4.3 Effect of raindrop diameters on microscopic characteristics
splashed soil aggregates
The microstructure of soil aggregates was different in two and three
dimensions after raindrop splash (Figure 2). The splashed soil,
especially by large raindrops, displayed an obvious fragmentation of
aggregates (Figure 2D). This result indicated that the main cause of
aggregate fragmentation was large raindrops. It is easier to form a
structure consisting of large aggregates after drying. The formation of
surface crust depended on the degree of surface aggregate fragmentation
and the stability of the soil structure (Wick et al., 2016; Gelaw et
al., 2015). The splashed soil aggregate particle size was smaller than
that of the undisturbed soil (Table 2). This result is consistent with
that of Ramos et al. (2003), in which the fragmentation of aggregates
produced smaller particles than the original soil, allowing the soil to
be displaced and reoriented into a more continuous structure; this
consequently clogged pores and formed a surface crust. Sajjadi and
Mahmoodabadi (2015) found that soil containing finer particle aggregates
had a higher transportability of preseparated particles than larger
aggregates. Moreover, as particle size decreased and the texture became
finer, the pore structure of the soil became denser. The impact of
raindrops, which caused physicochemical compaction and dispersion of the
surface soil, was the main reason for crust formation and reduction of
the infiltration rate (Assouline et al., 2004; Fu et al., 2017). In this
study, larger raindrop diameters resulted in greater corresponding
rainfall intensity and energy. As a result, soil tends to form a surface
crust after heavy rains. The formation of a soil surface crust not only
further reduces porosity (Pagliai et al.,2004) but also reduces surface
roughness, thereby exacerbating surface runoff and soil loss (Robinson
and Phillips, 2001; Assouline and Ben-Hur, 2006).