In many ways the flood season of 2007 mirrored that of 2006, with below average peak discharges, water levels and flood volumes, which at Kratie were significantly so. The definitive feature of the year was the exceptionally late start to the flood season throughout the Lower Mekong region. It generally starts during the early weeks of July but in 2007 the onset was as late as early August in Cambodia and Vietnam. This delay caused discharge and water levels towards the end of July to be some of the lowest observed since hydrological records began in the early 20th Century. The major impact of a shorter flood season, significantly low water levels over most of the season and the fact that maximum water levels were unseasonably late, not occurring as that did until October, was the reduction in the annual fish in the Great Lake by a reported 35%.
The only major regional storm event to cause widespread flooding over the Central Areas in Lao PDR and Thailand was Tropical Storm 'Lekima', which occurred in October and resulted in considerable damage and loss of life. This provided the only flood runoff during the year on a scale large enough to cause Mekong water levels to increase to anywhere near average.
Elsewhere, flash floods in Cambodia and Viet Nam during August and again in Viet Nam in November caused considerable damage and loss of life, particularly in the Upper Se San and Sre Pok Rivers.
Having contributed significantly to the understanding of Mekong mainstream floods and flooding in the 2006 Annual Flood Report, this document has made the logical progression and taken the hydrology of the flood regimes of the tributaries as its theme. These have been considered within three reaches of the mainstream, Chiang Saen to Vientiane, Vientiane to Pakse and Pakse to Kratie. The impacts of catchment scale, topography and reservoirs on the tributary flood regimes has emerged clearly as has the fact that extremely rapid rises and falls in water level and discharge, typical of flash floods, are not confined to small river basins. This hazard can occur on extremely large rivers in the region.
A major effort has been made to present the hydrological data available on the tributaries in a schematic way that shows where ‘functional’ data, that is those of sufficient length and reliability for meaningful analysis, are recorded. In the past the lack of such simple reference material has often confused. Unlike the mainstream data, those for the tributaries are not up to date so no analysis of conditions during 2007 is possible. Consideration should perhaps be given to the provision of current hydrological data for those rivers that contribute major volumes of flood water to the Mekong. These have been identified as part of this Report. Such data would add substantially to understanding current flood conditions, particularly in the event of an extreme flood year.
In 2007 the incidence of flash flooding and the death and damage that it causes has once again drawn particular comment, particularly through the National Flood Reports. There is general consensus that they are not well understood and that reducing the number of people and their property that is exposed to them is amongst the priorities for regional flood management and mitigation. It has also been observed in the national reports that investment in flood protection tends to take priority in the principal economic regions, which is inevitable, while far less is focussed upon the remoter areas where the flash flood hazard is significant. An improved balance has been suggested.
As far as hydro-meteorological data are concerned the Lower Mekong region has an excellent recording network by global standards. However, because storm rainfall is spatially very variable and the highest rainfall areas are in the remoter mountainous regions it is often not possible, as has been revealed here, to find any storm data from ground observations to link to many extreme flash floods that have occurred. This seriously limits efforts to understand them in terms of cause and effect. Satellite and weather radar are the obvious means of gaining this better understanding and efforts should be made to pursue such studies, perhaps under the remit of the FMMP.
Another data issue that drew comment is the lack of consistency between national water level datums and those held in the database at the MRCS. The latter should be audited in order to ensure that the water levels reported in documents such as this agree with the official national figures.
Finally, it has been recommended that consideration be given to the translation of the Annual Flood Report into the languages of the MRC member countries so that it can be more widely read and understood by the relevant stakeholders and line agencies and make a meaningful contribution to institutional strengthening and the available reference material.
FAO-CFOR (2005) 'Forests and Floods — Thriving on fiction or drowning on facts'. Centre for International Forestry Research. Food and Agriculture Organization of the United Nations. 340 pp.
Gruntfest, E. and J. Handmer (1999). 'Coping with flash floods'. Proceedings of the NATO Advanced Study Institute. Ravello. Italy. NATO Science Partnership Sub-Series: 2 Vol 77. Published by Springer. 340pp.
MRC (2007). 'Annual Mekong Flood Report, 2006'. Mekong River Comission. Vientiane. Lao PDR. March. 76 pp.