EVAPORATION AND SURFACE CONDUCTANCE OF 3 TEMPERATE FORESTS IN THE NETHERLANDS

This paper shows the behaviour of evaporation and surface conductance for three different forests in the Netherlands: a pine, larch and popl ar forest. Maximum evaporation rates of the forests are similar and ap proach 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, latch, pine forest. Observed maximum conductanc es follow this trend with the poplar having the highest conductance of 55 mm s(-1), the latch intermediate with 31 mm s(-1) and pine the low est 28 mm s(-1). Stomatal control was most strong in the pine forest a nd less strong in the poplar forest. The conductance of all three fore sts 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 al ready at solar radiation levels of 150 W m(-2), while poplar shows a m uch less rapid increase. The maximum conductance found here for pine c orresponds well with previously published values for the same species. The value for the latch and poplar stand are high compared to other p ublished results. This may be due to the relatively long sampling peri od of the present study, which increases the likelihood of obtaining r are high values. The results also suggest that at the local to regiona l scale large differences may be found in forest water use. For predic ting water yield of forests at this scale, the local variation in wate r use and stomatal control will have to be taken into account. ((C) In ra/Elsevier, Paris.).