Over the last five decades, the Lower Mekong Basin has been the scene of major human and environmental upheavals, especially in Cambodia, Laos and Vietnam where sustained political instability degenerated into major armed conflicts and significant migrations. The Vietnam War, whether official in Vietnam, or secret in Laos, arguably played a decisive role in the extensive deforestation observed from c. 1960 to c. 1980 in the Lower Mekong Basin. Deliberate massive removal of vegetation by bombing and chemical spraying was a military tactic to deny cover and land to opposition forces. In Laos only, from 1953 onward, approximately 1 million people were displaced, successively escaping the war, the communist takeover or following resettlement policies.
Detailed examination of bombing and hydro-meteorological data suggests that these extensive environmental damages and population movements altered watershed hydrology, especially in two sub-basins of the Mekong River that were either heavily bombed (in southern Laos) or depopulated (in northern Laos). The northern sub-basin (110,500 km2) adjoins the Xiengkhouang Plateau in the East. The southern sub-basin (50,300 km2) includes parts of the Annamite chain in the East and of the Boloven plateau in the Southeast and about one third of the latitudinal extent of the Ho Chi Minh trail in Laos.
Available information on air operations indicates that the southern sub-basin has been one of the most heavily bombed areas in Indochina. A total of between 690,000 and 2,948,000 tons of high explosive ordnance was expanded over this sub-basin between October 1965 and September 1973, clearing an estimated surface area of about 20,000 km2 (>one third of the sub-basin area). Most of this forest bomb-clearing occurred between 1970 and 1973.
The chronology of changes in the relationship between rainfall and river flows, in each of the 2 sub-basins, was studied using hydro-meteorological data covering the period 1960-2004, and hydrological modeling. Our analyses show that runoff started to increase in 1972 in the southern catchment and peaked in 1973-75. Although runoff stabilized afterward, it remained at higher than pre-war levels until the early 2000s.
In highly bombed areas of the southern catchment, land damage resulted in the displacement of significant amounts of soil, disrupting soil horizons, possibly fragmenting bedrock and exposing the water-table, through a process termed bombturbation. Bombturbation induces profound, long-term effects on ecosystem functioning. The original forest is replaced by succession vegetation types with comparatively sparse canopy and shallow root systems restricting evapotranspiration, hence the observed increase in runoff a few months after bombing climaxed. However, land-cover change is not the only cause for the higher water yields that persisted after 1987 in the southern catchment: with time, war damages become increasingly difficult to separate from the pressures of economic development, including commercial logging. However, bomb-degraded forest areas are still apparent in places.
In the northern catchment, runoff remained stable until 1975 and decreased thereafter. While air operations of the Vietnam War caused losses of forest, continuing ground battles in northern Laos over the period 1953-1975 induced important emigrations. Between 730,000 and 1 million people (one fourth of the country’s population as per 1970) were displaced over this period. This migration flow was sustained after the end of the war, as an immediate consequence of the communist takeover: between 1975 and 1985, more than 300,000 people fled to Thailand, escaping the new regime. This massive exodus particularly affected the northern provinces of Laos, traditionally inhabited by Hmong ethnics. The abandonment of large mountainous areas traditionally used for shifting or permanent cultivation, most likely induced a shift toward forest regeneration, explaining the runoff decrease observed after 1975. Forest regeneration in abandoned areas appears to have been sustained until more recent periods as the result of abandoned swidden areas to secondary forest, in response to the government’s policy of reducing shifting cultivation practices in the upland areas and promoting cash crop production. A plausible scenario therefore proposes that long-lasting conflicts depopulated the northern catchment of a significant proportion of its population, while bombing sorties, although far from anecdotal, were not sufficient to have ecocidal consequences such as that observed in the southern catchment.
Observed long-term hydrological variations are obviously consistent, in terms of magnitude, occurrence and spatial distribution, with the broad-scale land-cover changes that most likely occurred over the last half century in the two studied catchments. These results illustrate the high responsiveness of flow regime to forest cover changes in tropical areas where deforestation is expected to perpetuate at a high rate over the coming decades.
Lacombe G, Pierret A. 2013. Hydrological impact of war-induced deforestation in the Mekong Basin. Ecohydrology. doi: 10.1002/eco.1395
Lacombe G, Pierret A, Hoanh CT, Sengtaheuanghoung O, Noble A. 2010. Conflict, migration and land-cover changes in Indochina: a hydrological assessment. Ecohydrology 3: 382-391. doi: 10.1002/eco.166
About the authors:
Alain Pierret is a research scientist in the fields of soil biophysics, natural resource management and environmental archaeology. After working in Australia, with the CSIRO, Land and Water, from 1997 to 2003, he was appointed as a research scientist with IRD in December 2004. Alain has published over 50 scientific articles / book chapters among which 37 in peer-reviewed international journals. He is also a section editor for the journal Plant and Soil. Alain’s current research interests include the complex interactions between land use / land use change, plant growth and the water and carbon cycles.
Guillaume Lacombe holds a PhD in hydrology from Montpellier 2 University. He is an agronomic engineer graduated from AgroParisTech. His skills include watershed hydrology, climate change sciences, reservoir water balance modeling. He worked with short- and long-term assignments in Australia, Canada, Laos, Switzerland, Thailand, Tunisia, United Kingdom, USA and Uzbekistan. Since 2010, Guillaume Lacombe is appointed as a researcher at the International Water Management Institute, based in the Southeast Asia regional office in Vientiane, Laos.
Chu Thai Hoanh holds a Ph.D. in Agricultural and Environmental Sciences from the International Institute for Aerospace and Earth Sciences (ITC) and Wageningen University in the Netherlands. He has 40 years of experience in research and management with wide and varied subjects in agriculture and water management. In 1997 he joined the International Rice Research Institute (IRRI) for research on rice and land use in Asia countries. Since 2003 he has joined the International Water Management Institute (IWMI) and at present he is a principal researcher on water resources for many studies in Asia and Africa.
This Editorial was first published on the Asia-Pacific Migration and Environment Network on 4 September 2013.