Simulation of the interplay between resident and infiltrating water in partially saturated porous media

The interplay between resident water already in the subsurface environment (``old water") and infiltrating water (``new water") is examined. A smoothed particle hydrodynamics technique is used to simulate the interplay between old water and new water in a porous medium, over a cycle of drainage of old water and infiltration of new water. The effect of varying the average pore size is investigated via the Bond number, and several parameters (maximal mixing amount, minimal average size of old water pockets, mixing value for which the number of old water pockets decreases, and amount of old water remaining in the system for long times) are found to be independent of the average pore size, while the rate of change is always higher for larger pores. In particular, a certain amount of old water remains in the system within stable water pockets even after new water infiltration reaches steady state, and comprises about 2\% of the total water at steady state.

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