Gravity fingering control on evaporation and deep drainage in a 3D porous medium

Abstract:

Across climate conditions, from humid to arid regions, the interplay between rainfall patterns and evapotranspiration determines the rate of groundwater recharge and soil-water availability. When the soil is initially relatively dry, and for relatively low infiltration rates, the infiltration front becomes unstable, resulting in gravity fingers. The preferential flow channels allow for rapid percolation of water through the top layer of soil, thus reducing evaporation—a process which may control deep drainage into the subsoil in arid and semi-arid regions, where the potential evapotranspiration (PET) is high and the mean annual precipitation (MAP) is low.

Here, we conduct experiments in a 3D porous medium to study the interplay between water infiltration and evaporation, and the role of gravity fingering in shaping that interplay. We build a cylindrical vessel of 15 cm in diameter, packed with glass beads up to a height of 40 cm, and establish patterns of variable rainfall alternating with periods of intense evaporation by means of a heat lamp, reproducing climatic conditions corresponding to humid, sub-humid and semi-arid conditions (ratio β = MAP/PET in the range 0.3-2). We record the changing mass of water due to rainfall events and evaporation throughout the experiment and visualize the finger morphology through removeable mesh inserts which are used to later reconstruct their 3D representations. Our results suggest the finger structure is independent of the rainfall patterns and instead controlled by the porous medium and fluid properties. The onset time for the finger instability follows an inverse power-law relationship with β. For each climate condition investigated, the percentage of deep drainage before the finger forms is similar, indicating that surface evaporation is the dominant control. However, after the finger forms, we observe an increase in the percentage of deep drainage for all cases, especially in the driest conditions where deep drainage is enhanced by 4x. Our microcosm experimental results support the hypothesis that gravity fingers enable water to bypass the evaporative zone and enhance deep drainage, thus, helping to explain the structure and resilience of water-stressed ecosystems, moderating their response to climate variability.