Fingered water infiltration in soil suggests higher resilience of ecosystem in water-stressed climate

Abstract:

Gravity fingering is a powerful hydrodynamic instability that sets in during infiltration of water in dry soil, leading to the emergence of vertical channels of preferential flow. While this phenomenon has been characterized in laboratory experiments and documented in the field, quantitative modeling has proved challenging until recently. Building on earlier work, here we model unsaturated flow following a thermodynamic approach, which leads to a partial differential equation with a nonlinear fourth-order term.

Here, we assess the impact of gravity fingering on deep drainage, that is, the infiltration water that bypasses evaporation to reach deep aquifers. To do so, we compare modeling results between the classic infiltration model (Richards equation) and our fingered infiltration model. Both models of unsaturated flow are coupled with evapotranspiration, formulated as a spatially- and temporally-variable nonlinear sink term in the equation. We then apply the soil and climatic conditions in four semi-arid locations along the Kalahari transect. For each location, we reproduce the mean annual precipitation, the potential evapotranspiration as well as the rainfall statistics that characterize the intensity and frequency of the precipitation events. We then conduct unprecedented simulations of infiltration that are decades long with sub-minute temporal resolution and sub-centimeter spatial resolution.

Comparison of the two models show that the wetting front instability has a dramatic impact on deep drainage fluxes. Fingered infiltration model predicts significantly more deep drainage in all four locations. While both models capture the first-order influence of aridity level on deep drainage, the fingered infiltration model is significantly more sensitive to the temporal dynamics of precipitation, and thus is able to capture episodic aquifer recharge dynamics that are independent from rainfall seasonality and nonlinearly correlated with precipitation events.

Our results suggest that hydrological models based on simplified infiltration physics may underestimate groundwater recharge in dryland environments, and that gravity fingering moderates the response of water-stressed ecosystems to climate variability, increasing their resilience to a future scenario of higher aridity.