The First Application and Validation of the Hydrologic Model WaSiM-ETH at a Watershed Underlain by Permafrost

The Water Balance Simulation Model ETH, WaSiM-ETH, is a deterministic, spatially distributed hydrological model that has been successfully applied to numerous basins ranging from tropical rainforests in Asia to Alpine regions in Europe. This is the first time WaSiM-ETH is applied to the Arctic. Until now, the model has been lacking an active layer algorithm that is essential to successfully simulate the hydrologic regime in permafrost regions. Although the long-term goal of WaSiM-ETH is to dynamically couple the thermal and hydrological regimes, we implemented a simple empiric formula alpha*sqrt(snowfreedays) to represent the seasonal freezing and thawing of the active layer. The count of snow-free-days starts as soon as the snow water equivalent decreases below a defined value. If fresh snow falls, then two things can happen: 1) increasing the counter for snow free days will be paused until the snow melts away within a defined period. The snow free days are increased normally once the snow cover is ablated. 2) If the snow cover lasts for more than a specified time, the snow-free-days counter is reset to zero thus setting the thaw depth to zero. The latter is the simple refreezing model. We applied the model to an intensively studied wetland in northern Alaska that has all portions of the water balance measured. The watershed is mainly represented by a drained thaw lake basin and hence, exhibit low hydraulic gradients with the main topographical features formed by high and low centered polygons. Evapotranspiration (ET) represents the major pathway of water loss from the wetland despite several ET-limiting factors. Through its dynamic linkage between soil moisture and evaporation, WaSiM-ETH allows for a realistic representation of evaporation from the moss dominated wetland that experiences increased canopy resistance in late summer. WaSiM-ETH has a surface routing scheme that takes into account ponds as well as small scale topography that are typical features in the area. Lateral flows exhibit a strong seasonal variation with peak flow during snowmelt. The modeling of lateral flow is complicated by the disconnection of the early season flow paths, leaving most low centered polygons as individual hydrologic units a week after snow melt. We present a validation of the modified WaSiM-ETH on measured runoff, evapotranspiration and spatially distributed water table elevations from an arctic watershed underlain by permafrost. Our aim is to reduce the uncertainty in water table projections at finer scales (<1 m) and hence, refine the understanding of the future of the Arctic wetlands.