Présentations Colloques

    Oral Presentation
    5.05
    Session 5.05: Groundwater and Ecosystems – Hydrological role of forests and wetlands
    Lachassagne Patrick
    Quantitative interactions between forest and the water resources
    Afforestation is often considered as “the” solution to manage issues such as desertification, or even the local decrease of water resources. A comprehensive corpus of hydrological research on the relationships between the forest cover (or the forest cover changes, or, by extension, landcover changes) and water resources is available. It is however rarely used by afforestation (or deforestation) promoters. First of all, from the local to regional scales, forest has no influence on the amount of rainfall falling on a given watershed. Second, forest has a very significant influence on the fate and the hydrological distribution of rainwater and thus on water resources. Two main antagonistic – and usually successive – effects come up against one another- 1. Negative effect on water resources- compared to other types of land cover, forest intercepts and transpires a considerable portion of precipitation (evapotranspiration) as a consequence of both the large water needs of trees and their deep root exploration capacity- they can suck and then evapotranspire water from soils at a greater depth than other types of plant cover+ 2. Positive effect on water- forest locally reduces runoff and there improves aquifer recharge. Evapotranspiration occurs just after the precipitation event (rainfall). Recharge and replenishment of the water resource comes next, through infiltration of the residual water. In most hydrological configurations, overall the effect of additional uptake by a forest cover (evapotranspiration) is not fully offset by reduced runoff to the benefit of infiltration. Consequently, forest decreases the available water resource. However, not all forests and not all watersheds have the same hydrological functioning. Case by case study, for instance through modelling, is required to assess the impact, and also, what is more, the sensitivity of the watershed in question, especially that of its water resource, with regard to a modification (decrease) of the recharge.
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