Soil Moisture Response to a Changing Climate in Arctic Regions

Soil moisture is the land surface hydrologic variable that most strongly affects land-atmosphere moisture and energy fluxes. In Arctic regions, these interactions are complicated by the role of permafrost. Especially in northern regions, soil moisture therefore is important not only as a hydrological storage component, also as a result of its strong influence on the hydrological cycle through controls on energy fluxes such as evaporative heat flux, phase change in thawing of permafrost, and effects on thermal conductivity. With projected increases in surface temperature and decreases in surface moisture levels that may be associated with global warming, it is likely that the active layer thickness will increase, leading to subtle but predictable ecosystem responses such as vegetation changes. Field measurements of soil moisture have been collected on the North Slope of Alaska, with emphasis upon establishing macro and micro-topographic influences. Sites were installed in the foothill regions and on the coastal plain of the Kuparuk River basin. Spatially distributed model simulations are being conducted across a range of scales. Preliminary results indicate macro-topographic gradients greatly impact the importance of lateral versus vertical fluxes. Micro-topographic differences affect the small spatial scale differences in soil moisture, but have less impact upon flux direction. Permafrost in arctic regions exerts a significant influence on soil moisture through controls on vegetation and drainage. In relatively flat areas where the frozen layer is near the surface, the soil moisture contents are usually quite high. These areas have relatively high evapotranspiration and sensible heat transfer, but quite low conductive heat transfers due to the insulative properties of thick organic soils. As in more temperate regions, watershed morphology exerts strong controls on hydrological processes; however unique to arctic watersheds are complications arising from the short-term active layer dynamics and longer-term permafrost dynamics. As the active layer becomes thicker throughout the summer, it has a greater capacity to store water, resulting in a time-varying basin response to storm events. As the season progresses, the stream recession rates increase as more hillslope water flows through the soil rather than as overland flow. Peak flows are also more attenuated as the active layer increases in thickness or as permafrost areal extent decreases.