We studied water loss performance in a model plant, the Tufted sedge (Carex elata All.), which is an active water balance component of subsurface flow constructed wetlands. Due to active regulation of transpiration, the volume and dynamics of water loss in these constructed wetlands are difficult to plan without preliminary and targeted measurements and calculations with regard to the specific plant component. We estimated transpiration values in the laboratory based on daytime transpiration ranges for spring, summer and autumn, and examined the transpiration effect of the hydraulic load. During spring, water loss via transpiration can reach 83% of the hydraulic load on certain days. During summer, this value can increase to 100% of the hydraulic load, which means that the daytime transpiration can significantly affect effluent concentration. Air humidity proved to be the most critical environmental factor for water loss resulting from transpiration, therefore a water discharge plan designed in such a way as to be able to also adjust soil moisture is the key to optimal water circulation at the system level.