SBR- Content.pmd - INBO

3 - Water in the Mekong BasinThe four months from July to October account for 64 percent of the annual flow at Chiang Saen and74 percent of the average annual flow at Phnom Penh. During this period, extensive areas of thefloodplain, particularly in the region from the Khone Falls to the sea, are inundated. The total area offloodplain submerged in any given year ranges from 1 to 4 million hectares. 4The Tonle Sap Great Lake and the floodplain around Phnom Penh act as flood buffers for the deltain Viet Nam. From mid-May to early October, as the water in the Mekong rises, it spreads acrossthe floodplain and up the Tonle Sap River into the Great Lake. The depth of the Great Lake increasesfrom a dry season maximum of 3.6 m to more than 10 m, and the area of open water increases fromapproximately 2,500 km 2 to as much as 13,000 km 2 (Figure 6). As the Mekong River water levelsfall in later October and November, much of the water flows off the floodplain and down the TonleSap River. Water from the Great Lake continues to supplement the flow of the Mekong downstreamthrough the dry season, contributing 16 percent of the flow. 5In the dry season months of March to May, there has been an increase in monthly discharge overtime at Chiang Saen, Mukdahan and Pakse. In the wet season months of June to November, therehas been a decrease at these same sites based on data from 1950 (1961 from Chiang Saen). The trendbecomes stronger at the downstream sites.At Chiang Saen, the decline was statistically significant during August, but not for any other month(Figure 7). March, April and May have positive slopes for the regression lines, while the monthsfrom June to December have negative slopes.Figure 7. Pattern of change in discharge at Chiang Saen (1961-1999), Mukdahan (1950-2000)and Pakse (1950-2000)Note :Pink and red areas indicate negative slope to regression lines (i.e. discharge decreasingover time), green areas indicate positive slope (i.e. discharge increasing over time).Red and dark green areas indicate that the slope of the line was statistically significantlydifferent from 0 (p < 0.05).19