Geology Reference
In-Depth Information
Table 11.2
Average annual runoff, runoff coefficient, soil loss and sediment concentration from experimental
plots in ACIAR Project 9201 (from Coughlan & Rose, 1997b).
Runoff
(mm)
Runoff
co- efficient**
Soil loss
(t ha
−1
)
Sediment
concentration (kg m
−3
)
Site*
Treatments
Kemaman, Malaysia
Bare plot (sandy clay loam)
2245
0.62
127
5.7
4.5 years
No living ground cover
1287
0.35
90
7.0
17% slope
Grass and legume ground
cover
413
0.11
17
4.1
Average annual rainfall
=
3638 mm
Los Banos, Philippines
Bare plot (clay)
393
0.19
184
47
6 years
Clean cultivated farmers
practice
387
0.19
119
31
Avg. slope
=
18%
Alley cropping and
mulching
114
0.06
6
5.3
Average annual rainfall
=
2037 mm
VISCA, Philippines
Bare plot (clay)
55
0.02
69
125
2 years
Clean cultivated furrows
84
0.03
38
45
up- and- down slope
50% slope plots
Alley cropping and
mulching
16
<0.01
3
19
Average annual rainfall
=
2800 mm
Goomboorian, Gympie, Australia
Bare plot (loamy sand)
(landslope
=
14%)
286
0.27
216
76
3 years
Conventional plot, no
213
0.20
51
24
surface contact cover
Furrow slope <6%
Improved practice - furrow
mulching
150
0.14
3
2
Average annual rainfall
=
1045 mm
*Information on length of experimental period, slope, and average annual rainfall over the experimental period is given.
**Runoff coefficient, R
c
=
average annual runoff/average annual rainfall.
weeding, followed in descending order by results
for the local conventional farmer practice, and
lastly by results for an 'improved practice' judged
likely to be acceptable and also effective in con-
serving soil. Soil texture is also given.
Unacceptably high bare soil erosion losses of
well over 100 t ha
−1
y
−1
were measured at three of
the five sites. At VISCA (Presbitero
et al
., 1995),
the very high permeability of the soil yielded a very
low runoff coefficient (Table 11.2). Conventional
farmer practice at the three sites of excessive soil
loss did not reduce these high losses to tolerable
levels. Improved practices varied with site, but
these reduced the rate of soil loss to less than 20 t
ha
−1
y
−1
at Kemaman, Malaysia (Hashim
et al
.,
1995), and to less than 10 t ha
−1
y
−1
at other sites, a
figure which may possibly be tolerable, given the
expected high rates of soil formation in the tropics.
All improved practices were agronomic in nature,
making use of the considerable potential for bio-
mass production in the humid tropics, and involved
the use of cover sufficiently close to the ground
surface to impede overland flow, referred to as 'sur-
face contact cover'. This type of cover may be con-
trasted with 'canopy cover' provided by vegetation
in less intimate contact with the soil surface.
The effectiveness of surface contact cover in
reducing soil loss is illustrated by comparing
