Evapotranspiration from a riparian fen wetland

Evapotranspiration rates were measured in a riparian fen wetland dominated by vascular vegetation and surrounded by open agricultural areas and forests. The wetland is situated on a floodplain in central Denmark. Measurements were taken throughout the growing season (April–September) of 1999. Evapotranspiration rates were higher than those published for most other wetland types, with an average of 3.6 mm d −1 during the growing season and a peak rate of 5.6 mm d −1 . Daily average evapotranspiration was 110% of Penman9s potential open water evaporation. Evapotranspiration was the dominant sink in the energy balance of the wetland studied. During the day, evapotranspiration accounted for 82% of the available radiant energy, R n . Due to the presence of deposited fine-grained sediments, soil-water availability was kept high at all times which resulted in moderate canopy resistances, r c (overall mean =32 s m −1 ). Evapotranspiration was controlled by a combination of driving forces: R n , saturation vapour pressure deficit, D, and r c . It is hypothesized that the results presented in this study are conditioned by the proximity of the wetland to drier upland areas. During periods with high evaporative demand and low precipitation, warm, dry air is formed over the upland areas and wetland evapotranspiration rates are enhanced by local advection. Indicative evidence for the hypothesis is presented. Although the absolute magnitude of the results reported is only directly relevant to similar sites in Denmark, the processes and controls described are considered to be representative of riparian wetlands subjected to frequent flooding and/or with a high groundwater table, with vascular vegetation, and which are narrow corridors in open agricultural landscapes.