Geoscience Reference
In-Depth Information
Human impact
Natural impact
Precipitation risks
(intensity / pattern)
-
Natur
al h
azards
Floods and sedimentation
Deforestation and
land-use intensification
without conservation
measures
Precipitation risks
(intensity / pattern)
Increasing:
- soil erosion
- sediment load
- runoff
-
l
Floods and sedimentation
Precipitation risks
(intensity / pattern)
Local floods
in lower
course
-
River bed rising due
to water deviation
Floods and sedimentation
Fig. 2.15
General summary of the key elements of the Himalayan environmental degradation theory as proposed by Ives & Messerli
(1989). Schematic diagram showing the relationship between human-induced and natural processes at the micro-, meso- and
macro-levels. (Source: Ives & Messerli 1989; reproduced with permission from
The Himalayan Dilemma - Reconciling Development
and Conservation.
Ives, J.D. & Messerli, B. (1989). Routledge, London.)
to the uncertainties attached to gathering hard
evidence to test the model. Gerrard (1990) pro-
vides a useful review of the main criticisms of the
theory and poses several key questions requiring
new knowledge on the rates of deforestation,
the extent of deforestation, the main forms of
soil erosion and landsliding, the spatial extent
of erosion, the nature of the river sediment load
and whether the observed changes are the result
of land-use change or natural variability in the
sediment system.
Environmental sedimentology is ideally suited
to answering such problems through better under-
standing of the impact of human actions on the
sedimentary system. For example, Laban (1978,
1979) has shown that very high soil erosion rates
in Nepal can be tolerated within the landscape
and half the landslides are from natural causes.
Ironically areas least affected by landsliding are
the heavily populated and terraced agricultural
systems. In the lesser Himalaya evidence for
accelerated erosion has been found from studies
of the small Nana Kosi watershed where agri-
culture and deforestation have increased erosion
rates by a factor of five to ten (Rawat & Rawat
1994). Such discrepancies in erosion-rate evid-
ence from site-specific locations are hard to
accommodate in a general model, however, over-
all the evidence generally points to rates of soil
erosion that are generally less than predicted and
the sediment system is fairly robust under such
high rates of change.
