EVAPORATION FROM A RAISED PEAT BOG

Evaporation rates were measured for an oligotrophic raised peat bog do minated by the restionaceous rush Empodisma minus (Hook. f.) Johnson a nd Cutler in northern New Zealand. Evaporation rates were smaller than those found for other wetlands, ranging between 0.06-0.16 mm h(-1) du ring the daytime, and the average latent heat Aux accounted for only 2 3% of net radiation. Daily average evaporation was only 34% of the Pen man potential open water rate, yet the soil was permanently close to s aturation. Bowen ratios in the range 3-5 indicate that available energ y is primarily partitioned into sensible heat by the dense plant canop y. Bowen ratios in this range are normally associated with semi-arid z one climates rather than with a permanently moist raised peat bog. Acc ording to the Penman-Monteith model, the conservative evaporation regi me at this site is due to a very large canopy resistance, with daily a verages in the range 150-608 s m(-1) when the canopy is dry, This larg e canopy resistance is probably the result of plant responses to a nut rient-poor environment, combined with the effect of an extremely dense canopy preventing the diffusion of water vapour from the moist peat s ubstrate. Variations in canopy resistance behaviour are caused by vari able degrees of canopy wetness for several days following rain, This b ehaviour poses difficulties for modelling evaporation in water balance studies, hence a simple scheme utilising the Penman potential open wa ter evaporation is favoured. (C) 1997 Elsevier Science B.V.