Investigating Impacts of Hydrological Components to the Discharge of Lena Watershed Using a Land Surface Model (CHANGE)

In eastern Siberia, a number of changes of terrestrial processes have been documented from field observations and model simulations. The representative changes are the increasing river discharge, permafrost reduction, expanded growth period and wetness. To assess the impacts of the terrestrial processes to increased river discharge, a land surface model (CHANGE, coupled hydrological and biogeochemical model) was applied to Lena watershed over 1986 to 2004. The model does consider the effects of the components (i.e. snow processes, soil organic matter and ice within soil layers) of the hydrological and biogeochemical processes in the Arctic. The model also represents spatial heterogeneity in land cover by dividing each grid cell into three land cover types: lake, wetland, and vegetation. The vegetated portion of the grid cell is further divided into several patches of plant functional types. Multiple plant functional types can co-occur in a grid cell.

Dynamics in land surface processes during the modeling period were analyzed for Lena watershed, especially for two small watersheds (Aldan and Upper Lena) within the watershed. Air temperature, precipitation, and discharge in the watersheds have been increased since 1986. Simulation also indicated the increasing trends in active layer depth (ALD), snow water equivalent (SWE), and evapotranspiration (ET). The increases were especially significant in southern mountainous region of Lena watershed. In the two small watersheds, SWE was highly correlated to discharge, which suggests that the melted snow does greatly affect to the peak discharge in the early spring. The increased ALD also indicated similar pattern with discharge variation in the small watersheds. However, ALD does not directly affect to river discharge. The melted ice water caused by the thawed ALD is probably more related to river discharge. The melted ice indicated good correlation with discharge, as well as ET. This suggests that water produced by the thawed ALD was useful to ET and discharge during the summer season. Although precipitation in Lena watershed was in increasing trend, soil water storage did not indicate any trend. Of course, the inter-annual variability of soil water storage did correspond to one of precipitation. The trend of soil water storage suggests that the increased precipitation could contribute to discharge and ET.