The Lena River is one of the largest rivers in the Arctic that flow northward from mid latitudes to the Arctic coast (Figure 1), and it contributes about 15% of total freshwater flow into the Arctic Ocean [Shiklomanov et al., 2000; Prowse and Flegg, 2000]. The drainage area of the Lena basin is 2,430,000 km2, approximately 78–93% of which is underlain by permafrost [Brown and Haggerty, 1998; Zhang et al., 1999, 2000]. Lena basin is less developed in comparison to other large rivers in Siberia, such as the Yenisei and Ob rivers [Dynesius and Nilsson, 1994]. Studies show that anthropogenic diversions of the Lena River appear to be minor factor at present, and thus the change in river discharge may serve as a reliable indicator of regional climate change and variation [Shiklomanov, 1997; Shiklomanov et al., 2000; Savelieva et al., 2000; Dynesius and Nilsson, 1994]. Since the late 1930s hydrological observations in the Siberian regions, such as discharge, stream water temperature, river-ice thickness, dates of river freeze-up and break-up, have been carried out by the Russian Hydrometeorological Services and the observational records have been quality-controlled and archived by the same agency [Shiklomanov et al., 2000]. These data are now available from the R-ArcticNET, a digital archive (CD-ROM) for the Arctic drainage [Lammers et al., 2001]. In this analysis, long-term (1935–1999) daily, monthly and annual discharge records collected at the basin outlet (namely Kusur station, 70.70°N/127.65°E) were used. It is known that winter discharge measurements under ice conditions are less accurate, with the potential errors being 15–30% over the Arctic regions [Grabs et al., 2001]. In the former USSR, winter stream flow under ice conditions was determined by a standard procedure that involves direct discharge measurement, adjustment of the open water stage-discharge relation according to climatological data, and comparison of stream flow with nearby stations [Pelletier, 1990]. Application of this standard method in Siberian regions produces compatible and consistent discharge records over time and space. In addition, Lena basin mean monthly temperature and precipitation records derived from the global data sets [Jones, 1994; Hulme, 1991] are also utilized.
 The main objective of this study is to examine Siberian regional hydrologic change in the Lena River basin under a climatic warming condition. This analysis will quantify the annual and seasonal freshwater fluxes to the Arctic Ocean from the Lena River and their interannual variation and long-term trends, with an emphasis on defining the discharge regime and detecting its recent change (regime shift) through statistical approaches.
 We use correlation analysis and statistical significance test to assess the associations among climatic and hydrologic variables. Long-term changes are identified by linear trend analysis, i.e. a nonparametric Kendall-tau based method [Sen, 1968; Lettenmaier et al., 1994; Wang and Swail, 2001]. In addition, we will also present some complementary results on river ice thickness and permafrost data for the Lena River basin. The focus of this analysis is on the basin scale (as a whole at the outlet control station) in order to improve our understanding of the integrated, large-scale hydrologic processes in the northern regions. These analyses will enhance our capability of land-surface modeling particularly over large river basins in the high latitudes.