Evaporation from natural and modified raised peat bogs in New Zealand

The evaporation regimes of two northern New Zealand raised peat bogs with vegetation dominated by the restionaceous rush Sporadanthus traversii were investigated. One of the bogs is unmodified while the other is affected by the drainage practices of surrounding agricultural land. Measurements of the latent heat flux obtained concurrently with Bowen ratio and eddy covariance techniques were in close agreement, and good energy balance closure was demonstrated when there was adequate fetch. Evaporation rates and energy partitioning behaviour were similar for the two bogs despite differences in water table elevation and peat moisture content. Evaporation rates averaged 2.9 mm day−1, equivalent to 55% of the equilibrium evaporation rate. Energy 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 rainfall events. When the canopy was completely wet, evaporation rates approximated, or exceeded, the equilibrium evaporation rate. Evaporation rates were evidently constrained by a combination of plant physiological and canopy structural factors which combined to prevent high rates of evaporation from the moist peat surface. These findings provide evidence of strong canopy-hydrological feedbacks within the raised peat bogs of northern New Zealand which result in conditions favourable for peat formation.