Geology Reference
In-Depth Information
2.0
1.5
1.0
Fig. 11.4 Values of b calculated
using data collected from each
erosion event on steep plots with
observed rilling at the VISCA
site, plotted against the mean
streampower for the event.
Reproduced with permission
from Presbitero et al . (2005).
0.5
0.0
0
1
2
3
4
5
Stream power
Ω
(W/m 2 )
Los Banos, the Philippines : At all tropical sites, in
order to maintain a bare soil plot for substantial
time periods, weed growth had to be controlled by
cultivation, usually by hand as at the Los Banos
site. Cultivation of the different land manage-
ment treatments (including current farmer prac-
tice and alley cropping systems) used oxen and
up-and-down slope cultivation. At Los Banos, the
erodibility parameter b was evaluated for the bare
plot over the four years 1990 to 1993. As shown in
Paningbatan et al . (1995) and Rose et al . (1997),
b was very responsive to the cultivation required
to remove weed growth, especially in the first
year. At the end of a fallow period, during which
soil had consolidated, b was as low as 0.4, but
then would rise to a little above 1.0 for the first
erosion event following weeding. Variation in
b was not as extreme in subsequent years, declin-
ing from 0.92 to 0.7 during 1991, and from 1.2 to
0.65 during 1993. Change through time in f (the
average sediment settling rate) was monitored,
increasing from 0.096 to 0.165 m s −1 during 1990.
Soil strength was measured for pre-saturated soil
using a torvane apparatus, and strength was found
to increase monotonically with time since the
last cultivation. Despite some scatter in the value
of b , there is some support for the expected decline
in b as soil strength increased (Rose et al ., 1997).
Also, over a 5-year period of measurement the soil
organic matter fell by approximately half from
some 4% to 2%, depending on treatment details
to some extent. Since commencement of the
GUEST program, the significance of interactions
between soil erosion and loss of carbon stored in
the soil has received much greater recognition.
VISCA, the Philippines : At VISCA there were both
low-slope plots at 10%, and plots with quite steep
slopes typical for the hand-hoe cultivation carried
out on the island of Leyte (Plate 7). All experimen-
tal plots at the VISCA site were 6 m wide and 12 m
long in the downslope direction. For the low slope
(10%) plots, the average value of b for all 13 events
during 1991 was 0.87 with a standard deviation of
0.12 (Presbitero et al ., 1995). For the four events
with obvious rills, the average value of b was a lit-
tle lower (0.78), perhaps due to rills eroding into
somewhat more consolidated soil. On the adjacent
steep-slope plots of 50%, 60% and 70% slope (Table
11.2), b was calculated for 18 events where rilling
was recorded on all three slopes, yielding 18 × 3
=
54 datasets, with results shown in Fig. 11.4.
Search WWH ::




Custom Search