A comparison of evaporation rates from two fens of the Hudson Bay Lowland

Abstract

Measurements of evaporation were made over two fens, a sedge-covered mineral-rich fen and a Sphagnum carpet mineral-poor fen, located in the southern portion of the Hudson Bay Lowland of Canada. Summertime evaporation rates, determined by the Bowen ratio energy balance method, were very close for both fens, averaging 161 W m −2 for the mineral-rich fen compared with 153 W m −2 for the mineral-poor fen. However, the controls on evaporation differed between the wetlands. There was a strong linear relationship ( r 2=0.96) between available energy and evaporation for the mineral-poor fen, which could be used to accurately predict evaporation rates in the absence of direct measurements. The strength of this relationship resulted from two factors: (1) the smooth aerodynamic nature of the surface; (2) an adjusting peat surface which maintained a relatively constant water supply to the fen surface. The mineral-rich fen also demonstrated a linear relationship between available energy and evaporation. However, the slope was smaller and the data were more scattered about the regression line ( r 2=0.62) than for the mineral-poor fen. The exchange of moisture in the mineral-rich fen related more to the physiology of the vascular plants. Evaporation from both wetlands was found to be less than that computed by Penman's open water evaporation formula, suggesting that these wetlands do not evaporate at the same rate as lakes.