SURFACE EXCHANGE OF WATER-VAPOR 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 Minnes ota (U.S.A.) from May to October in 1991 and 1992. The daily ET rate r anged 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 rat e of open water evaporation when the vascular plants were actively gro wing and the water table level was within or above the rooting zone. U sing a dual-source modification of the Penman-Monteith equation (Massm an, 1992), we partitioned the measured ET into evaporation from the no n-vascular Sphagnum surfaces and transpiration from vascular plants. T he analysis indicated that about two thirds of the water vapour flux t o the atmosphere was from evaporation when the Sphagnum surface was we t. Such an evaporative flux was expected because of vertical distribut ion 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 d ay. The remainder of R(n) was thus available for evaporation from Spha gnum. Evaporation significantly decreased as the Sphagnum surface drie d out. When the water table was within the rooting zone (0-0.4 m), the vascular plants absorbed Sphagnum-generated sensible heat, which amou nted up to one third of their transpiration energy flux. Under these c onditions, the total water vapour flux remained near its potential rat e 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 plan t senescence resulted in ET rates lower than the potential rates by 5- 65%.