Geography Reference
In-Depth Information
Zealand, find that 28° is the threshold for post-
deforestation landslides, and most occur on slopes
steeper than 32°. Average surface lowering was 0.2
m in 10 years. However, in many situations, the
relationship between such environmental controls,
forest cover and landslide activity is difficult to
prove. The author's studies of the impacts of new
hill roads in the Himalaya found that forested
slopes were more prone to landslide activity than
those where the forest was replaced. The reason
was that, in this environment, forest survived only
on those slopes that proved too unstable for other
development (Haigh et al. 1995).
Hydrological balance
It is widely accepted that deforestation results in
increased flood hazard. However, the case is more
easily argued than proved. The hydrological
impacts of deforestation are diverse and, to date,
do not register on the record from the world's
largest forest basin, the Amazon (Marengo 1995).
The role of deforestation in causing the increasing
flood problems of north India is debated (Haigh
1994).
However, the human impact, including
deforestation, is thought to have an important
impact on sediment yields from the major rivers
of South and East Asia. It also has an impact on
the flow of smaller streams (Bruijnzeel 1990).
Theory suggests that deforestation leads to
increased annual water yields and greater
groundwater recharge and greater dry season flow.
Certainly, it is usually associated with increased
flood discharge, if only because the volume of
water lost to evapotranspiration, commonly 40 per
cent of that received in precipitation, remains on
the ground. Nevertheless, in the seasonally dry
tropics, deforestation is often linked to dry springs
and streams (Bartarya 1991; Valdiya 1998).
Sandstrom (1995) attempts to show that the
hydrological consequences of deforestation vary
with land management, hydroclimate and
landscape characteristics. Others believe that the
key is the depth of the soil (Haigh 1994). Forests
create deep open-textured soils, with a large
water-holding capacity, even on steep slopes.
When the trees are cleared, this soil becomes
reduced and compacted. More rainwater is
converted to runoff and near-surface flow and less
enters the soil or groundwaters (
cf
. Shibano 1998).
Where soils are deep, through flow is the
dominant supplier of storm flow. It is unusual for
rainfall to exceed the infiltration capacity of forest
soils, except where there has been compaction, but
saturated overflow is not unusual.
Forests return a great deal of the rainfall they
receive to the atmosphere. Annual
evapotranspiration in tropical moist lowland
forests ranges up to 1500 mm y
-1
, with
transpiration accounting for a maximum of
Channel response
Landslide sediments often have an important
impact on stream channels in mountain regions.
Right across the Himalaya and its Siwalik fringe,
deforestation and development are associated
with changing the dominant processes in stream
channels from incision to sedimentation. Affected
streams may be converted to bedload channels.
They tend to become wider (sometimes braided),
more shallow and less sinuous. Frequently, surface
flows decrease and become restricted to flood
conditions, and the water flows through the
channel bed's fill. In the Himalayan fringes, there
have been dramatic increases in stream width,
and it is likely, but not yet proven, that this has
led to further undercutting and destabilisation of
hill slopes (Froelich and Starkel 1993; Haigh
1994). Elsewhere, in situations where the
sediment supply to the rivers is not much
increased, the increased runoff, especially the
increased severity of the mean annual flood, that
accompanies deforestation, coupled with reduced
vegetative protection and increased compaction
of the soil surface, may lead to exactly the reverse
effect. In semi-arid environments, deforestation
often leads to stream channel trenching, decreases
in the width/depth ratio, and increased channel
sinuosity. The impact of the affected channels
depends upon the balance and the character of
the local change in the supply of water and
sediment.
