MEKONG BASIN RESEARCH (SE-ASIA)

Hydrology

Around 80 % of the total mean annual Mekong runoff of 470 km³ reaches the South China Sea during flood season from June to October making floods a recurrent event in the MRB (MRC, 2005; Pham Trong et al., 2009). Depending on type, frequency, duration and severity, the socio-economic costs of extreme floods in the MRB are similar to other great and heavily populated basins in Asia (e.g. Indus, Ganges etc.), where the river basin is principally used for agricultural purposes. However, one aspect must be paid particular attention to: Even though floods can impose large socio-economic costs to the people of the Mekong (e.g. loss of lives and property, loss of livelihoods, decrease of purchasing and production power etc.), the environmental, social, and economic benefits of flooding (e.g. fish production, provision of nutrient rich sediments, recharge groundwater tables etc.) by far outweigh the costs related to damages. In the Lower Mekong Basin, for example, it is estimated, that the annual costs of flooding amount on average to US$60–70 million a year, whereas the average flood benefits are accounted to up to US$8–10 billion per year (MRC, 2010a).
Nevertheless, these benefits should not hide the fact that the damages, which are caused by floods, are still substantial in the Mekong region. In this chapter we will firstly present summary information about the main causes, effects and impacts of trans-boundary flood issues which have the potential to considerably affect or interfere in the socio-economic performance of the Mekong riparian states. Subsequently, the most significant negative socio-economic impacts of floods are outlined with specific reference to the individual countries and sub-regions, while the last part analyses in detail the vulnerability to flood hazards as the key reason for recurrent flood related damages.
In 2005, the Mekong River Commission released its first ‘Annual Mekong Flood Report’ based on the national flood reports and data of the LMB Countries. The report has been updated annually to supply a reliable source of data for a sound understanding of the system and the opportunity for an enhanced future management in flood related affairs like land use planning (MRC, 2006).

Hydrology of Floods

While during dry season at Vientiane the largely snow-melt driven part of discharge originating in China (Yunnan component) plays a significant role (75 % of generated runoff) this flow component is getting less important further downstream and it is generally not significant during the flood season (share of contribution of less than 15 % at Kratie during wet season; compare MRC 2005). Thus, the major causes of flood hazard are torrential rainfalls associated either with the southwest monsoon (May-September) or later in the year with tropical cyclones. The magnitude of flood pulses can vary significantly from year to year (MRC, 2005). The figure provides the recurrence intervals of flood events for the station of Vientiane. It emphasizes the huge inter-annual variability of peak flood (10.000 to 26.000 m³/s) and flood volume (50 km³ - 150 km³).

Next to the temporal variability of flood events there is a significant spatial variability of floods related to regional patterns of runoff generation. Snowmelt and rainfall on the Upper Mekong Basin generate the first case while the second case is dominated by excessive rainfall events in the highlands of Thailand, Lao PDR and Vietnam. Both cases originate from two different atmospheric processes and hence show a distinct behavior (Delgado et al., 2010).
To classify the severity of a flood and create a basis for future assessments, historical flood events have been taken into account. The 1966 flood was the worst in the upper part of the LMB, 1978 was the worst flood so far around Kratie. In 1996 a severe flood has been recorded in the area around Stung Treng at the confluence with the Mekong River, the Mekong Delta suffered most during the floods in 1961, 1966 and 2000. 2005 was the most devastating for the central area of Lao PDR and Thailand. Also the floods in 1971, 1974, 1984, 1991, 1995, 2001and 2002 can be considered as severe for several sections of the LMB. Thus, a severe flood in the LMB is quite a common situation, but the location of the severe impacts is varying within the basin from year to year. The flood in 2011 is not considered in the following table, but it ranks among the highest discharge rates in the LMB (MRC, 2011b).
While peak discharge and total wet season flood volume are useful approximations to quantify the flood hazard, there are other hydrological characteristics which play a role in determining the severity of a flood in terms of impact or damage; these are:

• The area and depth of inundation
• The time of occurrence of the flooding (e.g. delayed floods, season and cropping stage; frequency)
• The speed the water rises (time for humans and animals to react to flood)
• The stream velocities of the flood water (destructive force, bank erosion etc.)
• The duration of the flooding (e.g. complete crop yield failure after prolonged flood)

Drought Vulnerability and socioeconomic impacts

The agricultural sector in the Mekong region is highly sensitive to water shortages as a result of below average flow during both the dry and rainy season. A decrease in water supply during dry season can lead to a reduced crop yield due to reduced soil moisture availability and irrigation possibilities. During rainy season, water shortages can diminish the volume and extent of essential floodwaters for controlled field inundation and can lead to salt water intrusion in the delta region (MRC, 2010a; Navuth, 2007). 

Drought events in the Mekong River Basin pose serious socio-economic threats to those dependent on secure water availability and supplies. The devastating drought of 2004, for example, affected millions of farmers and the low-income population and caused substantial agricultural deficits in Northeast Thailand and Cambodia, a considerable reduction in the second rice crop in Lao PDR and very critical levels of saline intrusions in the Mekong Delta (Navuth, 2007). The potential socio-economic impacts of droughts for each riparian state of the MRB are summarized.

 

References and further reading:

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ADB (2011) Lower Mekong Basin Component Flood Vulnerability Indices,TA-7276-REG Supporting Investments inWater-Related Disaster Management, Draft final report Asian Development Bank, 1-59.

Beechham, R., Cross, H. (2005) Modelled Impacts of Scoping Development Scenarios in the Lower Mekong Basin, Mekong River Commission, Cited by: MRC (2009d), Hydrological and Flood Hazards in the Lower Mekong Basin, Mekong River Commission Secretariat, Vientiane, Lao PDR, 1-324. 

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De Bruijn, K. M. (2005) Resilience and Flood Risk Management, A Systems Approach Applied to Lowland Rivers, Delft University Press, Delft, Netherlands.

Delgado, J. M., Merz, B., Apel, H. (2010) Flood trends and variability in the Mekong River, Hydrology and Earth System Sciences, 14, 407-418.

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Hoanh, C. T., Guttman, H., Droogers, P., Aerts, J. (2003) ADAPT: Water, climate, food and environment under climate change, The Mekong basin in Southeast Asia, International Water Management Institute, Mekong River  Commission, Future Water, Institute of Environmental Studies. Colombo, Phnom-Penh, Wageningen.

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