Toute l'actualitéSoils and water resources are two vital elements of the environment. They function as a system and are intrinsically linked. Indeed, most of the water that reaches the surface of terrestrial ecosystems, notably through precipitation, infiltrates into the soil or runs off the surface. What are the flows and interactions within the soil-water system? How do soils, depending on their structure, texture, state of health, type of cover and use, influence the quality and quantity of water available to plants and living organisms? These questions are crucial to understanding the impact of forestry and agroforestry restoration projects, both in terms of increasing the infiltration of rainwater into water tables and reducing runoff and its risks.
The link between soil and water
The role of soil in the water cycle
As a living part of the geosphere, soil is both the support and the product of living organisms, since it results from the transformation of the surface layer of bedrock, degraded and enriched with organic matter. Soil is thus one of the planet's main carbon sinks.
It also plays a vital regulatory role in the water cycle. Although soil water only represents 0.064% of the total freshwater available on the planet, it is essential for plant nutrition and the photosynthesis process, as well as for the life of the wide variety of micro-organisms found in the soil, including animals (earthworms, arthropods, nematodes, protozoa), fungi and bacteria.
Interactions between soil and water
Generally speaking, the amount of water available in the soil for plant, animal and fungal biodiversity comes from precipitation. More than half of annual precipitation passes through the soil and plants (crops, forests, plants, etc.), before being evapotranspired for the most part. This is green water. The rest of the rain reaches rivers, lakes and infiltrates the water table. This is blue water.
The amount of water temporarily stored in the soil circulates in microporosities, i.e. small cavities in the soil, and constitutes the soil's water reserve. This capacity depends on climate, soil structure and texture, depth and organic matter content (carbon content).
The main particles (derived from the weathering of rocks) that make up soil texture are sand, silt and clay. Each has a different water retention capacity (low for sand, high for silt and clay).
By calculating the granulometric distribution (texture), i.e. the proportion of each of these fundamental fractions within a given soil, it is possible to define a soil's hydrological group and thus gain a better understanding of its water retention capacity.
The influence of soils on water availability
The role of the available water storage capacity in the soil
By storing part of the excess rainwater and making a useful water reserve available to vegetation and micro-organisms, the soil has the capacity to delay and mitigate the water deficit that can occur when rainfall volumes are restricted.
When the useful reserve is too low, it is considered to be limiting for living organisms, and is unable to feed the various tree and plant species in the absence of precipitation. The photosynthesis process can then slow down or even come to a halt, causing vegetation to die back.
Conversely, during extreme climatic events, excess water in the soil can be detrimental to plants. Waterlogging causes root asphyxia and reduces tree rooting depth.
Water flows in ecosystems
Knowing the useful reserve of a soil, it is possible to determine deep drainage flows, i.e. groundwater recharge, or conversely the depletion of soil water resources.
To do this, we need to compare the volume of precipitation with the potential evapotranspiration. This bioclimatic index translates into millimeters of water the evaporative capacity of the atmosphere, controlled by solar radiation, atmospheric drying and wind speed.
If the volume of precipitation is less than or equivalent to the potential evapotranspiration, then blue water flows are non-existent (there is no runoff and the water tables are not recharged), even if green water flows continue (plant transpiration takes place until the available water storage capacity in the surrounding soils is depleted). On the other hand, if the volume of precipitation exceeds the potential evapotranspiration, then the available water storage capacity begins to recharge. Runoff, i.e. the flow of water over the soil surface (as opposed to water entering through infiltration), is directly correlated with precipitation. It occurs when the intensity of rainfall exceeds the infiltration rate, or when the soil profile is saturated with water (saturation of the soil's available water storage capacity, which may trigger transfer to the water table).
The impact of runoff on soil and water quality
Erosion
Limiting runoff and its environmental risks, such as erosion, is one of the challenges of forestry and agroforestry ecosystem restoration projects.
Runoff is one of the driving forces behind erosion: flowing water carries with it particles of varying size, depending on the quantity of water in motion and the slope coefficient, creating an abrasive effect on the land subject to runoff. Erosion can thus be the source of soil and nutrient losses that are detrimental to agronomy.
Flooding
Runoff can also lead to river flooding, landslides, mudslides and downstream flooding.
Agricultural pollution
Last but not least, runoff can exacerbate pollution linked to agriculture: fertilizers and other treatment products are washed into watercourses, then into coastal environments and the seas and oceans, instead of staying where they are spread, leading to a deterioration in water quality and biodiversity as a whole.
Towards sustainable resource and ecosystem management
It is essential to project the impact of forestry and agroforestry ecosystem restoration projects in order to take into account the health of the soil-water system in the design and management of projects, and to assess the benefits generated on water resources through their implementation.
To do this, Reforest'Action establishes a baseline scenario and a project scenario, the study of which enables us to understand how changes in land use and soil cover will impact on water resources. In other words, by reintegrating trees into a landscape, where they were absent or barely present, using different techniques, what impact can be observed on the water cycle, increasing the infiltration of rainwater into the water table and reducing runoff and its risks? To find out more, read our article “Modeling the impact of (agro)forestry projects on the soil-water system”.