Reconstruction and evaluation of changes in hydrologic conditions over a transboundary region by a regional climate model coupled with a physically-based hydrology model: Application to Thao river watershed.

The differences among countries in terms of physical features, governmental policies, priorities in short- and long-term water resources management may lead to conflicts in managing and sharing of water resources over the transboundary regions. Due to no formal data sharing agreement between countries, there has been usually no data availability at transboundary regions. In this study, a methodology, in which a physically-based hydrology model was coupled with a regional climate model, is proposed to reconstruct and evaluate hydrologic conditions over transboundary regions. For the case study, Thao river watershed (TRW), within Vietnam and China, was selected. The Watershed Environmental Hydrology (WEHY) model was implemented based on topography, soil, and land use/cover information which was retrieved from global satellite data resources. The watershed model-WEHY was first validated over the TRW, and then was used to reconstruct historical hydrologic conditions during 1950-2007. The results of this study suggest no significant trend in the annual streamflow over the target watershed. In addition, there is a time shift in the wet season between the upstream sector in China and the downstream sector in Vietnam over the TRW. The annual flow contribution from the upstream sector in China to the outlet of TRW is estimated to be around 66%, and the remaining 34% contribution comes from the downstream sector in Vietnam territory. Last but not the least, the annual flow as a function of return period varies not only with the return period but also as a function of the time window, reflecting the effect of the changing regime on the streamflows at the TRW. The evolution of the flow frequency through time is an evidence of the ongoing non-stationarity in the hydrologic conditions over TRW.