Deforestation and flooding

							Deforestation and flooding

Jose Agustin Breña Naranjo


The topic of defforestation and its relationship with floods has been very discussed over the last
decades though it is during the last years that the mentionned theme took more interest since it
became an officially declared “priority” issue in several international conferences, forums and
declarations. Thus, the debate between ecologists, scientists, planners, decision-makers and
politicians has been marked by different perspectives about the deforestation and its
consequences but mainly on what should be done. These perspectives come from the different
scientific groups, this is, while the forestry/agro forestry community states that forests reduce
floods, some research studies carried out by hydrologists show that there is scarce scientific
evidence that support this statement. So we will see in this report that the public perception is that
forests are of great benefit in reducing floods but also it will be showed why it can be considered
as a popular belief.

For example, one of the most disseminated causes that explain the magnitude of the disastrous
floods in Bangladesh and northern India have been almost always associated with “deforestation
of the Himalayas”. Also, in Europe floods are often attributed by the media to “deforestation in
the Alps”. However this led to a significant contradiction since it is known that the increased
forest evaporation may reduce floods, and that interception of rainfall by forests reduces floods
by removing a proportion of the storm rainfall and by allowing the build up of soil moisture
deficits. In addition, forest management activities may increase floods such as: cultivation,
drainage and road construction which tend to increase stream density and soil compaction during
logging

However, there are some early hydrological studies in: USA (Hewlett and Helvey, 1970), South
Africa (Hewlett and Bosch, 1984), UK (Kirby et al., 1991; Johnson, 1995) and New Zealand
(Taylor and Pearce, 1982) which show little linkage between land use and storm flow.

Also there are more recent studies in: USA (La Marche & Lettenmair, 2001), UK (Robinson, M
and Dupyrat, 2003) and India (Sikka et al., 2003) that only show benefits of forests at small
catchment size and for small events, little or negative benefit for largest events.

Other studies at large catchment size held for the Himalayas (Hofer, T. 1998) show no
measurable effects for Ganga-Brahmaputra-Megha catchment as well no increase in frequency or
the magnitude of flooding over the last few decades (reported in Calder et al., 2003).

Similar conclusions can be find out in a paper published in the Journal of Land use and Water
Resources Research which indicates that in the absence of complete information,
overgeneralizations or myths about links between land use activities and hydrological processes
have emerged, such as ‘deforestation causes flooding’ regardless of basin size, or ‘deforestation
causes erosion’, even where natural erosion rates are high. These generalisations, which cannot be
verified, and in some cases have been disproved scientifically, are still frequently used as
justifications for policy and programmes.

Such myths may selectively single out particular causes from multiple ones in order to support
institutional and political agendas, and may be a convenient basis for advice because their
assumptions are unverifiable. This leads to misguided policies and remedial approaches, and
often results in poor and minority populations in upland areas being made scapegoats, despite
their relatively minor contributions to the problem.

General statements about land–water interactions need to be questioned continuously to
determine whether they represent the best available information and whose interests they support
in decision-making processes. Due to the complexity of landscape processes and the long time lag
between cause and effect, uncertainty is inherent in any scientific findings and assumptions about
land-water interactions. This uncertainty needs to be made explicit to avoid the emergence of new
myths.

According to Prof. Ian Calder from the University of Newcastle, for the largest, most damaging
flood events, there is little scientific evidence to support anecdotal reports of deforestation as
being the cause. Whereas on a micro-scale (small watershed) the effects of human interventions
such as forest cutting can be directly documented in terms of higher discharge peaks or higher
sediment load, on a large-scale natural processes are dominant, and the impacts of human
activities are neither detectable nor measurable. Field studies generally indicate that often it is the
management activities associated with forestry (as mentioned previously) that are more likely to
influence flood response, rather than the presence or absence of the forests themselves.

A good example is the case of the Ganges-Bramaputhra basin in which media headlines for this
region claim that: “the severe floods in Eastern India and Bangladesh are not the result of a
natural disaster, but of a ruthless exploitation of wood which has been practised over centuries in
the forests of the Himalayas”. Headlines such as these are based on assumptions that the forest
cover in the Himalaya is rapidly decreasing, which only holds true for certain areas, e.g. the
Western Himalayas of Pakistan. Also, that there is a direct link between forest removal in the
Himalayas and flooding in the lowlands of the Ganga and Brahmaputra river systems, and that the
mountain people with their forest management practices are responsible for the inundations in the
plains. Thus, the newspaper statement reflects the still widespread wrong assumption that land–
water linkages observed in a small and medium sized watershed can be extrapolated to large
watersheds.

In many studies, it can be documented that in small watersheds the human impact on land–water
influences is dominant. In medium sized watersheds it is already difficult to distinguish between
man-made and natural impacts on the land–water linkages. In large watersheds, natural factors
(e.g. heavy rainfall events and deep landslides) are clearly the dominant links between land and
water. There is, of course, a significant contribution of ‘base flow’ from the highland catchments
of the Brahmaputra and the Ganga to the floods, but this input is just one element of many others
and is not a flood-triggering one. The natural rates of weathering and erosion in this tectonically
active steep land zone are high, and sediment transport is a dominant process, irrespective of
vegetation cover. Finally, the authors conclude that inappropriate land use practices may still
have disastrous consequences within a highland watershed, but conservation practices should not
be undertaken with the expectation that they will prevent floods in the lowlands.

This report should be considered as an information report and not as a scientific report because of
its short length so I was not in measure to enter in technical details. However, I think this report
shows the lack of understanding between the hydrology & other sciences and the
planning/management/policy issues. If we examine the conventional land use actions carried out
by the local and national governments with the support of some international organizations in
order to decrease substantially the flood risks, then we will see several cases of short and mid-
term plans or projects that are not necessarily adequate or efficient for this aim.
More scientific research must be done in different regions, with different climatic and
hydrological conditions and for catchments of different sizes. In my opinion, the scientific results
showed in this report led to a shift of ideas concerning the interaction between forests and water.
Furthermore, this homework made me realize the importance of IT for the spread of the
knowledge, due to the fact I could not find out this information in any water research center or
academic institution in Mexico.


References


http://www.cluwrr.ncl.ac.uk/SIWIwater%20weekIRC.pdf (June 2 2005)

http://www.luwrr.com/uploads/paper02-01.pdf (June 2 2005)

http://www.iwmi.cgiar.org/pubs/SWIM/SWIM03.PDF (June 4 2005)