Elsevier

CATENA

Volume 85, Issue 1, April 2011, Pages 34-47
CATENA

Effect of ground cover on splash and sheetwash erosion over a steep forested hillslope: A plot-scale study

https://doi.org/10.1016/j.catena.2010.11.005Get rights and content

Abstract

The contributions and relationships of erosion by splash and overland flow over a steep slope in a Japanese beech forest in plots with different percentages of ground cover were examined. Three erosion plots (2 m wide × 5 m long) with average understory coverage of 1%, 45%, and 94% were installed. Sediment transported by rain splash and by overland flow was sampled separately. For the plots with sparse, moderate, and high understory coverage, the average proportions of splash soil to total soil erosion during the monitoring period were 16%, 32%, and 18%, respectively. A significant correlation between the amount of splash soil and precipitation was found in the plots with 1% and 45% understory coverage, whereas no statistical relationship was identified for the plot with high understory coverage. At the sparse ground cover plot showing the largest sediment movement, the contribution of splash transport decreased with increasing precipitation. The amount of sheetwash erosion was significantly correlated with the amount of splash soil under the condition of sparse ground cover. This relationship was more pronounced during high precipitation events and the rainy season. Splash contribution to the sediment transport was in the range of 0.8%–76.7%, 2.8%–81% and 2.1%–60.8% for plot with high, moderate and low ground cover, respectively. The sparse ground cover showed the largest variation of splash and sheetwash contribution in soil erosion. This variability was due to variation in ground cover and soil surface wetness condition which led to a variation of detachment and non-linear relationship of sheetwash splash.

Research Highlights

► Separate measurements of rain splash and sheetwash erosion. ► Measurements were carried out under natural rainfall conditions. ► Clarifying the effect of ground cover on hillslope soil erosion mechanisms (sheet wash and splash). ► Quantitatively clarifying the importance of rain splash and sheet wash erosion. ► Conceptualization of downslope rain splash erosion processes.

Introduction

Understanding soil detachment by precipitation drops is an important first step in clarifying soil erosion processes on hillslopes (e.g. Young and Wiersma, 1973, Abrahams and Parsons, 1991, Van Dijk et al., 2003, Kinnell, 2005). Numerous studies have indicated that soil erosion processes involve detachment by both falling raindrop impacts and overland runoff, followed by the movement of detached particles by shallow overland runoff (Meyer et al., 1975, Nearing et al., 1989, Kinnell and Wood, 1992, Kinnell and Cummings, 1993, Zhang et al., 1998, Foster, 1990, Huang, 1998, Foster, 1982, 1984; Morgan, 2001, Nord and Esteves, 2005). Complex interactions of splash erosion, overland flow erosion, and other factors may affect erosion processes. Particles detached by raindrops can be transported by splashing or overland flow. For interrill erosion, overland flow during moderate precipitation may not have enough flow depth and consequently enough power to break down the cohesive force between soil particles and then transport detached particles (Emmanuel and Dunne, 2003).

Furthermore, although the subject of fewer studies, sediment movement by splashing has also been found to be important in the interrill erosion process of forested steep landscapes (Miura et al., 2003). Sidle et al. (2007) showed that a significant amount of storm water movement occurred in the near-surface soil layer of a forested hillslope with sparse ground cover (Gc). Sparse cover could result from inappropriate forest management or overgrazing of understory vegetation by domestic or wild animals. When overland flow is very shallow, not only soil detachment but also splash-induced soil movement (sediment transport by the splash mechanism) may play significant roles in the sediment transport of the hillslope. Furthermore, in cases of shallow overland flow, the flows are often not uniform. Therefore, to examine erosion processes, it is necessary to clarify the importance of splash and sheetwash erosion (transported sediment by unconcentrated overland flow) by sampling each process separately. Recent laboratory experiments have advanced our understanding of the physics of splashing (Furbish et al., 2007, Ma et al., 2008). However, knowledge of splashing on steep forested hillslopes is limited. Although previous studies have examined relationships among various rainfall indices, overland runoff, ground cover, and soil erosion (Marston, 1952, Cerdà, 1998, Cantón et al., 2001), and Miyata et al. (2009) recently increased our knowledge of ground cover effects on hillslope erosion processes, few studies have examined splash and sheetwash erosion separately (within an event) with regard to their relationships to rainfall, runoff, and ground cover as well as their relationship to each other.

