Floods and droughts can occur anywhere in the Lower Mekong Basin. A flood is a highly visible natural disaster that clamours for attention and better management. In contrast, drought is a ‘quiet’ and largely invisible disaster that develops and intensifies over time; an Act of God, something to be endured rather than managed. Both disasters impose large economic and social costs on the peoples of the LMB. However, the economic benefits of floods far outweigh their economic costs: the average annual cost of flooding in the Lower Mekong Basin is USD 60-70 M/year; the average annual cost of flood benefits is USD 8-10 B/year, i.e. some 100 times greater. The challenge for better flood risk management is to reduce the costs and impact of flooding whilst preserving the benefits. The average annual cost of drought in the Lower Mekong Basin is at least as large as the flood cost and possibly considerably bigger.
Droughts in the Mekong Basin can occur at any time during the year. Meteorological droughts are defined by low rainfalls over the wet season (May to November) and reduce the yield of rain-fed rice and other crops. (Over 90 percent of rice production in Lao PDR, Thailand and Cambodia is rice-fed). Hydrological drought is defined by a reduction in surface and groundwater resources. The agricultural impact of a hydrological drought is most severe during the dry season, when less than normal stream flows reduce irrigation opportunity and the yield of dry season crops. Hydrological droughts also occur during the wet season, when less than normal stream flows reduce the volume and extent of floodwaters stored in the Great Lake and the yield of its fishery.
The annual flood in the Mekong River is the most pervasive physical event in the Lower Basin.
It has shaped the environment and ecology of the basin, especially across the Cambodian Lowlands and the Cuu Long Delta, including the nature, culture, welfare and economy of riparian societies, and the vegetation, animals and land-use of flood-prone areas. Between July and October, a massive flood wave moves down the Mekong River pass Lao PDR and Thailand, growing in volume on its downstream journey. At Kratie in North-eastern Cambodia, with a volume of some 300 km3 (average conditions), the flood wave moves out onto the Cambodian Lowlands, where some 30 km3 flows upstream along the Tonle Sap River into the Great Lake, and the remainder flows down the Mekong and Bassac Rivers and into the Cuu Long Delta of Viet Nam before entering the South China Sea. From October onwards, the Great Lake drains back into the Mekong and Bassac Rivers, sustaining the recession limb of the flood at downstream locations. In total, an average of some 460 km3 of water flows out into the South China Sea each year. The Mekong flood is a regular annual event, driven by the southwest monsoon and supplemented by tropical weather systems generated in the Northwest Pacific and the South China Sea.
Mainstream flooding across the Cambodian Lowlands and Cuu Long Delta persists for some two-four-months each year and affects several million people. Flooding also occurs in the various tributaries of the Mekong River, but is of a more sporadic nature and with a typical duration of several days to one week.
Droughts can occur at any location in the Mekong Basin. Meteorological droughts (characterized by rainfall deficits) are especially devastating for rainfed agriculture. Two areas of the LMB particularly prone to meteorological droughts are the Western region of the Khorat Plateau in North-eastern Thailand and the South-eastern area of Cambodia. Hydrological droughts (characterized by stream flow deficits) allow ocean salinity to penetrate further up the waterways of the Cuu Long Delta, thereby limiting irrigation use of these waters during the low flow season.
Climate change has the potential to worsen both flooding behaviour and drought impacts. This report examines and identifies shortcomings in several studies of climate change in the LMB. A more considered approach to deal with climate change impacts in the basin is outlined.