Floods, Droughts, and the Human Transformation of the Mekong River Basin

The Mekong River basin suffered large flood and drought damages in the second half of the 20th century, raising widespread concerns about possible streamflow trends resulting from the unprecedented land cover and use changes of this period - especially deforestation. Conflicting opinions and lacking scientific evidence on streamflow trends, both quantity and composition, have hampered policymaking. Combining empirical and modeling approaches, we demonstrate that streamflow trends have been significant but modest. They were strongest in the dry season and differed between regions, depending on the extent of agricultural expansion, rain-fed and irrigated areas, dry-season cultivation (double-cropping), which historical landcover was replaced by agriculture, and possible precipitation trends. More significant trends are observed in sediment transport. Still greater streamflow modifications are expected from the planned cascade of reservoirs in the Chinese Mekong, whose snowmelt contributes a large fraction of dry season discharge downstream. The role of streamflow trends in the destructiveness of recent floods and droughts is overshadowed in importance by a different trend: an increase in the vulnerability of crops and human settlements to hydrologic extremes and variability.