The vadose zone is a dynamic storage element for soil water as inputs from precipitation are lost to the atmosphere through evapotranspiration, and to the deeper subsurface through infiltration. The vertical distribution of soil water content (SWC) can be readily monitored using non-invasive geophysical techniques due to the high dielectric polarizability of the water molecule. We report on a study of short-term changes in SWC during a 4.2 cm storm event using ground penetrating radar (GPR) at a long-term geophysical test site in Southeastern New England. A time-dependent "snapshot" of the vertical distribution of SWC is provided from the vertical profile of GPR velocity using a sequence of high quality common mid-point (CMP) soundings. In addition to the usual procedure of interpreting signal traveltimes (hence velocity and depth) from the time delay of the principal reflected phases, we find that information from the direct air, air refracted, and ground refracted phases can be invaluable.

Our study covers a period of variable precipitation from July 23 through July 26, 2003, and describes the time-dependent partitioning of SW between the nominally 1 m thick topmost organically-rich soil layer and an underlying nominally 3 m thick organically-poor gravelly sand. There were two rainfall episodes: 2.6 cm fell during the initial 3 days; a final cloudburst of 1.6 cm fell on Day 4. The first episode increased the SWC of the soil layer from a pre-storm level of 11.4 % (water volume per unit soil volume) to 26.6%, however, the SWC of the gravelly-sand returned quickly to its antecedent level. The second precipitation episode (1.6 cm cloudburst) did not significantly change the SWC of the surface soil layer, however the SWC of the deeper gravelly-sand increased from 6.8% to 10.2%. The refracted GPR phases corroborate this rapid re-partitioning of SWC in the subsurface.