Understanding the causes of streamflow changes in the Eurasian Arctic

Abstract

Since the 1930s, streamflow from Eurasian rivers discharging to the Arctic Ocean has been increasing. Much of this change has been attributed to increased precipitation, but recent studies show inconsistencies in long-term streamflow and precipitation trends. Alternatively, these inconsistencies may imply a release of water from storage via permafrost degradation. For many of the Eurasian rivers, an increase in annual streamflow volume has been accompanied by a shift in seasonality. However, the signature of these changes in forcings and/or internal hydrologic dynamics are potentially confounded by the construction and operation of large reservoirs on many of the region's major rivers. A combination of data analysis and macroscale hydrological modeling are used to explore the controls on observed changes in annual and seasonal streamflow for several basins in the Eurasian Arctic. This work comprises three related studies. First, by comparing trends in observed streamflow and precipitation for a number of periods between 1936 and 2000, we find that streamflow trends generally exceeded precipitation trends for the permafrost basins (likely due to permafrost degradation), whereas precipitation trends generally exceeded streamflow trends over the non-permafrost basins (likely due to increased summer evapotranspiration). Second, using a physically-based reservoir model coupled to the Variable Infiltration Capacity (VIC) macroscale hydrology model, we find that, while reservoir construction and operation apparently had little effect on long-term annual streamflow trends, they are responsible for much of the observed increases in winter and early spring streamflow from the regulated basins. Third, by applying the VIC model over the Eurasian Arctic, we explored the roles of evapotranspiration, snowpack storage, and soil moisture storage (both liquid and frozen) in contributing to discrepancies between monthly precipitation and streamflow trends. Evapotranspiration influenced streamflow trends between May and September, while the effects of snowpack dynamics (the greatest contributor to seasonal streamflow changes) were greatest during the spring snowmelt and the fall onset of snowpack accumulation. The effects of soil moisture dynamics on streamflow changes were dominant during the snow-free season, although for some permafrost basins, an increase in active layer depth likely resulted in greater baseflow into the fall and early winter.