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Water Balance


To accurately establish mass balance, in this case for water, it is necessary to:

  1. Define a physical system
  2. Quantify the total inflow and outflow of water fluxes on a fixed time scale
  3. Apply the water balance equation as follows:

Total Inflow - Total Outflow = Stock variation

Selecting the system

Our water balance model is designed to account for a homogeneous area, generally one that measures 1 m2 or 1 square hectare. This system includes tree crowns and all soil layers containing root systems. Water fluxes are expressed in mm (1 mm = 1 L/m2).

In-flowing water fluxes

This is largely defined as precipitation, but capillary rises should also be considered, notably if there is a water table. In and out-flowing water fluxes
Lateral water fluxes (run-off or lateral drainage) might also exist, but the model will only consider conditions in which the in-flowing and out-flowing water fluxes for the plot are equal.

Out-flowing water fluxes

The results of these different types of fluxes either increase or decrease the total water content of the system found essentially in the soil (1).

Examples and supporting data

  1. Example of a beech forest in the Lorraine region of France comparing water balance fluxes taken during a wet year (2006) and during a dry year (2003). Annual values are shown in mm.
    Water fluxesWet year (2006)Dry year (2003)
    Tree transpiration220197
    Rainfall interception11696
    Soil and herbaceous sublayer evaporation4546
  2. Example of temporal variation in a beech forest in lowlands over a period of two years showing the leaf area index, tree transpiration (T), actual evapotranspiration (AET), daily rainfall levels and relative water content of the soil (REW). Granier et al. (2000)

    Example of temporal variation in a beech forest in lowlands

Useful references

Peiffer M, Le Goff N, Nys C, Ottorini J-M, Granier A (2005) Bilan d’eau, de carbone et croissances comparées de deux hêtraies de plaine. Revue forestière française, LVII, 205-216.

Granier A, Badeau V, Bréda N (1995) Modélisation du bilan hydrique des peuplements forestiers. Revue forestière française, XLVII, 59-68.

Aussenac G, Boulangeat C (1980) Interception des précipitations et évapotranspiration réelle dans des peuplements de feuillu (Fagus silvatica L.)et de résineux (Pseudotsuga menziesii (Mirb) Franco). Ann. Sc forest., 37(2), 91-107.

Bréda N, Huc R, Granier A, Dreyer E (2006) Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences. Annals of Forest Science, 63, 625–644.

Granier A, Biron P, Lemoine D (2000) Water balance, transpiration and canopy conductance in two beech stands. Agricultural and Forest Meteorology, 100, 291-308.

Courbet F, Doussan C, Limousin J-M, Martin-St Paul N, Simioni G (2022) Forêt et changement climatique - Comprendre et modéliser le fonctionnement hydrique des arbres. Editions Quae, collection Synthèses, 144 p.

(1) but not exclusively from soil: the different organs of the tree, mainly the sapwood of the trunk and to a lesser extent, the branches and roots, can act as an additional source of water in the case of drought. When the water stress is alleviated, this phenomenon can act in reverse.

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