Field investigation and hydrological modelling of a sub-arctic wetland system by SLURP and WATFLOOD

The wetlands existing as bogs, fens, swamps, marshes and shallow water comprise 14% of Canadian land. Recently, there are growing research interests in the hydrological characteristics of arctic and subarctic wetland systems in the need for more efficiently conserve wetlands and assess climate change related impacts. This research targeted the Deer River watershed near Churchill, Manitoba, which presents a typical subarctic wetland system in the Hudson Bay Lowlands. An extensive field investigation was first conducted from 2006 to 2008 to facilitate in-depth understanding of the wetland hydrology; and two semi-distributed hydrological models, SLURP and WATFLOOD, were employed to simulate the hydrologic cycle in the targeted subarctic wetland. -- The 28-year historic data (1978 - 2005) revealed a steady elevation of mean temperature and accumulative precipitation in the summertime (late June - early October). The 3-year field observation (2006-2008) also provided evidence to indicate a warming climate in the watershed. Frost table, soil moisture and streamflow were monitored and analyzed to advance the acknowledgement of the climatic, geographical and hydrological characteristics of the subarctic wetlands. The frost tables at the monitored transects were declining and reciprocal at their distances to the stream channels because of the subsurface flow within organic layer moving towards the stream and accelerating the thaw of frozen soil. Following the major recharge period during the snowmelt, soil moisture contents in the shallow layers of the wetland kept declining over time throughout the summer. The water discharges were low before September due to low precipitation and strong evapotranspiration as well as expansion of storage capacity of the organic soil layers, and then gradually increased due to the intensive precipitation in the fall. All the monitored streams showed prolonged responses to precipitation due to the combined effects of shallow impermeable frost table, porous soil, and varied soil storage capacity. -- Based on the watershed delineation by River Tools and TOPAZ, SLURP and WATFLOOD were applied to further justify the conclusions from field investigation and examine their applicability on subarctic wetlands. The results also revealed the distinguishable hydrological features of sub-arctic wetlands. It was observed that the snowmelt in the spring season produced the highest peak discharges and contributed to the majority of the annual streamflow. Peaks of the simulated spring flows from both models were to some extent lower than the observed ones. This could be attributed to the effects of extensive wetland ponds and shallow permafrost tables which could restrict the infiltration of rainwater and drive the snowmelt to form spring flow peaks. It was also shown that most of the small or moderate rainfall events during the summertime were unable to generate noticeable surface runoff possibly due to canopy interception, depression storage, porous soil layers, descending permafrost table and intensive evapotranspiration. A thorough comparison between SLURP and WATFLOOD was conducted from the aspects of modelling structure, formulation, parameters, and results, which indicated that SLURP presented a slightly better overall performance than WATFLOOD in most of the years at both watershed- and sub-basin level simulation.