Water tracer model-assessed contributions of source waters to changing circumpolar Arctic terrestrial evapotranspiration and river discharge

• Tracer model quantitatively separated contributions of source waters to evapotranspiration and discharge in the Arctic river basins. • Rain and snowmelt water are major sources to evapotranspiration and discharge, respectively, and discharge shows seasonally varying sources. • Rain plays a key role in driving interannual, seasonal, and regional variability of evapotranspiration and discharge. • The individual roles of rain and groundwater to evapotranspiration and discharge are increasing during 1979 to 2016. Climate change has resulted in alteration of snow cover, permafrost degradation, vegetation infilling, and precipitation apportionment across the Arctic terrestrial region, which has in turn affected the balance of hydrological processes including seasonality and interaction of snowmelt, rain, and soil water storage. Effects on Arctic river discharge have been widely observed, although relatively few studies have provided detailed assessments of the underlying causes of change, including adjustments in the timing and relative contributions of source waters (i.e., snowmelt, rainwater, soil water, permafrost thaw) and the effects of altered evapotranspiration regimes. Principal challenges have included limitations in the observational networks, which have often frustrated efforts to reliably model complex changes that are underway. This study explores a tracer-aided ecohydrological model that was used to quantitatively assess source water contributions to evapotranspiration and discharge from the circumpolar Arctic river basin, based on three meteorological forcing datasets from the period of 1979 to 2016. The model, which provides additional constraints on water partitioning using isotopic evidence, revealed that rain and snowmelt water accounted for 67% and 39% of the annual evapotranspiration and discharge averaged over the study period, respectively. Rainwater contributions to annual evapotranspiration and discharge were found to have increasing trends over the study period, whereas snowmelt water showed fairly stable or insignificant negative trends. Rainwater was determined to be a dominant source of evapotranspiration across the growing season, while discharge was sustained by seasonally-varying dominant water sources. Earlier snowmelt events also appear to have increased the proportion of snow and rain in peak discharge, with rainwater sources being linked largely to autumn storage in the previous year. We attribute higher evapotranspiration in summer to reduction in the proportion of rain and snowmelt water in summer discharge, resulting in negative interannual trend. Both soil water and permafrost thaw likely have contributed to increases in cold season discharge, although the proportion accounted for by permafrost thaw sources was mostly low. Our analysis renders a new perspective on underlying changes to source water partitioning, especially enhanced rainwater contributions, as a key driver of seasonal, interannual and regional changes in terrestrial evapotranspiration and discharge across the Arctic region.