MONITORING INFILTRATION AT AN ARID SITE WITH A DEEP FRACTURED-ROCK VADOSE ZONE

Characterizing and understanding moisture flow and transport in deep vadose zones remains a daunting task. A tensiometer network has been operated for the over 5 years at an arid site with fractured rock (basalt) where instruments have been operated at depths to 75 m. This network monitors a 97 acre site comprised primarily of basalt intercalated with thin layers of sediment. Water potentials have ranged from a near-saturated -30 cm to approximately -400 cm of water. Water potential data from most of the sites indicate either steady or a long-term moisture redistribution trend (drying) in the subsurface, within both the basalt and sedimentary interbeds that extends to a depth of 73 m. This response is believed to the related to lower than normal precipitation at this site for the past 5 years. Three of monitoring sites indicate increasing water-potential trends, with one of the sites showing a pulse of water moving to about 20 m from localized infiltration in a depression from snowmelt and rainfall. Several sensors indicate both wetting and drying time intervals, indicating they are sensitive to perturbations in seasonal infiltration events. This tensiometer network has been important in understanding the dynamics of moisture movement in this deep complex geologic media.