Pore doublet micromodel experiments of evaporation influence on pre-event water entrapment and pre-event and event water interaction

The stable isotopic signature of xylem water differs from those of rainfall and river water. Without considering the water exchange through the vapor phase, this phenomenon implies that when the event water (new water) infiltrates the soil, a part of the pre-event water (old water) is isolated. The isolation and immobilization of old water are suspected because of the low matrix potential. Because evaporation is the main process that can reduce the matrix potential after quick gravitational drainage, we conjectured that after evaporation, the old water hides in small pores and increases the likelihood of air entrapment, which separates the new and old water. To examine and visualize the potential effect of evaporation on the interactions between old and new water, we performed a series of wetting–drainage–evaporation–wetting experiments within pore doublet micromodels (PDMs). The experimental results showed that air bubbles are trapped when the menisci of the residual liquid are sufficiently far from the inlet and outlet of the PDM. Evaporation can increase these distances, resulting in the formation of larger air bubbles. The air bubble trapped upstream prevents the invading liquid from quickly flushing the original residual liquid out of the PDM. Nevertheless, the invading wetting liquid is still able to bypass the trapped air bubbles and mix with the residual wetting phase via corner flow. In addition, the remobilization of the upstream bubble enhances the process of removing the old trapped liquid.