Geology Reference
In-Depth Information
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Fig. 11.7
Relationship between
soil erodibility
b
and peak runoff
rate for the Chiang Rai site.
Reproduced with permission
from Yu
et al
. (1999).
0.0
0
5
10
15
20
25
30
35
Peak runoff rate (mm/h)
flow being more concentrated than assumed,
and by the influence of rainfall detachment and
re-detachment. This latter possibility is sup-
ported by the finding that for three of the four
sites, statistical analysis supported a role for peak
rainfall intensity in effecting an increase in the
values of
b
. This could point to the purposeful or
chosen limitation in the Type B analysis of
GUEST that yields a value of
b
which does not
explicitly recognize the role of rainfall in evaluat-
ing soil erodibility. Whilst this choice was made
partly as an appropriate approximation in the
spheres of higher-slope application, and partly on
the grounds of simplicity in application compared
with Type A analysis, it may limit the physical
interpretation of the meaning of
b
. However, the
influence of rainfall in this dataset on
b
was
highly variable from site to site, with no clear
pattern emerging.
Table 11.4 gives information on soil texture at
the four ASIALAND sites, together with the arith-
metic average and standard deviation in calcu-
lated values of
b
. Since soils with abundant fine
materials tend to be well structured (increasing
depositability and yielding lower sediment con-
centration), and have greater strength (increasing
resistance to removal by rainfall or flow-driven
processes, also decreasing sediment concen-
tration), we would expect that soil erodibility
would decrease as the proportion of fine mate-
rial increases. There was some experimental sup-
port that increased soil strength was associated
with a decrease in the soil erodibility parameter
b
(Misra & Rose, 1995; Misra & Teixeira, 2001).
Furthermore, since size distribution determined
using the wet-sieving technique most resembles
the actual size distribution of soil particles dur-
ing rainfall and runoff events, this technique was
used to provide an appropriate size distribution.
We then took the ratio of percentage of particles
(both primary particles and aggregates) > 0.5 mm
to that
, as a measure of the particle
size. Figure 11.8 shows a scatterplot of this ratio
of coarse to fine materials against the average soil
erodibility. It can be seen that as the particle size
from wet-sieving analysis decreases, the average
soil erodibility at these sites also tends to
decrease. The linear relationship shown as the
≤
0.5 mm,
Θ
