HR: 14:40h
AN: H43J-05    [Abstracts]
TI: Differences in Water Migration through the Unsaturated Zone due to Antecedent Conditions Prior to Multiple Natural Precipitation Events Detected by GPR
AU: * Jacob, R W
EM: Robert_Jacob@brown.edu
AF: Geological Sciences Department, Box 1846, Brown University, Providence, RI 02912 United States
AU: Hermance, J F
EM: John_Hermance@brown.edu
AF: Geological Sciences Department, Box 1846, Brown University, Providence, RI 02912 United States
AB: Soil water in the vadose zone is in constant flux; it is replenished by precipitation and lost to the atmosphere through evapotranspiration, as well as to the deeper subsurface through infiltration. The high dielectric polarizability of the water molecule makes it possible to monitor the vertical distribution of soil water content (SWC) using non-invasive geophysical techniques, such as ground penetrating radar (GPR). We report on a study of short-term changes in SWC during multiple storm events at a long-term geophysical test site in Southeastern New England. 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 reflected signal traveltimes (hence velocity and depth) from subsurface interfaces, we find that information from the air refracted and ground refracted phases can be invaluable. We describe the time-dependent partitioning of SWC between the nominally 1 m thick topmost organically-rich soil layer and an underlying nominally 3 m thick organically-poor gravelly sand in relation to three precipitation events. These three events are of interest due to the significant total rainfall amount; Event 1) 4.2 cm, Event 2) 7.9 cm, Event 3) 7.1 cm. The antecedent SWC of the organic-rich soil layer prior to each event were significantly different, 16.4% (water volume per unit soil volume), 13.2%, and <6%, respectively. The SWC increased, as expected, for each rain event in the soil layer, however, regardless of differences in the total rainfall amount the SWC increased to 26% in all three cases, which is significantly less than the laboratory measured retentivity of 34%. In contrast to the overlying soil layer, the SWC of the gravelly sand increased for only the first two precipitation events, the 4.2 and 7.9 cm events. We conclude that the field retentivity of the soil layer is reached for all three storms and the different responses observed in the gravelly sand are due to differences in the antecedent SWC of the soil layer. During the week following Event 1 and 3, a series of GPR measurements indicate that the SWC recovers slowly in the organic-rich soil interval to its antecedent conditions. After Event 1, the SWC of the gravelly sand is systematically lower than its pre-storm value, suggesting the possibility that the SWC of the deeper layer is being replenished by the continuing drainage from the organic soil layer above. However, after Event 3, the SWC of the entire gravelly sand did not change significantly, suggesting that the water in the soil layer is not infiltrating into the deeper subsurface for this rain event. By understanding the dynamic response of the subsurface to each of these three precipitation events, it is not only possible to map the time-dependant vertical distribution of SWC in relation to antecedent conditions but may be possible to observe changes in subsurface layering due to hydrologic properties of the geologic material.
DE: 1818 Evapotranspiration
DE: 1838 Infiltration
DE: 1854 Precipitation (3354)
DE: 1866 Soil moisture
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: Fall Meeting 2005