EFFECTS OF RIPARIAN VEGETATION ON THE HYDROPERIOD OF SEASONAL WETLANDS

Seasonal wetlands are common in regions where topographic depressions trap surface runoff, such as the prairie wetlands in the northern glaciated plains of North America. The bottom sediments of seasonal wetlands are subjected to frequent oxidation, which enhances nutrient recycling. The duration of surface water, or hydroperiod, is a critical habitat parameter for seasonal wetlands. The infiltration "loss" is among the most important factors determining the hydroperiod especially for those wetlands that do not have surface outflow. Riparian vegetation affects infiltration rates by taking water from the root zone, thereby inducing subsurface flow towards the riparian zone. Studies were conducted in the St. Denis National Wildlife Area in Saskatchewan, Canada to evaluate the infiltration loss and the effects of the riparian vegetation. Surface and groundwater levels were monitored in an experimental wetland. A bromide tracer was released in the wetland, and concentrations in surface water, groundwater, and plant tissues were monitored. Hydraulic measurements and bromide mass-balance calculations indicated that the infiltration represented a much larger water loss from the wetland than evaporation. However, the water infiltrating under the wetland mostly stayed within the top 1-2 m and flowed laterally towards the wetland perimeter. The infiltration rate increased as the ratio of the perimeter length to the area increased, indicating the significant effects of the shoreline-related loss, due to water uptake by the riparian vegetation.