainfall intensity and rock fragment cover on soil hydrological responses in Central Chile

“…Our results also agree with those presented by Wang et al (2012), regarding the delay in the initiation of surface runoff produced by increasing rock fragment cover area, regardless of rainfall intensity. This decrease and delay on initiation of surface runoff can be attributed to increased soil surface roughness, reduced crosssectional area and increased tortuosity produced by rock fragments on the soil surface (Van Wesemael et al, 1996;Cerdá, 2001;Wang et al, 2012), which in turn increases detention of surface runoff. According to Rieke-Zapp et al (2007), rock fragments increase hydraulic roughness and friction, leading to decreases in surface runoff and sediment accumulation.…”

Effects of surface rock fragment characteristics on interrill runoff and erosion of a silty loam soil

“…The general trend found agrees with that reported by Poesen and Lavee (1991), even though the small rock fragments of their experiments were larger than those tested in our experiment. The effect of rock fragment size was also found to be important with respect to soil compaction and soil surface roughness evolution during rainfall (van Wesemael et al, 1994Wesemael et al, , 1996.…”

“…The effects of size of rock fragments on overland flow and interrill sediment yield were studied by Poesen and Lavee (1991) and Lavee and Poesen (1991) under indoor rainfall simulation conditions. Also under laboratory conditions, van Wesemael et al (1994) and van Wesemael et al (1996) studied the effects of size of rock fragments on soil compaction and soil surface roughness changes during rainfall. These two soil physical aspects control to a certain level the intensity and the extent of rainfall erosion processes.…”