During the second half of the 20th Century, South-East Asia was the arena of a series of armed conflicts, direct consequences of the Second World War, decolonization and the Cold War, followed by political instability which continued up to the 1990s. The region's history has left its scars: extensive forests erased from the map by bombing, populations displaced or forced to emigrate, entire areas abandoned although vegetation is steadily taking over again.
Research scientists from the IRD and its partners1 recently showed the discharge rate of the Mekong has oscillated in close correlation with the major events that had taken place. Runoff increased by over 50% in southern Laos between 1972 and 1975, at the height of the Vietnam War. Conversely, the north of the country saw it decrease by 30% between 1995 and 2004, following people's exodus from the area to escape from the communist forces' advance. Only the extensive changes in land-use and vegetation pattern can explain such variations in discharge of the Mekong.
Twenty years of war in South-East Asia, from 1955 to 1975, followed by political instability up to the end of the 1980s have strongly marked the environment. Analysis of Mekong Commission2 hydro-meteorological data showed IRD researchers and their partners1 that the changes in the great river's discharge rate were correlated with the dramatic events which marked the whole region's history.
To characterize the effects of these events on the runoff which feeds the river system, the researchers focused on two war zones: one in southern Laos, subjected to heavy bombing, the other in the North of the country, the arena for intensive land-based military operations.
Destruction of forest cover to expose the enemy
Between 1965 and 1975 South-East Asia was the theatre of the heaviest bombing campaigns in human history. In the first area studied, as many bombs were released as in the whole world over during the Second World War. In the face of elusive guerrilla forces, bombing systematically destroyed the vegetation cover that sheltered the enemy and camouflaged the Ho Chi Minh trail, a network of roads and tracks used by the communists in the North to supply South Vietnam. One kilogram of artillery wipes out over 12 m² of vegetation cover. The amplitude of the environmental effects of such an attack was appalling: between 8 000 and 40 000 km² are estimated to have been deforested just over this study zone, amounting to 70% of its surface area.
This massive destruction of tropical forest was followed by recolonization of land by herbaceous or scrubby vegetation that had been less completely uprooted. The whole process brought about a drastic reduction in average annual evapotranspiration3 and a substantial increase in runoff in that area: over 50% more between 1972 and 1975, then 15% more between 1975 and 2004.
Flight from the war and its political consequences
The second study area, in northern Laos, experienced a massive exodus of its people: 730 000 to one million fled Laos, escaping from war and from the subsequent power take-over by the Pathet Lao4 in 1975. These figures represent one-fifth of the country's inhabitants of the time. This region was also home to a large number of Hmongs, who fought alongside the American army. Many of them fled to Thailand and the rest of the world at the end of the war.
The land in the area had traditionally been cultivated, but once abandoned it was recolonized by forest. The research team observed that the resulting rise in evapotranspiration and improved infiltration of water in the soil afforded by the regenerated vegetation brought a reverse trend, an average 30% decrease in runoff, between 1995 and 2004.
Only history can explain such changes
In spite of data deficiencies, the results of these studies show clear trends towards increase or decrease of the Mekong's discharge that were closely correlation with historical events and extensive land-use changes the region was subjected to during the 20th Century.
What other factors could have the potential for exerting an impact on runoff? The climate, often invoked as the major determinant of hydrological change, is not involved here: rainfall remained stable throughout the study period. As for hydroelectric dams, constructed from 1970 on the Mekong and sometimes blamed, they drain only 2% of the catchments investigated and therefore have only a slight influence on the river's discharge. Finally, urban expansion, a cause of possible increase in runoff, did not begin until the 1980s and is still a marginal phenomenon in Laos, where the demographic pressure remains low. Only the armed conflicts of the last Century can therefore explain such radical changes in runoff.
This study provides lessons from the past for reasoned use of land and sustainable management of water resources. Today, for other reasons, human impact continues to have repercussions on the region's hydrological regime. Intensive mining and extraction of timber, notably teak which is exported to the countries of the North, and clearance for agriculture. These new land uses could generate major problems in the short term, such as flooding or conversely water shortages, or pollution. They should be incorporated in public policies for water management in Laos.
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