Geology Reference
In-Depth Information
Table 11.4 Summary description of the four experimental sites from China, Malaysia and Thailand, together
with calculated soil erodibility b (after Yu et al ., 1999).
China
Malaysia
Thailand
Site
Luodian, Guizhou
Kuala Dal, Perak
Chiang Dao, Chiang Mai
Doi Tung, Chiang Rai
Soil texture
Silty clay loam
Clayey
Silty or clay loam
Silty clay or clay loam
Median particle size* (mm)
0.327
0.765
0.200**
1.15**
Depositability f (m s −1 )
0.0942
0.0856
0.0224
0.0738
Slope (%)
40
15
28
30
Plot length (m)
25
25
36
36
Mean soil erodibility b
0.421 ± 0.144
0.4 21 ± 0.166
0 .248 ± 0.165
0.962 ± 0.172
*From wet-sieving analysis
**Measured in 1997
these sites, Yu et al . (1999) showed that for any
particular runoff event, consistent values of b are
obtained using runoff rates inferred from use of
program GOSH, or from Q e inferred using either
of the methods just mentioned. This finding pro-
vided no direct proof that the values of b obtained
using such inferred values of Q e agree with values
of b based on direct measurement of runoff rate,
on which Q e is physically based (Equation (11.18) ).
Examination of such agreement could be obtained
in the future using the ACIAR database. Should
such agreement be justified, then this would pro-
vide a yet firmer basis for employing GUEST
methodology to data lacking the measurement of
runoff rate.
Whilst measurement of runoff rate is a signifi-
cant experimental burden in soil erosion research,
as mentioned earlier in Section 11.5 there are
advantages in its measurement using techniques
outlined in that section and in Ciesiolka et al .
(1995) and Coughlan and Rose (1997b).
Soil samples taken from the four ASIALAND
sites were subject to wet sieving from which soil
depositability was inferred using the equation of
Cheng (1997). In all but the Malaysian site, weeds
on the bare plot were controlled chemically.
Rubber tree seedlings were planted into the
Malaysian site, probably affecting later results
from that site. Plot steepness ranged from 40% at
the site in China to 15% at the Malaysian site
(Table 11.4). As in the ACIAR projects referred to
earlier in this chapter, values of b were found to
vary with successive erosion events at each site.
An important feature of this study was to exam-
ine statistically the possible role of factors that
might have an effect on the values of b obtained.
At the Chinese and Malaysian sites there was
sufficient length of record to show a general down-
ward trend with time in b , despite considerable
variability. This time trend could indicate a grad-
ual loss of the more erodible components of the
soil, or some form of soil consolidation or strength-
ening, as weed control was achieved without cul-
tivation. This time trend in decreasing erodibility
has been observed elsewhere in similar regions.
However, the decline in values of b in Malaysia
could be partly due to the cover provided through
time by rubber tree seedlings established in the
second year of the experiment. The value given
for b by GUEST is an effective erodibility figure;
however, if the soil has some cover, then the value
obtained for b is less than that of bare soil.
Although the length of record was only
five months at the Chiang Rai site in Thailand,
the values of b , calculated assuming an essen-
tially planar land surface, followed a down-
ward trend with peak runoff rate (see Fig. 11.7,
and a similar trend in Fig. 11.4). Whilst the expla-
nation of this trend is uncertain, at lower runoff
rates higher values of b could be given both by
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