Evaporation and surface conductance of three temperate forests in the Netherlands

Abstract

This paper shows the behaviour of evaporation and surface conductance for three different forests in the Netherlands: a pine, larch and poplar forest. Maximum evaporation rates of the forests are similar and approach the equilibrium evaporation rates for large extended surfaces. There is a tight relationship between available energy and evaporation for poplars, less so for pine and larch. Average evaporation declines in the order: poplar, larch, pine forest. Observed maximum conductances follow this trend with the poplar having the highest conductance of 55 mm s-1, the larch intermediate with 31 mm s-1 and pine the lowest 28 mm s-1. Stomatal control was most strong in the pine forest and less strong in the poplar forest. The conductance of all three forests follows a strong near-linear decrease with humidity deficit until 8-10 g kg-1, with a slowly reducing conductance afterwards. For pine and larch the surface conductance reaches the 50 % reduction value already at solar radiation levels of 150 W m-2, while poplar shows a much less rapid increase. The maximum conductance found here for pine corresponds well with previously published values for the same species. The value for the larch and poplar stand are high compared to other published results. This may be due to the relatively long sampling period of the present study, which increases the likelihood of obtaining rare high values. The results also suggest that at the local to regional scale large differences may be found in forest water use. For predicting water yield of forests at this scale, the local variation in water use and stomatal control will have to be taken into account.