Providing optimal root-zone fluid fluxes: Effects of hysteresis on capillary-dominated water distributions in reduced gravity

Management of water, air and nutrients in coarse-textured porous plant-growth substrates relies not only on the relative amounts of fluids but also on their distribution within porous media. Integration of plants in future life support systems for space exploration raises the question of how fluid distributions in porous plant-growth substrates are altered under reduced gravitational conditions. Central to addressing this issue is the behavior of the water retention characteristic (WRC). WRC encapsulates fluid-porous medium interactions and is key for control of water supply to plants. The hysteretic nature of WRC implies non-homogenous water distributions between its primary draining and wetting curves. During dynamic drainage and wetting cycles, considerable water content gradients develop at separations of only a few pore lengths. In the absence of a gravity force, these non-homogeneous distributions give rise to considerably different fluid distribution relative to terrestrial observations in the same porous media. Consequently, such altered fluid distributions may reduce connectivity and increase tortuosity of gaseous pathways sustaining diffusive gas transport compared to terrestrial conditions for similar air-filled contents. Such changes in fluid pathways may induce limitations to exchange of respiratory gases for plant roots, and highlight a range of other potential changes in microgravity behavior of capillary dominated processes important for distribution and transport of fluids in porous media.