SBR- Content.pmd - INBO

3 - Water in the Mekong BasinTwo other useful indicators of the variability of flood flow behaviour are the Index of Variation (I v),which is the standard deviation of the annual flood series in the log domain, and the ratio of the floodwith a 100 year return period to the mean annual flood. For the Mekong at Chiang Saen, the I vis0.12 and the ratio of the 100 yr ARI flood to the mean annual flood is 2.3. By comparison, the worldaverage I vis 0.17 and the Asian average 0.16, while the ratio of the 100 year ARI flood to the meanannual flood, is 4.5 for world rivers and 3.4 for those in Asia. 3 These data demonstrate that, relativeto many other rivers, when exceptional floods do occur in the Mekong, they are not very much largerthan the normal average flood.3. Seasonal runoff patternsPatterns of discharge in the mainstream of the Mekong are seasonal (Figure 5a & 5b) with the floodpeaking in August in the upper part of the lower Mekong basin and in September in the lower part.Note that the flow peak in Phnom Penh does not coincide with peak rainfall, which occurs in October(Figure 5c).Figure 5.Average monthly flows for the Mekong River at (a) Chiang Saen and (b) Phnom Penh,and (c) monthly average rainfall for Phnom Penh.17