Reply to comment on “Statistical and physical analysis of soil detachment by raindrop impact: Rain erosivity indices and threshold energy” by M. Styczen

Abstract

[1] Salles et al. [2000] conducted laboratory experiments to study the effects of various simulated rain properties on soil detachment rates due to raindrop impact. Two soils materials were tested: i.e., a cohesive silt loam and a noncohesive fine sand. A statistical analysis was made in order to evaluate which rain parameter best predicts the splash detachment rate. For both soils the product of raindrop momentum and raindrop diameter yields a slightly higher correlation with splash detachment rate than all other tested rain parameters. [2] The mass of detached sediment was measured with the splash cup technique. During the experiments with the silt loam we observed that after the splash cup was exposed to the rain, some sediment was sticking to the outer wall of the splash cup. This sediment was transported from the soil surface by the shallow overland flow occurring when the rain intensity exceeded the infiltration capacity of the silt loam soil. Therefore two erosion parameters were measured: i.e., the rate of raindropdetached silt loam transported by aerial splash (Ds) and the rate of raindrop-detached silt loam transported by surface wash (Df). The related processes are as follows. The thin water layer appeared at the soil surface in the splash cup (the water depth never exceeded 1–2 mm) when rain intensity exceeded infiltration rate, causing some runoff from the splash cup. Since the soil surface within the splash cup was horizontal, the detachment capacity of the runoff was negligible. Nevertheless, owing to the thin water layer, some of the splashed material was splashed subaqueously to very short distances (smaller than the splash cup size) and trapped by the water layer. The subaqueously splashed sediment could already be detached by raindrop impact and offered less resistance to detachment by the subsequent raindrops that impacted the soil surface or to transport by the runoff. [3] According to our objectives (i.e., to determine the rate of detached sediment by raindrop impacting a bare soil surface), we consider that the sum of the two components that we first separate as Ds and Df is the most representative of total sediment detached by raindrop. [4] Styczen’s comments discuss the inappropriateness of combining Ds and Df when conducting a physical analysis of soil detachment. First of all, Styczen considers the process of splash erosion on a noncohesive soil to be different compared with a cohesive soil: The first process is assumed to be an elastic collision, whereas the second one is considered to be an inelastic collision. The first assumption does fit with observations reported in the literature. The kinetic energy of a falling raindrop is much larger than the sum of kinetic energies of the splashing droplets even for sandy topsoils, as shown by several authors, indicating that 80–95% of the incoming raindrop energy is being dissipated in the impact area (i.e., transformation into health, noise, deformation of the surface during crater formation, and change of soil water potential) [e.g., Zaslavsky, 1970; Van Asch and Roels, 1979; Mutchler and Young, 1975; Ghadiri and Payne, 1980]. Therefore, irrespective of the soil cohesion, splash erosion would be better considered as an inelastic collision. [5] Styczen and Hogh-Schmidt [1988] produced a relationship for estimating soil detachment including two types of parameters: soil dependent and rain dependent parameters (see equation (1) of Styczen’s comments). The raindependent parameter is the square raindrop momentum DV 2 with the notation we used in our paper. One of the soil dependent parameters is Pr. It is ‘‘the probability that detached material becomes measurable as splash (that it moves out over the edges of the splash cup or into a moving water layer)’’ according to Styczen’s definition. With respect to our experiment this definition includes the subaqueously splashed sediment transported in the thin water layer. We agree that Pr is soil dependent and probably varies within a rain event. We did not particularly check this aspect during our experiments. Nevertheless, for some of the rain intensity values tested, the duration of rain exposure varied between replicates. No discrepancies appeared between the rates derived from these replicates and variations in Ds could not be demonstrated from our results. [6] According to Styczen, Ds is more appropriate to represent raindrop splash. She assumed that the measurements describing pure splash erosion for the silt loam would be best predicted if one uses a function of (drop diameter). Copyright 2002 by the American Geophysical Union. 0043-1397/02/2000WR001577$09.00