Abstract: A large number of studies, using water chemistry and stable isotopes to conduct hydrograph separation, indicate that majority of stream water during rainfall is derived from ‘old water’ that is stored in the catchment before the rainfall. This phenomenon is known as ‘old water paradox’. Pressure wave in unsaturated zone driven by precipitation is regarded as a potential underlying mechanism. In this study, we established four numerical soil column experiments with different soil types (sand, loamy sand, sandy loam and loam). We attempted to explore pressure wave propagation and the possibility of rapid groundwater discharge driven by the pressure wave with demonstrative experiments. Our results show that after the first intensive rainfall, pressure wave were generated in the four columns and were observed at the bottom after 1.86, 2.84, 5.24 and 7.46 days. In comparison, the relevant soil water arrived at the bottom after 6.12, 10.33, 18.05 and 30.00 days. The differences between celerities and velocities are 74.71, 51.05, 27.08 and 20.13 cm/d. Our study indicates that celerities and velocities are strongly affected by soil hydraulic conductivities and soil retention curves, and pressure waves in sand can be propagated more easily than those in other soil types. As the thickness of unsaturated zone is usually linearly proportional to the distance from river bank, pressure wave generated during precipitation can propagate into groundwater and drive rapid groundwater discharge. This process would be easier in more permeable soils. Overall, this study advances our understanding of catchment hydrology and helps discover underlying mechanisms in a theoretical manner. 

Key words: old water paradox, groundwater discharge, unsaturated zone, pressure wave, solute transport