ESS CC Team leader Eddy Moors will defend his thesis entitled: "Water Use of Forests in the Netherlands" on May 22, at VU University Amsterdam.
Forests are complex ecosystems with a large variability in the horizontal as well as in the vertical space. To study the dissimilarities in water use for different forest types, the water and energy balance of five forest stands in the Netherlands were observed during periods varying from two years to more than 15 years. The main tree species of the stands were: Scots pine, poplar, oak, larch and at one site a mixture of pine, birch, beech and oak.
Two conditions were distinguished for the analysis of the driving processes of the evaporation rate: dry and wet. Under dry conditions the opening and closing of the stomatal conductance was the main process controlling the evaporation rate. The drivers controlling the opening and closing were different between tree species and for undergrowth and tree species. Overall the most important driver was the vapour pressure deficit. The inclusion of temperature did not improve the optimization results and it is advised not to include temperature as a driver to simulate the stomatal
conductance for the present climatic conditions in the Netherlands. The contribution of the undergrowth varied with time between 5% and 100% during the year. The smallest contribution of the undergrowth to the total evaporation appeared in the middle of the summer. The highest contribution appeared in spring and autumn.
To improve the feedback of water stress by lowering groundwater tables during prolonged periods of drought a conceptual model is introduced incorporating two separate soil water signals. The model has a default feedback based on the water content at the deepest roots, and a site specific feedback through the soil water content of the surface layer containing 80 to 90% of all roots. The default feedback is based on data of multiple forest stands over Europe.
Under wet conditions it is shown that the evaporation at the end of the shower and just after the shower is much larger than often assumed. In most models this underestimation of the evaporation rate of intercepted water is compensated by an underestimation of the water storage capacity of the leaves. The evaporation rate under wet conditions is better simulated by taking into account the vertical variation in the surface roughness lengths for heat and momentum.