SBR- Content.pmd - INBO

State of the Basin Report - 2003Figure 8. Pattern of change in rainfall for three sites with continuous records since 1950Note: As for Figure 7, in months indicated in red there was a statistically significant decrease in rainfall overthe period, while months indicated in pink had a decrease in rainfall that was not statistically significant.Months indicated in bright green showed a statistically significant increase in rainfall, while those inpale green showed an increase which was not statistically significant.Downstream, at Mukdahan and Pakse, the patterns are similar for most months (Figure 8). AtMukdahan, significant dry season increases occurred in March and April, and significant decreasesoccurred in September and October. At Pakse, there has been a significant decline in discharge inOctober and November and increases in March, April and May. The pattern is more stronglydeveloped at Mukdahan and Pakse, than at Chiang Saen, with significant changes apparent in fourmonths at Mukdahan and five at Pakse. The same pattern was apparent, but less prominent,especially in the dry season at Nong Kai. Data from Luang Prabang did not show the pattern at all,but there are concerns about the quality of data at both of these last two sites.This pattern of decreased flows in the wet season and increases in the dry season, was not causedby changes in rainfall patterns. An analysis of the monthly rainfall records from Vientiane,Mukdahan and Ubon Ratchathani did not find trends consistent with the trends in discharge (Figure8). At Ubon there was a significant decrease in rainfall in March, which is not consistent with theincrease in discharge at Pakse in the same month. At Mukdahan a significant increase in rainfalloccurred in November, a month for which there was a significant decrease in discharge. The onlyconsistency is a significant decrease in rainfall at Vientiane in September that is consistent with adecrease in discharge in Mukdahan in the same month. From this data it appears that changes inrainfall have not been responsible for the pattern of change in river discharge.The other possible causal factor for the change is human activity. The pattern is consistent withthe operation of reservoirs storing wet season flows and releasing them during the dry season.The pattern does not appear to have been caused by a single large event, such as the commencementof operation of a single large dam. Rather, it appears to have been relatively continuous overseveral decades, but to have possibly stabilised since the early 1970s. Changes of this kind havebeen demonstrated in the Mekong mainstream, for example following the enlargement of NamNgum Dam in Lao PDR 6 , but the many smaller water control structures known to have beenconstructed in the basin have probably had far greater impact (Fig.9).20