Geography Reference
In-Depth Information
1045 mm y
-1
(Bruijnzeel 1990). In most
environments, these figures are very much
lower. Hence, when forests are removed, a great
deal more water remains in the environment
than before, and the total discharge of affected
streams tends to increase. In the tropics,
increases of water yield equivalent to between
110 and 825 mm y
-1
are reported in the year
following clearance. After reviewing 145 studies,
Sahin and Hall (1996) estimate for southwest
Australia that, given a 10 per cent reduction in
cover, removing deciduous hardwoods would
increased annual yield by 17-19 mm yr
-1
;
removing conifers, 20-25 mm yr
-1
; bu t
eucalypts, only 6 mm yr
-1
. Afforestation by scrub
would effect a 5 mm yr
-1
decrease in yield.
In Nigeria, Lal (1997) reports results from a
seven-year study of a secondary tropical rain
forest watershed (44.3 ha) that yielded 2.2 to 3.1
per cent of its annual rainfall as runoff.
Deforestation of 7 per cent of the watershed area
increased water yield to 7.0 per cent of annual
rainfall. Total water yield following deforestation
and conversion to agricultural land use rose from
9.6 to 21.3 per cent. Base flow increased after
deforestation by 5-18 per cent of annual rainfall,
while peak flow decreased by 2.3-6.2 per cent.
Previously, dry season flows decreased as the dry
season progressed, but in the years following
deforestation it increased. Forest lysimeters
showed higher seepage losses than those in
cropland.
Back in Western Australia, Bari
et al
. (1996) used
paired experimental catchments, with a six-year
calibration, to examine the effect of logging. This
resulted in an increase in groundwater levels. Deep,
permanent groundwater levels rose for four years
to a maximum of 5 m and then began to decline.
Springs that flowed two-three months each year
before logging flowed five-six months afterwards.
Stream flows increased dramatically one year after
logging due to decreased interception and
evapotranspiration, increased recharge, decreased
soil moisture deficit and consequently an increase
in through flow. The increase in baseflow was
twice that of quick flow.
Water chemistry
Deforestation affects many aspects of water quality
(Anderson and Spencer 1991). These include its
load (suspended, dissolved and bed), its turbidity
and organic matter content, its thermal and
hydrological regime, and its ecology—often
shifting the ecosystem from heterotrophy to
autotrophy. Naturally, these impacts affect the
water chemistry. Deforestation is traditionally
linked to a decline in dissolved organic matter and
a flush of released plant nutrients (
ibid.
: 15). An
attempt to apply the MAGIC model to a central
Amazonian catchment indicated that, in a thirty-
year period, deforestation would lead to the release
of SO
4
2-
, inducing acidification and a general
increase in ionic concentration (Forti
et al
. 1995).
In the longer term, the stream water ionic
concentrations might be expected to reach new
equilibria at levels higher than before deforestation
and leading to eventual acidification.
When Nigerian lowland alfisols are converted
to agriculture, their soil chemical quality and pH
decreases cultivation time (Lal 1996). At 0-5 cm,
the rate of decrease at 0-5 cm was 0.23 pH units
per year, 0.05 per cent per year for SOC, 0.012
per cent per year for total nitrogen, 0.49 cmol kg
yr
-1
for Ca
2+
, 0.03 cmol kg yr
-1
for Mg
2+
, 0.018
cmol kg yr
-1
for K
+
, and 0.48 cmol kg yr
-1
for
CEC. At 5-10 cm depth, while Mn
3+
concentration and phosphate increased due to
fertilisation; there was also an increase in total
acidity.
However, even acid rain does not cause
acidification of streamwaters in Japan's mountain
forests. They seem to be immune and only recently
has some explanation for this begun to emerge
(Box 14.2).
Soil quality and erosion
Deforestation exposes vulnerable tropical soils to
erosion and fosters their degradation through the
loss of soil nutrients and reduced soil vitality (Theng
1991). Logging may have especially dramatic
impacts. From Malaysia, Lal
et al
. (1985) report cases
of erosion climbing to 13 Mg ha
-1
month
-1
from
