Evaporation from a raised peat bog

Abstract

Evaporation rates were measured for an oligotrophic raised peat bog dominated by the restionaceous rush Empodisma minus (Hook. f.) Johnson and Cutler in northern New Zealand. Evaporation rates were smaller than those found for other wetlands, ranging between 0.06-0.16 mm h−1 during the daytime, and the average latent heat flux accounted for only 23% of net radiation. Daily average evaporation was only 34% of the Penman potential open water rate, yet the soil was permanently close to saturation. Bowen ratios in the range 3-5 indicate that available energy is primarily partitioned into sensible heat by the dense plant canopy. Bowen ratios in this range are normally associated with semi-arid zone climates rather than with a permanently moist raised peat bog. According to the Penman-Monteith model, the conservative evaporation regime at this site is due to a very large canopy resistance, with daily averages in the range 150-608 s m−1 when the canopy is dry. This large canopy resistance is probably the result of plant responses to a nutrient-poor environment, combined with the effect of an extremely dense canopy preventing the diffusion of water vapour from the moist peat substrate. Variations in canopy resistance behaviour are caused by variable degrees of canopy wetness for several days following rain. This behaviour poses difficulties for modelling evaporation in water balance studies, hence a simple scheme utilising the Penman potential open water evaporation is favoured.