Abstract
This study employs stable oxygen and hydrogen isotopes as natural tracers to assess the headwater of a landslide next to a drainage divide and the importance of the slope's headwater in the study area. The study is undertaken near WuโShe Township in the mountains of central Taiwan. Because a reservoir is located on the other side of the divide, this study evaluates the relationship between the reservoir water and headwater of the landslide as well. Over a 1โyear period, water samples from September 2008 to September 2009, including local precipitation (LP), WuโShe Reservoir's water (WSRW), slope groundwater (SGW), upperโreach stream water (USTW), and downโreach stream water (DSTW), were analysed for deuterium (ฮดD) and oxygen (ฮด^18^O) stable isotopes. Results indicate that WSRW is the predominant component in SGW: approximately 70% of SGW originates from WSRW and 30% from LP based on a two endโmember massโbalance mixing model for ฮด^18^O. The similar two endโmember mixing model is also employed to assess the contributions of USTW and SGW to DSTW. Model results indicate that SGW is the major source of DSTW with a contribution of about 67%. Accordingly, about 47% of DSTW sources from the WSRW. In short, owing to reservoir leakage, WSRW contributes the greater part of both SGW and DSTW. Plentiful WSRW in SGW threatens the stability of the slope in the divide area. To avoid subsequent continuous slope failure, necessary mitigation steps are required. Copyright ยฉ 2011 John Wiley & Sons, Ltd.