Geoscience Reference
In-Depth Information
Table 15.3
Soil loss for
various land uses and slopes
(tons/hectare/year)
Slope category (%)
Land use
18-30
>
30
Rice
50
100
Corn with fallow
50
150
Other agriculture
25
50
Forest
1
1
One of the major volcanic eruptions is that of Mt. Pinatubo in 1991 emitting
enormous amounts of ash laden steam clouds reaching as high as 20 km above the
event. An estimated 6 B m
3
of pyroclastic materials were deposited over a 4,000 km
2
area including the eight major river basins that drain the volcano. The interim
effect of this eruption is the occurrence of heavy ashfalls and
lahar
(lava) making
vast tracts of lands planted to rice and sugarcane in Central Luzon unfit for agri-
cultural production and several residential areas unsuitable for human settlement.
Furthermore, every year, particularly during the wet season, heavy rainfall continues
to erode the pyroclastic material deposited on the slopes of the volcano causing
fast-moving lava to wreak havoc and severe damage on an estimated 300,000 ha
comprising mostly of residential and agricultural communities.
4.2
Human-Induced
Primary causes of land degradation have been identified as:
4.2.1
Intensive Logging
Forest cover in the Philippines decreased from 34 % in the 1970s to 22 % in
1987. The 1987 forest resource inventory showed a forest cover of 6.6 Mha leaving
10.8 Mha of possible degraded forest lands. Estimates and distribution of degraded
lands are highly variable. Most of the area is mountainous and faces severe erosion
problems with vegetation removal. The underlying causes of deforestation are
rooted in a complex web of social, economic, and institutional problems both within
and outside the forestry sector.
4.2.2
Unsustainable Agricultural Practices in the Sloping Uplands
This is due to intensification of agricultural land use without compensating invest-
ments in soil conservation and fertility. A major causal factor is upland migration
(Table
15.3
).
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