Rising minimum daily flows in northern Eurasian rivers: A growing influence of groundwater in the high-latitude hydrologic cycle

A first analysis of new daily discharge data for 111 northern rivers from 1936 - 1999 and 1958-1989 finds an overall pattern of increasing minimum daily flows (or low flows'') throughout Russia. These increases are generally more abundant than are increases in mean flow and appear to drive much of the overall rise in mean flow observed here and in previous studies. Minimum flow decreases have also occurred but are less abundant. The minimum flow increases are found in summer as well as winter and in nonpermafrost as well as permafrost terrain. No robust spatial contrasts are found between the European Russia, Ob', Yenisey, and Lena/eastern Siberia sectors. A subset of 12 unusually long discharge records from 1935-2002, concentrated in south central Russia, suggests that recent minimum flow increases since similar to 1985 are largely unprecedented in the instrumental record, at least for this small group of stations. If minimum flows are presumed sensitive to groundwater and unsaturated zone inputs to river discharge, then the data suggest a broad- scale mobilization of such water sources in the late 20th century. We speculate that reduced intensity of seasonal ground freezing, together with precipitation increases, might drive much of the well documented but poorly understood increases in river discharge to the Arctic Ocean.