Evaporation from natural and modified raised peat bogs in New Zealand

The evaporation regimes of two northern New Zealand raised peat bogs with v egetation dominated by the restionaceous rush Sporadanthus traversii were i nvestigated. One of the bogs is unmodified while the other is affected by t he drainage practices of surrounding agricultural land. Measurements of the latent heat flux obtained concurrently with Bowen ratio and eddy covarianc e techniques were in close agreement, and good energy balance closure was d emonstrated when there was adequate fetch. Evaporation rates and energy par titioning behaviour were similar for the two bogs despite differences in wa ter table elevation and peat moisture content. Evaporation rates averaged 2 .9 mm day(-1), equivalent to 55% of the equilibrium evaporation rate. Energ y balance partitioning favoured the sensible heat flux, and Bowen ratios of around 2 were common. The importance of the latent heat flux in the energy balance depended on the state of canopy wetness following frequent rainfal l events. When the canopy was completely wet, evaporation rates approximate d, or exceeded, the equilibrium evaporation rate. Evaporation rates were ev idently constrained by a combination of plant physiological and canopy stru ctural factors which combined to prevent high rates of evaporation from the moist peat surface. These findings provide evidence of strong canopy-hydro logical feedbacks within the raised peat bogs of northern New Zealand which result in conditions favourable for peat formation. (C) 1999 Elsevier Scie nce B.V. All rights reserved.