The correlations of multi-timescale characteristics of water level processes in Lancang-Mekong River

With the recent dramatic advances of the significant regional cooperations in economy around Lancang-Mekong River Subregion, the changes, regularity and influences of the water resource in this area have received remarkable international attentions and become a focus in the research community of water resource. Based on the daily water level from 1986 through 2004 observed at Yunjinghong station and Chiang Saen gauging station, a quantitative examination has been made for the multi-timescale characteristics and correlations of the water level variations between the upper and lower reaches of Lancang-Mekong River through binomial coefficient weighted average method, five-year moving average method, and other statistical methods. Besides, the major influences of cascade hydropower dams in the main upstream on the transboundary water level during key years are also reviewed. The results demonstrate that the annual variations of water level in Lancang River exhibit multi-timescale characteristics. And positive correlations are observed between the multi-timescale variations of the Lancang River water level in lower reaches and the Lower-Mekong River water level in upper reaches. Annual variations of the water level in upper-lower reaches exhibit prominent interannual and interdecadal features, during the recent 18 years, while differences mainly happen to the smaller timescales. That shows the water level annual variations at both stations are influenced by the climatic factors (e.g. precipitation) and solar activity. The normal operation of cascade hydropower dams in the main upstream of Lancnag River have some interim effects on downstream monthly or yearly water level. It may have more impacts on short-term daily water level, such as of few day's varitations. It also can be found that the individual monthly-high water level between two stations show prominent positive correlation, as well as the dates of that. But as far as the multi-timescale correlations of water level in Lancang and Lower Mekong River is concerned, the major causes of considerable differences in water level variations are attributable to both runoff flowing from the upstream (the Lancang River) and the high local inflow between two stations.