In forested landscapes, ground cover reduces soil erosion and overland flow (Marston, 1952, Hattori et al., 1992, Zhou et al., 2002, Miura et al., 2002, Miura et al., 2003, Kawana et al., 1963, Murai and Iwasaki, 1975, Murai and Iwasaki, 1976, Yoshimura et al., 1981, Yoshimura et al., 1982). Because the size distribution of throughfall drops is greater under the canopy (Chapman, 1948, Tsukamoto, 1966, Nanko et al., 2006) rain splash erosion could be an important erosion mechanism. But still there is not enough knowledge of soil erosion in interrill area of forest floor (Miyata et al., 2009).

There are previous studies of ground cover effect on soil erosion, but those were not examining splash and sheetwash process separately. Miura et al. (2003) reported the dominance of splash in mass transport of steep forested hillslope without separating splash and sheetwash erosion.

There are laboratory or field experiments with simulated rainfall that examined splash and sheetwash erosion by comparing the results of experimental sets which one of rainfall or overland runoff was isolated (e.g. Hudson, 1957, Ziegler et al., 2000, Rouhipour et al., 2006). It is rare to find previous works on separate sampling of splash and sheetwash by field experiments under natural rainfall. Though Young and Wiersma (1973) in a laboratory experiment with simulated rainfall sampled splash and sheetwash separately on a low slope (9%). Sutherland et al. (1996) in a laboratory experiment with small plots (0.18 m2) separated splash and sheetwash on slopes up to 20°. Separately studying splash and sheetwash with regard to ground cover effect was neglected in previous researches of field experiment on natural hillslopes (Hattori et al., 1992, Miura et al., 2002, Miura et al., 2003, Kawana et al., 1963, Murai and Iwasaki, 1975, Murai and Iwasaki, 1976, Yoshimura et al., 1981, Yoshimura et al., 1982, Miyata et al., 2009).

It is commonly accepted that downslope sediment splash transport is high over steep slopes. Although now the contribution of splash in sediment transport processes is well known, we do not know what is the quantitative contribution of splash and sheetwash in total sediment transport in our study area. How are their relationships and variations with changing of ground cover?

The objectives of this study were to examine a) sediment production by rain splash erosion in relation to totally produced sediment to show relative importance of splash erosion, b) the relationship between splash and sheetwash erosion, and c) the function of forest floor cover over a steep hillslope in relation with splash and sheetwash mineral soil material movement.

We conducted field monitoring of soil erosion by splash and sheetwash mechanisms under natural precipitation in steep hillslope plots with different percentages of ground covers (including understory vegetation and litter). To examine the interaction between splash and sheetwash, we sampled and measured splash and sheetwash soil material separately. The relationships of storm precipitation characteristics and overland flow to splash and sheetwash erosion were investigated to identify factors affecting soil loss from the plots. The findings of this study will provide more knowledge of splash contribution to sediment transport over steep forested landscapes in which ground cover is sparse due to forest management or overgrazing by animals. Clarifying the relative importance of splash and sheetwash can help to get a better evaluation of steep hillslope erosion. Various classes of understory coverage were examined to identify the role of the forest floor in soil erosion and the results can be applied to soil conservation planning and modeling.

Section snippets

Methods and materials

A number of experiments have already been applied to measure soil detachment rate and splash transportation by raindrops. These methods mainly involved the use of cups and trays. Salles and Poesen (2000) used a splash cup technique in a laboratory experiment to determine the mass of detached sediment. Nanko et al. (2008) also used cups to measure the rate and mass of splash erosion. Van Dijk et al. (2003), in a study in West Java, Indonesia, measured the splash mass and rate on terrace beds and

Characteristics of precipitation

During the monitoring period from May to November 2007–2009 for plot 3 and May to November 2007 and May to August 2008 for plots 1 and 2, total precipitation of 2007, 2008 and 2009 was 1980 mm, 2275 mm and 2042 mm respectively, 64%, 38% and 28.7% of which occurred in the typhoon season from July to November respectively for 2007, 2008 and 2009. Intense and heavy precipitations (1264 mm, 862 and 587 mm of total precipitation for 2007–2009) were recorded during typhoons. Large storm events occurred

Conclusion

Plot-scale contributions and relationships of sheetwash and splash soil erosion on a steep forested hillslope were investigated during 2007 to 2009. In the field monitoring, we sampled erosion by rain splash and overland flow separately. The findings of this study are summarized as follows: (1) Considerable soil splash transport occurred in hillslopes with sparse understory vegetation. (2) The contribution of splash transport to total sediment movement mainly depended on vegetation and litter

Acknowledgments

This study was partially funded by the Kanagawa Prefecture Natural Environmental Center. We appreciate four anonymous reviewers and the editor, for their comments. We thank the graduate students of Tokyo University of Agriculture and Technology, Laboratory of soil erosion and hydrology for their assistance through field works.

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