Forest water balance model
- Silva Joint Research Unit
The actual evapotranspiration (AET, mm) of plant cover is the sum of all the water vapour fluxes to the
soil evaporation and understory herbaceous transpiration
evaporation of water intercepted through the canopy
Climate variables which determine AET of plant surfaces are : :
Tree transpiration is the predominant term in actual evapotranspiration. When the stand is closed and the leaf area index is high, soil and understory evapotranspiration is often low, even negligible, as evaporative demand is reduced below the canopy.
There currently exists a proven method for measuring the water vapour flux above a canopy: the eddy covariance method. This method combines a three-dimensional sonic anemometer and a fast infra-red gas analyzer.
Above : three-dimensional sonic anemometer installed on the top of a eddy-flux tower (eddy covariance measurement system).
Left : eddy-flux tower of the forest of Hesse (57), a site managed by INRA-Nancy.
Another method, known as water balance, is used to estimate the AET considering changes in soil water content and
rainfall from below the canopy. This method does not provide accurate temporal resolution as the time required is
generally a week or longer. In addition, the high spatial variability of the soil water content due to the tree's
uptake requires a high number of replicates.
Transpiration is directly dependent on the energy intercepted by tree crowns. Potential evapotranspiration (PET),
which drives transpiration, quantifies the atmospheric demand (see the
PET is a function of global radiation, temperature, air humidity and wind speed. We have shown that a tree's
transpiration in a closed stand with high leaf area index and no water stress can reach a maximum of 75% of
PET. When the leaf area index is lower (below 6), transpiration decreases. For broad-leaf stands during leaf
expansion, leaf yellowing and the leaf fall phase, transpiration is less than maximal.
In the case of drought, relative extractable soil water (REW) decreases. When REW drops below 0.4, trees consequently experience water stress which induces stomatal closure and reduces transpiration. It is interesting to know that this threshold of soil water deficit can be applied to a large number of species and soil types.
Another component of AET is the soil layer evapotranspiration, which is equal to the sum of soil evaporation and understory transpiration. Here, we call understory the herbaceous layer. The woody understory layer, i.e. the understory as forest managers often refer to as coppice (e.g. composed of hornbeams in an oak or beech stand) is integrated in the leaf area index of the stand. Herbaceous sub layer evapotranspiration depends principally on the energy available below the tree's canopy. As such, the BILJOU© model is designed to calculate global radiation which reaches the sub layer. This calculation involves both the stand's global incident radiation (see the page meteorology and the stand's leaf area index.
In a forest stand, transpiration varies considerably between trees, as we have shown in sap flow measurements
on the opposite figure.
Variation in transpiration from one tree to another relates mainly to the trees' social status, the dominant trees
showing higher transpiration rates due to their higher leaf area and the fact that their crowns are located in more
favourable microclimatic conditions than smaller trees.
In a forest stand, there may also exist spatial variations relative to soil heterogeneities and gaps in the canopy.
However, modelling for water balance at stand scale requires constant conditions. Therefore, a heterogeneous plot should be divided in more homogeneous sub-elements. Forest managers generally achieve this by setting basic management units.
During the course of a sunny day with high soil water conditions, stand transpiration typically reaches 2 to 4 mm / day (= 20 - 40 m3/ha/day). When a stand is closed, tree age becomes less a factor in transpiration. In other words, if there are 2,000 trees (i.e. a young stand) per hectare, each tree will transpire on average between 10 and 20 litres of water per day. If there are 200 trees (i.e. an older stand), transpiration of one tree will be around 100 to 200 litres per day.
Aussenac G., Granier A. (1979) Etude bioclimatique d'une futaie feuillue (Fagus silvática L. et Quercus sessiliflora Salisb.) de l'Est de la France II. Etude de l'humidité du sol de l'évapotranspiration réelle. Annales des Sciences Forestières, 36, 265-280.
Bréda N., Cochard H., Dreyer E., Granier A. (1993) Water transfer in a mature oak stand (Quercus petraea): seasonal evolution and effects of a severe drought. Canadian Journal of Forest Research, 23, 1136-1143
Vincke C, Bréda N, Granier A, Devillez F (2005a) Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. I. Trees and forest floor daily transpiration. Annals of Forest Science 62(6): 503-512