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Water cycle

Water is essential to life. Continually recycled through the ecosystem, water is the main carrier of nutrients. A water surplus or a dry spell affects the carbon cycle as well as the cycles of other chemical elements.

Evaporation from the oceans and plant transpiration are the two main sources of water vapour in the atmosphere.

The water cycle begins with precipitation. Water infiltrates into the ground or flows over its surface until it enters a stream and eventually reaches the oceans. Some of the water in the ground is absorbed by plants and returns to the atmosphere via transpiration. And then the cycle starts all over again.

Water cycle

In forest ecosystems

In a forest ecosystem, a fraction of the rainwater that falls is intercepted by the plant cover formed by the forest crown and evaporates immediately. Oftentimes, a brief, light rainfall cannot reach the floor of a dense forest.

Forest and clouds

In our forests, barring exceptional circumstances, all the water that reaches the ground infiltrates into it. Once in the ground, the rate at which water circulates depends on the type of soil and its organic matter content. The water surplus eventually drains toward a stream or reaches the water table.

Plants absorb soil water because they transpire. The evaporation of water from leaves draws water from the roots toward the foliage, like a long drinking straw. Part of this water is used for photosynthesis. At our latitudes, forests lose from 400 to 500 mm of water every year through transpiration and through evaporation of rainwater intercepted by the forest crown.

In winter, when the temperature remains below the freezing point for several weeks, water accumulates on the surface of the ground in the form of snow; plant transpiration and absorption of water are then reduced to a minimum.

In spring, the water produced by melting snow saturates the soil just before plants become very active and need large amounts of water. The water surplus disperses toward streams or enters the water table.