ИСТИНА

A long–term daily streamflow dataset, covering the period from 1925 to 2013, was compiled for the Lena River basin, with an average record length of 49 years. Time–series of mean annual daily flow (MADF) and extreme (maximum, Qmax, and minimum, Qmin) daily flows were subject to trend detection and change–point detection analysis. Significant changes result mostly from rapid 'breakpoint' homogeneity disruptions. Thirty one time–series showed trends in MADF, 10, in Qmax, and 32 of the 55 records with non–zero Qmin, significant at p ≤ 0.05. Upward trends prevail in both mean annual and extreme flows, with average magnitudes of 47% (MADF), 56% (Qmax) and 68% (Qmin). Two to three stations in each subset showed downward trends (averaging –20%, –39% and –38%, respectively). Abrupt changes are observed in the 1990s and early 2000s mostly in the headwaters of the basin, underlain by discontinuous permafrost. Step trends are responsible for the major part of the present runoff increase in the Lena River basin, suggesting an abrupt shift in this hydrologic system. This transition started in the late 1980s with a rapid change in winter baseflow, and was followed by an increase in mean annual flows a decade later, though there may exist no causal relation between these trends. The rapidity of change casts reasonable doubts on the capacity of natural drivers to produce such effects in a short amount of time, besides catastrophic events. In this case, an appeal towards an a priori assumption that the observed trends are mainly caused by human influence, either direct or indirect, is hard to refute.