Our Research Gravity fingering during water infiltration in soil: Impact on the resilience of crops and vegetation in water-stressed ecosystems

More than one third of the world’s population lives in regions with arid or semi-arid climates, where people face challenges in sustainable water management and food production. The ecology and water budget of these ecosystems depend critically on the dynamics of soil water. Current conceptualizations of water infiltration in semiarid regions limit the existence of deep drainage, that is, water percolation below the shallow root zone, to exceptional geological, climatic, or biological scenarios. These predictions affect our understanding of plant ecology, groundwater recharge, and nutrient cycling at the regional and global scales, and are in contrast with observations of deeply rooted woody plant coverage and active recharge in semiarid regions worldwide.

This project investigated a previously overlooked hydrodynamic instability (gravity fingering during water infiltration in soil) that exerts a powerful control on evapotranspiration, and allows for the presence of subsoil water and deep drainage fluxes in arid and semiarid climates, where potential evapotranspiration far exceeds mean annual precipitation. The team hypothesized that fingered flow causes water to quickly traverse the shallow root zone, bypassing most of the soil column and effectively reducing evaporation and transpiration from shallowly-rooted plants.

The overall objectives of this project were:

1. To understand how water infiltration and deep drainage are controlled by gravity fingering under different soil textures and climatic conditions.
2. To use this fundamental knowledge to understand the resilience of natural vegetation and agricultural crops to a changing climate in arid and semi-arid environments, and to eventually “engineer” irrigation processes to minimize evaporative losses.

The research team tested the working hypothesis that gravity fingering is of critical importance for water-vegetation dynamics in arid and semi-arid environments by designing a plan of work organized in two main thrusts combining (1) precision controlled laboratory experiments, and (2) mesoscopic modeling of the infiltration process and macroscopic consequences at the regional scale.

The mechanistic description of the hydrodynamic determinants of deep drainage in water-stressed ecosystems should be incorporated into recent efforts towards the understanding, monitoring and stewardship of the Earth’s critical zone. By linking a powerful hydrodynamic mechanism with the ecology of water-stressed environments, this research team has provided a new tool to understand and predict the response of vulnerable ecosystems in a future scenario of increased aridity.