L’eau forestière de qualité : comment agir sur les quantités produites ?

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Forests have a key influence on the water cycle through properties related to their structure, in particular the development of the canopy and of root systems. Any examination of forests as suppliers of water must consider the following two elements: — overall, the more productive the forest, the greater its water consumption, which accordingly reduces drainage flow. In other words, there is a trade-off between biomass production and the amount of water returned to the environment; — water drained under forest canopies to streams, springs or the water table is generally of good quality. A self-evidence worth remembering is that quality and quantity of water cannot be dissociated: the service of “quality water” implies availability of sufficient quantities over time. Knowledge and management of forest-sourced water are based on knowledge of the water balance and its variations in space and time. This article discusses the various causes of variations in water balance connected with climate, soil conditions, forest species, silvicultural practices, all of which have an impact on the amount of water drained. It also provides some comparisons with other major vegetation types. To answer the question of how to quantitatively assess the amount of water supplied by forests and how to modulate that quantity by management, we implemented a modelling approach that uses a water balance model with a daily time step. Using site-specific parameters and meteorological data, the model simulates the flow from actual evapotranspiration, rainwater at ground level, the water content of the soil and drainage. Two forest sites were selected for the study described in this article. These are forests in eastern France with contrasting rainfall patterns for which we simulated, under current climate conditions, the effect on the volume of water drained of five different silvicultural scenarios.