HR: 11:50h
AN: H42A-05    [Abstracts]
TI: The Vadose Zone's Dynamic Response to a Natural Precipitation Event: Water Migration Through the Unsaturated Subsurface Monitored by GPR
AU: * Jacob, R W
EM: Robert_Jacob@brown.edu
AF: Geological Science Department, Brown University - Box 1846, Providence, RI 02912 United States
AU: Hermance, J F
EM: John_Hermance@brown.edu
AF: Geological Science Department, Brown University - Box 1846, Providence, RI 02912 United States
AB: Water movement and its retentivity in the vadose zone is a fundamental link between precipitation, stormflow runoff and groundwater recharge. Moreover, the vertical distribution of water in the subsurface is fundamentally linked to vegetation since, following a precipitation event, the retentivity of near subsurface layers will determine the amount of water in the root zone. We report on a study of changes in the distribution of water in the subsurface during the hours and days following a precipitation event employing a sequence of high quality ground penetrating radar (GPR) measurements. The velocity of a radar signal in the subsurface is strongly modified by the soil water content (SWC), so that the technique provides an attractive means for non-invasively characterizing subsurface conditions using a Topp-like relation to infer SWC from GPR velocities. In this study, we focus on the vadose zone response to a natural precipitation event, by collecting a series of common mid-point (CMP) soundings at various times before, during, and after a 3 day period of rain at a geophysical test site in Southeastern New England. The shallow subsurface at the field site is unsaturated, and characterized to a depth of four meters by three layers: an organic rich soil layer (thickness = 0.9 m), a gravelly-sand layer (thickness = 2.9 m), underlain by a fine silty-sand, with bedrock at approximately 10 m deep. The CMPs were analyzed by carefully hand picking the first break of all observed phases: direct air, reflected, air refractions, and ground refracted phases. CMP soundings are ideal for this experiment allowing a combination of cross-checks. In particular, we have found it essential to complement the conventional analysis of reflected phases with the interpretation of ground refracted phases. Our study period involved a time of variable precipitation over 3 days, from July 23 to July 26, 2003, resulting in a total of 4.2 cm of water being added to the field site. The radar velocity of the organic rich soil layer decreased from 0.12 m/ns to 0.08 m/ns over the course of the 3 days, indicating a 0.14 m³/m³ increase in SWC in the upper meter of the subsurface. In addition, over the same period, the radar velocity of the deeper gravelly-sand decreased from 0.14 m/ns to 0.12 m/ns, corresponding to a 4% increase in SWC, from 0.065 m³/m³ to 0.1065 m³/m³. Of course, the systematically higher velocity of the gravelly-sand is consistent with the lower SWC expected for a low retentivity material. During the week following the precipitation event, a series of GPR measurements indicate that the SWC recovers slowly in the organic rich soil layer from its highest value of 0.26 m³/m³ to its pre-event value of 0.11 m³/m³ with a time constant of approximately 80 hr. Over the same period, the SWC of the top of the gravelly-sand layer (as inferred from the radar velocity of the refracted phase) recovers more quickly from 0.097 m³/m³ to its approximate pre-storm value of 0.066 m³/m³, with an approximate 20 hr time constant. However, the SWC of the entire gravelly-sand (as inferred from the reflected phase from its base), 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 layer above. The overwhelming issue in our view is the fate of water in the organic soil layer. As the SWC diminishes by 0.14 m³/m³ over the post-storm period of 1 week, how much of this water drains vertically to the gravelly-sand, and how much is evapotranspirated to the atmosphere?
DE: 0933 Remote sensing
DE: 1860 Runoff and streamflow
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2005 Joint Assembly