Alpine wetlands regulate water storage and release in mountain headwater systems, yet their hydrological functioning remains poorly quantified, particularly with respect to evapotranspiration (ET) and groundwater-surface water exchange. As climate warming alters snowpack, melt timing, and vegetation dynamics, improved understanding of wetland ET processes is essential for predicting downstream water availability. This study examines the seasonal drivers of ET and groundwater contributions to ET in a sub-alpine wetland in the Canadian Rocky Mountains using eddy covariance, chamber-based flux measurements, and hydrometeorological observations. Site-scale ET was measured using a flux tower, while microsite-scale evaporative losses were quantified using portable chambers across contrasting surface and vegetation conditions. Groundwater dynamics were assessed using water table, soil moisture, and precipitation data. With seasonal periods delineated using carbon flux dynamics to distinguish snowmelt, growing season, and late-season dry-down. Results show strong seasonal variability in ET magnitude and controls. During snowmelt, elevated water tables sustained high evaporative losses despite low atmospheric demand, indicating a strong subsurface control. Growing-season ET was increasingly governed by radiative forcing and atmospheric demand, while late-season ET declined as water tables receded. Chamber measurements revealed spatial heterogeneity in fluxes but scaled closely with tower-derived ET. Groundwater contributions to ET were greatest during early season melt and diminished as storage was depleted. These findings highlight the role of alpine wetlands in redistributing snow-derived water and their importance for headwater hydrology under a warming climate.