Effect of antecedent conditions and fixed rock fragment coverage on soil erosion dynamics through multiple rainfall events

Abstract

The effect of antecedent conditions and specific rock fragment coverage on precipitation-driven soil erosion dynamics through multiple rainfall events was investigated using a pair of 6-m × 1-m flumes with 2.2% slope. Four sequential experiments -denoted E1, E2, E3 and E4, involved 2-h precipitation (rates of 28, 74, 74 and 28 mm h -1 , respectively) and 22 h without rainfall -were conducted. In each experiment, one flume was bare while the other had 40% rock fragment coverage. The soil was hand-cultivated and smoothed before the first event (E1) only, and left untouched subsequently. Sediment yields at the flume exit reached steady-state conditions over time scales that increased with sediment size. Experiments were designed such that both steady and non-steady effluent sediment yields were reached at the conclusion of E1. Results from subsequent experiments showed that short-time soil erosion was dependent on whether steadystate erosion was achieved during the preceding event, although consistent steady-state effluent sediment yields were reached for each sediment size class. Steady-state erosion rates were, however, dependent on the rainfall intensity and its duration. If steady-state sediment yields were reached for a particular size class, that class's effluent sediment yield peaked rapidly in the next rainfall event. The early peak was followed by a gradual decline to the steady-state condition. On the other hand, for size classes in which steady state was not reached at the end of the rainfall event (i.e., E1), in the following event (E2), the sediment yields for those classes increased gradually to steady state, i.e., the sharp peak was not observed. The effect of rock fragment cover (40%) on the soil surface was also found to be significant in terms of the time to reach steady state, i.e., their presence reduced the time for steady conditions to be attained. Effluent sediment yields for the bare and rock fragment-covered flumes (E1) showed steady conditions were reached for the latter, in contrast to the former. We used the Hairsine-Rose (H-R) model to simulate the experimental data as it explicitly models soil particle size classes. Experiments E1 and E2 involved soil compaction by raindrops, and in this case the model predictions were found to be unsatisfactory. However, compaction was effectively completed by the end of experiment E2, and the model provided reasonable predictions for experiments E3 and E4.

Solute and sediment transport at laboratory and field scale: Contributions of J.-Y. Parlange

“…Jomaa et al [2012b] HR model adjusted taking the rock fragments cover into account and tested using experimental data. Jomaa et al [2013] Antecedent soil conditions' effects on soil erosion investigated and the HR model tested through multiple rainfall events.…”

Linear scaling of precipitation-driven soil erosion in laboratory flumes

“…For example, for experiment H7-E2, the early peak of sediment concentrations for three largest size classes (> 100 µm) disappeared for bare soil conditions (Flume 1) compared with the surface-protected flume (Flume 2). This is due to the different antecedent conditions (H7-E1 was followed by 22 h of air drying) where surface sealing, compaction and roughness did not develop similarly for both flumes (Jomaa et al, 2013). The presence of surface rock fragments on Flume 2 increased the water depth due to the reduction of cross-sectional area available for flow, which increased the infiltration rate and reduced detachment of the soil surface (raindrop detachment is attenuated by the increased water depth).…”

“…For experiments H6, H7-E1-E4, and H8, Flume 1 was bare soil while Flume 2 was covered by surface rock fragments (Table 1). Jomaa et al (2012bJomaa et al ( , 2013 While noting that there are exceptions for some size classes, this proportionality appears to hold also under unsteady conditions, i.e., for the entire erosion event. In particular, Figs 2, S1…”

“…Previously, we modelled experiments H6, H7-E1, H7-E2, H7-E3 and H7-E4 using the Hairsine-Rose model (Hairsine and Rose, 1991;Jomaa et al, 2013;Jomaa et al, 2012b;Rose et al, 1983a,b), where the shielding effect of rock fragment cover was considered. The linear scaling model deduced from the area-based predictions presented here was compared with these existing results (plots not shown).…”

“…Four sequential experiments, denoted H7-E1, H7-E2, H7-E3, and H7-E4 are taken from Jomaa et al (2013;2012b), and experiment H6 from Jomaa et al (2012b). Experiment H6 used two flumes, H6-F1 (bare soil) and H6-F2 (20% rock fragment coverage), each subjected to 3 h precipitation at a rate of 74 mm h -1 .…”