Forests and water - case studies
Himalayan deforestation and downstream floods
Throughout the 1970s and 1980s, there was widespread belief that deforestation in the Himalaya - which had started in about 1950 - was the primary cause of seriously increased monsoon flooding in Gangetic India and Bangladesh. ”Ignorant” mountain subsistence farmers, with their uncontrolled population growth and continued dependency on the forest for fuelwood, were blamed for this presumed deforestation, which was predicted to leave Nepal with no remaining accessible forest cover by 2000.
Gradually this belief took hold worldwide, and was reported as fact in the popular media, conservationist publications and even scholarly/scientific literature. Many publications provided lowland Indian and Bangladeshi politicians and decision-makers with a perfect scapegoat: the Nepalese mountain farming communities.
During the 1990s, much research tested the validity - at the micro and macro levels - of the association between deforestation and human interference in the Himalaya and floods in the lower Ganges and Brahmaputra plains.
Research on forest cover in Nepal found that forests on the upper slopes had decreased significantly in the 1970s and 1980s. Contrary to the prevailing assumption, however, it was also found that the tree cover on land controlled by local farmers had increased in several areas of the Nepalese Middle Hills. Much of this reforestation had been spontaneous, with no government or foreign aid assistance. Community forest activities at lower altitudes had a beneficial impact on the local environment and the stability of land-use systems; for instance, by converting shrub and grasslands to more productive forest land.
Sophisticated geographic information system (GIS) modelling based on analysis of thousands of soil samples suggested that human intervention in Himalayan watersheds plays a significant role in sediment transfer by redistributing downstream the losses incurred by cultivation on steep slopes upstream. Upstream farmers control the physics and chemistry of their fields through an elaborate system of retaining and redistributing sediments. Downstream, the hydrological impacts of local farming, beyond small sub-watersheds, proved to be significant only when there were extreme rainfall events.
A detailed geomorphologic study of Middle Hills terrace irrigation determined that terracing does not change hydrological behaviour on hill slopes, and single terrace failures during torrential monsoon downpours do not contribute sediment to downstream flow. The mechanisms of large-scale terrace slope failures, which involve many terraces, are much more complicated, but even at this scale, farmers’ management of terraces and irrigation channels can correct most damage, induce lower sediment yield and reduce overland flow.
At the macro level, a review of historic flood data, changes in river courses, available hydrological and climatic data, and socio-economic information covering the Brahmaputra basin indicated that neither the magnitude nor the frequency of flood events had increased significantly over the last 120 years. The study concluded that lowland floods in India and Bangladesh are largely natural processes, independent of human activities in the Himalayan catchments. Instead, the primary cause of flooding is direct rainfall over India and Bangladesh. Rainfall patterns in the Meghalaya hills are the main cause of fluctuations in the Meghna river.
The same research found that local populations felt more threatened by dry-season low water levels, riverbank erosion and cyclones in the lower delta, than by natural monsoon flooding. There is therefore little sense in investing in massive public works to change river courses, which would cost many times more than the gross national product (GNP) of Bangladesh, because their technical feasibility is arguable given the size and power of the Brahmaputra river.
The Himalaya deforestation and downstream flooding debate is a classic case of apparently simple explanations of upstream/downstream linkages turning out to be far more complicated when they are subject to rigorous investigation. In particular, this case highlights the dangers of dramatic reporting based on unproved assumptions, emphasizes the need for scepticism when reviewing alarmist recommendations drawn from short-term, emotional impressions, and stresses the importance of conducting long-term historical analysis. It also underlines the need to identify self-interest in international politics concerning upstream/downstream linkages.
Adapted from J. Ives. 2001. Highland-lowland interactive systems.<p>Download the full-text document<br><br></p>
Want to know more? Read:<p>What are the impacts of deforestation in the Himalayas on flooding in the lowlands? Rethinking an old paradigm, , by Thomas Hofer.<p>Forests and floods. Drowning in fiction or thriving on facts? , Forestry Perspectives 2.