Hydrologic controls on the survival of Water Howellia (Howellia aquatilis) and implications of land management

Water Howellia (Howellia aquatilis) is a rare and endangered wetland plant listed as a threatened species under the Federal Endangered Species Act. 70% of the world population is found in the 1.7×105 ha Swan Valley, Montana, USA in wetlands of usually less than 1-ha. Survival of this aquatic species requires emergence during mid-summer, the drying out of occupied portions of the wetlands, a slight refilling of the wetlands in fall, and wetland re-establishment in the spring. This project was designed to establish the water source and seasonal controls on stage variation. Research approaches included water budgets derived from extensive micro-basin field measurements, including evaluation of ground water-surface water interaction, and standard geochemical analyses. Potential impacts to the wetland water balance from timber harvesting or stand replacement fires were evaluated using the WRNSHYD® model. Results from two field seasons indicate that ground water inflow and plant transpiration control wetland stage. Snow survey recharge estimates, horizontal and vertical hydraulic gradient observations, seepage meter flux rates, wetland and ground water quality and the ground water inflow component of the water balance support the hypothesis that seasonal changes in the wetland are driven by a localized ground water flow system. Modeling predicts an increase in micro-basin water yield due to decreased plant transpirational water loss when watershed trees are removed by harvesting or a stand replacing fire. The duration of a potential increase in water yield is unknown.