Surface exchange of water vapour between an open Sphagnum fen and the atmosphere

Water loss by evapotranspiration (ET) is a principal component of the hydrologic cycle in wetlands. Using micrometeorological techniques, we measured ET from a Sphagnum-dominated open fen in northcentral Minnesota (U.S.A.) from May to October in 1991 and 1992. The daily ET rate ranged from 0.2-4.8 mm d(-1) with a growing season average of 3.0 mm d(-1). The evapotranspiration rate of the fen was near the potential rate of open water evaporation when the vascular plants were actively growing and the water table level was within or above the rooting zone. Using a dual-source modification of the Penman-Monteith equation (Massman, 1992), we partitioned the measured ET into evaporation from the non-vascular Sphagnum surfaces and transpiration from vascular plants. The analysis indicated that about two thirds of the water vapour flux to the atmosphere was from evaporation when the Sphagnum surface was wet. Such an evaporative flux was expected because of vertical distribution of vascular plant leaves which had a small leaf area index (0.4-0.7) and intercepted only about 30% of net radiation (R(n)) during the day. The remainder of R(n) was thus available for evaporation from Sphagnum. Evaporation significantly decreased as the Sphagnum surface dried out. When the water table was within the rooting zone (0-0.4 m), the vascular plants absorbed Sphagnum-generated sensible heat, which amounted up to one third of their transpiration energy flux. Under these conditions, the total water vapour flux remained near its potential rate owing to the enhanced transpiration from vascular plants. A drop in water table of 0.15-0.2 m below the hollow bottom during vascular plant senescence resulted in ET rates lower than the potential rates by 5-65%.