ReliefWeb

1. Synopsis and Content

This report considers the general hydrological conditions in the Lower Mekong Basin during the 2010 flood season, which saw the lowest volume of flood season flow within the 87 year period of record on the Mekong mainstream at Kratie1 in Cambodia. The flow deficit therefore exceeded that of 1992, which was regarded as the worst observed regional hydrological drought. Water levels across the Cambodian flood plain in the Viet Nam Delta were well below average with the inevitable consequences for natural flood plain irrigation and salt intrusion.

Geographically, the 2010 flood season deficit was less severe in the northern parts of the region, but downstream of Pakse and Kratie the situation was unprecedented both in terms of seasonal flow volumes and the duration of the season itself.

On average, the flood season lasts between four and five months in the Lower Mekong Basin. In 2010, it lasted for just three. The principal reason is that the onset of the annual flood was delayed by a month and more due to a lack of runoff producing storms during the early weeks of the SW Monsoon. In addition, total seasonal rainfall in Southern Lao PDR and over the Se Kong, Se San and Sre Pok Basin in Cambodia was 40% below average making it one of the weakest monsoon seasons on record in this part of the region. These conditions during 2010, when the annual flood volume was 40% below normal at Kratie, complete an eight year sequence of below average floods.

The theme of the Report considers the relationship between some aspects of the regional climate and the annual flood. Previous Annual Flood Reports have considered many of these linkages as a matter of course. The major subject area addressed here is based upon seminal studies of the long term structure and pattern of the Asian Monsoon over the last millennium, based on regional tree ring chronologies. The reconstructions reveal that drier and wetter phases can last for a decade or longer and significantly that the overall pattern has remained the same for the last 1,000 years.

A major revelation is how the historical ‘mega droughts’ that are revealed through the tree ring studies correspond with events referred to in historical chronicles, which for example in Viet Nam and Thailand go back as far as the 11th and 12th centuries. Such episodes led to famine, social unrest, rebellions and on occasion to regime change such as the end of the Ming Dynasty in China in the mid 17th Century. Such correspondence with the chronicles not only confirms the overall accuracy of the long term climate reconstructions but also emphasises the key role that the monsoon has played in the history of the region and the ongoing dependence of society upon it.

The report concludes with a summary of the four National Flood Reports, produced by the respective responsible Line Agencies of the MRC Member Countries. A major point to emerge here is that although regional conditions were generally dry, the passage of tropical storms across the region resulted in narrow belts of significant flooding, particularly in the Mun- Chi Basin in Thailand during October.