321 
,nd cor res 
in the 
:e occurs 
and sub— 
In the second case saturation of a shallow upper horizon of the soil 
profile plays a role. A relatively impermeable layer occurs at shallow depth. 
The erosion will start soon after the rains have largely saturated the 
upper part of the profile. After this interval, increasing runoff will occur 
with increasing distance from the divide. (Weyman 19735 Carson et.al. 1972) 
ergsma 1970)> 
•elief. 
The horizon which is relatively impermeable is in many cases the plough- 
sole. In other cases a horizon may have the function of a relatively 
impermeable horizon. 
ion types 
strong 
yse it 
slope the 
ions of 
Lateral flow in the upper part of the soil profile can give a significant 
contribution to the saturation of profiles lower down the slope (Hewlett 1961 , 
Hewlett and Hibbert 1967 )* These lower places where runoff starts will occur 
in a contour zone, or.in laterally concave surface topography (seepage 
lines, percolines, Young 1972). 
The case of saturation of a deep layer of the profile above a rela 
.al tillage, 
tore often 
ist case the 
if the animals. 
tively impermeable horizon is considered one of the cases of the saturation 
overland flow (Young 1972). 
Saturation overland flow occurs when a rising watertable presents 
limitations to the infiltration. When the watertable which limits the infil 
tration of runoff is related to a stream or valleybottom, the overland flow 
i, for 
.fferences 
will occur in certain positions in the landscape. This may play a part in 
the formation of the lower part of an erosion toposequence.. 
r give a 
lion to a 
In many cases no erosion toposequence can be observed. Where no regularity 
in the occurrence of the erosion features is present this is in some cases 
due to the frequent interruption of the slope by field boundaries, conser 
.1 erosion 
isures. 
vation terraces, farm compounds, roads across the slope, etc. The regular flow 
of the runoff is prevented by the field lay out and conservation measures. 
'y much, the 
m he 
Land use may play a similar role. Plots of forest, with a good soil 
surface cover, grass fields and crops which provide protection during at 
least an important part of the erosive rainfall can strongly reduce the 
erosion and interfere with a regular erosion toposequence. 
¡ometimes 
Local 
¡ytone 
In other cases the absence of an erosion sequence is to be attributed 
to the irregularity of factors determining the erosion conditions, like 
ihe slope 
es may he 
gullies 
r between 
relief, parent material and soil profile. When reliefforms are varying much, 
and hill slopes have different forms, or where the influence of the sub 
stratum does not follow the contour, an erosion toposequence will be absent. 
Theoretical considerations to explain the occurrence of an erosion sequence. 
Le divide 
i, resulting 
ition of a 
The explanation of the occurrence of an soil erosion sequence is based 
firstly on the work of Horton (1945)» Starting from a case of certain simple 
conditions of climate, topography, soil and soil cover, he developed a formula 
that shows that the erosive power of the surface runoff from rainwater increases 
with the distance from the divide. This erosive power depends also on the depth 
in the 
h 
.ty of 
of flow, the inclination of the land and the roughness of the surface. 
The forces acting against the erosion are related to the coherence and 
stability of the structural elements of the soil, and the hydraulic surface 
roughness, in which vegetation and soil play a role.