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The influence of vegetation on the water cycle

What is the water cycle?

The water cycle corresponds to all water transfers (in liquid, solid or gaseous form) between the water reservoirs on Earth (oceans, atmosphere, lakes, rivers, groundwater, glaciers). The engine of this cycle is solar energy which, by promoting the evaporation of water, drives all the other exchanges.

In detail, the water contained in large water reservoirs such as oceans, lakes or rivers, evaporates (in the form of water vapour) under the action of the sun (via heat) in the atmosphere (Fig. 1). Subsequently, this water vapor condenses to form clouds which, as they cool, generate rain events. Some of this water will be included in surface flows (rivers, streams, lakes); the other part will percolate in the soil. The latter will then be used by the vegetation for its water needs. The vegetation will then transpire (or evapotranspiration) under the action of the climate and its metabolism in the form of water vapour, which will again be integrated into the atmosphere.

 Diagram of the water cycle

What is the role of vegetation in the water cycle?

It is essential to take vegetation into account in the water cycle because it is an integral part of this cycle. It has been estimated that globally, approximately 70 trillion liters of water evaporate from terrestrial ecosystems each year (Stoy et al. 2019). About 70% (varying between 62 and 74% depending on the locality) of this water is redistributed in the form of rain on terrestrial ecosystems (Tuinenburg et al. 2020). Models have also shown that the vegetation cover could, at most, transpire 7.5 million litres of water per hectare per year (Vose et al. 2003). These figures are highly dependent on the species present and the environmental conditions of the site studied. Consequently, the transpiration from the vegetation, coupled with the shade generated by the canopy, makes it possible to reduce the temperature of the environment via the increase in the relative humidity of the atmosphere. This parameter corresponds to the water vapor content of the atmosphere. The more the atmosphere is saturated with water vapor (close to 100%), the “cooler” the temperature will feel.

 

It is an ecosystem benefit of primary interest for fighting against urban heat islands, particularly in the context of climate change. Indeed, the temperature in urban areas is generally 2.5°C higher than in rural areas (Nuruzzaman 2015). Thanks to evapotranspiration and shade, a vegetated area can locally reduce the temperature by 1.5 to 2°C. This effect may be greater depending on the climatic conditions of the area studied and the type of vegetation planted (up to 4 to 6°C, Perini and Magliocco 2014).

Urbanization and its impact on the water cycle

Another phenomenon linked to urban areas is the artificialization of soils, due to the spread of urbanization. This consists of transforming natural soil, agricultural or forest, through development operations, resulting in partial or total waterproofing. This can, in time, disrupt the water cycle, at the local level, by reducing the percolation of rainwater in the ground and intensifying the possible problems of flooding of a territory. Thus, renaturalization via the establishment of forest areas, can be an economical and effective way to improve the water cycle at the local level and reduce flood risks. Indeed, it has been demonstrated that the presence of root systems in the soils makes it possible to reduce the resistance (parameter allowing to determine the resistance of the soils to a pressure) of the latter (Nawaz et al. 2013). Thus the more a soil is resistant, the less the percolation of water in the latter will be effective. Consequently, the addition of a plant cover allows the maintenance of good water percolation and a reduction in surface runoff.

 

To conclude, a forest plantation can reduce island phenomena heat but also improve the flow of rainwater at the local level.