SBR- Content.pmd - INBO

State of the Basin Report - 2003The pattern is not consistent with changes caused by irrigation or catchment vegetation clearance.Irrigation extractions involving direct pumping from the river or diversions from a reservoir would beexpected to reduce dry season flows. In some cases, increased water is released downstream froman irrigation dam during low flow periods to ensure sufficient water is available for farmers to pumpfrom the river, but this pattern of water release is not known from this region. Forest clearing wouldtend to increase runoff during wet periods and reduce dry season runoff, the opposite of the patternfound here.The negative regression slopes are far steeper than the positive slopes (Figure 10). This indicatesthat there has been a more pronounced change in wet season flows than in the dry season, since theregression lines indicating the trend are steeper. If water during the wet season were being stored inrice fields or reservoirs and then lost either to groundwater or through evaporation, that wouldexplain this pattern.Figure 10. A plot of the slopes of the regression lines of monthly discharge vs year for data fromChiang Saen (short dashes), Mukdahan (solid line) and Pakse (long dashes).Note: The slope of the line is plotted against month, with positive values indicating atrend to increasing monthly discharge and a negative slope indicating a decreasingmonthly discharge.In summary, there are clear patterns of change in the hydrology of the river since about 1950. Thechanges do not appear to be consistent with the perceptions and fears of the river community – inparticular there is no evidence of reduced dry season flows. The causes of the changes are not clearbut appear to be from direct human interventionin the catchment rather than climate change.Clearly there is a need for a great deal morefield investigation before we can be confidentthat we can predict and manage changes inflow in the Mekong River.MRC and its predecessor the MekongCommittee, have been monitoringMekong River water levelssince the 1960s22