Toute l'actualitéSoil and water function as a system and are intrinsically linked. It is essential to take the health of this system into account when designing projects, and to assess the benefits generated by their implementation. By reintegrating trees into a landscape, where they were absent or sparsely present, what impact can be observed on the water cycle, increasing the infiltration of rainwater into the soil profile and reducing runoff and its risks? To obtain spatialized data for these impact indicators, Reforest'Action uses a projection tool based on remote sensing.
A tool for projecting impact on soil and water
Modeling water flows on a watershed scale
In order to model the quantity of water infiltrated and the quantity of water runoff on the scale of an ecosystem and to project the evolution of these metrics after the implementation of a project, Reforest'Action's Impact and R&D division uses the VWBA (Volumetric Water Benefit Accounting) model developed by the World Resources Institute, and relies on the Invest tool which takes into account a wide range of factors, some of which thanks to remote sensing:
- Precipitation volume
- Potential evapotranspiration
- Soil hydrology group
- Topography
- Type of land use and cover
- Runoff coefficient (curve number)
Our three impact projection indicators
The tool used by Reforest'Action illustrates the circulation of water within a watershed (the surface area that receives water flowing towards a single watercourse or groundwater table). This tool can also be used to map water flows. It is then possible to model the landscape after project implementation, obtaining spatialized data for three impact projection indicators: evapotranspiration, avoided runoff and local groundwater recharge in milliliters.
A project design tool
“In concrete terms, this tool is an impact projection tool, as it enables us to model the amount of runoff that could be avoided by implementing projects,” explains Richard Sourbès, GIS and remote sensing expert at Reforest'Action. “The model we use enables us to adapt project design to the local soil-water resource, by targeting, upstream of the project, areas where reforestation or the integration of trees in agroforestry will be of great interest. It's also a project design tool and a planning aid.”
Expected impacts of projects on the soil-water system
Reduced runoff and water purification
Planted trees require a significant input of water as they grow, temporarily reducing the amount of water available in the soil. However, from their very first years of growth, they reduce runoff by absorbing excess water from the soil surface, thus ensuring the proper functioning of the water and nutrient cycle necessary for the development of flora and fauna.
Preventing runoff also helps to clean up the water that infiltrates the soil, a particularly beneficial impact in agricultural environments. In fact, water filtration by trees prevents the transfer of pollutants from the soil to the water system.
“In concrete terms, planted trees reduce runoff, significantly increase the potential evapotranspiration of the ecosystem and, to a lesser extent, local groundwater recharge. They also reduce the connection between the agricultural environment and the hydrographic network, and thus limit potential pollutant transfers (excess organic matter, fertilizers, pesticides, etc.). It is this triple function that reduces the risk of degradation of water bodies. The trees planted therefore help to influence the quality and quantity of water in the hydrographic network adjacent to the project,” explains Jean-Christophe Renaud, in charge of regenerative agriculture projects for Reforest'Action.
Increase in actual evapotranspiration
In the medium term, the restoration or creation of a forestry or agroforestry ecosystem reduces the response time of a watershed to rain stimulus, by reducing runoff: in other words, the drop of water reaches the outflow less quickly. What's more, in the first 30 years following the implementation of a project, an increase in actual evapotranspiration (the total amount of water that evaporates from the soil and plants present in an area when the soil is at its natural moisture content) can be observed, giving rise to cloud formation and an increased volume of precipitation at watershed scale.
“Based on the results of our impact projections, forestry and agroforestry ecosystem restoration projects can ultimately increase both the quality and quantity of water (green and blue) circulating at watershed scale,” concludes Jean-Christophe Renaud.
Practices borrowed from regenerative hydrology
Through this research and development work, Reforest'Action aims to take into account the regeneration of freshwater cycles in the design and implementation of projects, and thus deploy practices that improve the distribution, infiltration and storage of rainwater and runoff, in order to improve the resilience of ecosystems to water-related issues (drought, erosion, desertification, flooding...) - all key practices in the science of regenerative hydrology.