Forests can store water without dams, so why are they called “green reservoirs”?

On March 30, 2022, General Secretary Xi Jinping pointed out during a voluntary tree planting activity in the capital that forests are water reservoirs, money reservoirs, grain reservoirs, and carbon reservoirs, vividly illustrating the fundamental and strategic position and role of forests in national ecological security and sustainable economic and social development of mankind. Forests are known as "green reservoirs" because they have a strong water conservation capacity. There is a Chinese folk saying: "Planting trees on the mountain is equivalent to building a reservoir." This sentence vividly depicts the important role of forests in maintaining water and soil and purifying water quality.

Forests intercept, absorb and store precipitation through a multi-layer structure - canopy layer, shrub layer, herbaceous plant layer, litter layer and soil layer. This layer-by-layer interception mechanism effectively reduces surface water and soil erosion. At the same time, the root system and soil layer of the forest also have strong water retention and permeability.

(Photo source: National Forestry and Grassland Administration)

Forest canopy: The forest canopy is where the forest intercepts precipitation for the first time. It returns part of the precipitation to the atmosphere through physical evaporation. At the same time, the formation of penetrating water, stem flow water and intercepted water further regulates the distribution of precipitation.

Litter layer: The litter layer is like a "ground cover", protecting the soil from the direct impact of raindrops. At the same time, it is like a "sponge", absorbing water when it rains and releasing water when it does not rain, thus regulating the moisture of the surface soil.

Soil layer: Soil permeability directly affects the storage and infiltration of precipitation. The granular structure of forest soil helps to improve the storage capacity of precipitation.

According to statistics, each square kilometer of forest can store 5-10 tons of water. Dense forests can intercept 15%-40% of precipitation on rainy days. The maximum amount of water that can be stored in the forest litter layer each year is 186.5-223 mm (fir forest) and 314.5-377.4 mm (mixed forest), showing its significant water storage function.

Forests not only conserve water resources, but also regulate runoff and reduce flood peak flow. In small watersheds, increasing forest coverage can significantly reduce flood peaks, delay the arrival of flood peaks, increase dry season flow, delay the arrival of the dry season, and reduce the flood-dry season ratio.

Studies have shown that every 2% increase in forest coverage in a small watershed can reduce the flood peak by about 1%. When the forest coverage reaches the maximum value of 100%, the flood peak can be reduced by 40%-50%. This regulatory effect is of great significance for reducing flood disasters and protecting the safety of downstream areas of rivers.

Forests are an important part of the life community of mountains, rivers, forests, fields, lakes, grasslands and sands. Through their own hydrological regulation function, they form a continuous internal circulation system, providing a continuous supply of water for forest economy, farmland irrigation, livestock and poultry breeding, etc. Forests are precious natural resources on the earth. In the form of "green reservoirs", they provide irreplaceable ecological services to human society.