CHARACTERIZATION OF WATER USE PATTERNS IN THE DEEP VADOSE ZONE THROUGH GEOPROBE DRILLING INTO WEATHERED BEDROCK

The Sierra landscape is characterized by high variability in both soil and saprolite thickness. There is considerable uncertainty about the depth of water storage within these layers and the accessibility of these storage pools to roots within various layers of weathered bedrock material below the soil layers. Geoprobe drilling was used to collect cores from weathered bedrock material down to eleven meters depth. Drilling locations were chosen through interpretation of previous geophysical work. Analyses of these cores show considerable in situ porosity development, allowing for a large pool of stored water. Investigations into rooting depth of trees in the surrounding area show the majority of roots in the top two meters but indirect evidence points to a limited number of roots extending deeper into the saprolite. A physical root excavation and associated imaging and modeling of a White Fir tree show > 90% of the roots distributed from 30-150cm with limited roots available to access deeper soil water and water stored in the saprolite. This leaves open the question of how trees access deep saprolite water and to what extent this plays a role in late summer tree function. Tensiometer measurements down to 2.5 meters show gradients developing in late summer that drive flow toward the active root zone. Investigations of stable isotopes of water at depth and in vegetation at the surface, help to understand water use from different storage pools as conditions dry at the surface. Access to deeper water storage pools is also confirmed by results from coupled modeling across the soil/tree/atmosphere continuum. We seek to answer inter related questions of the role of vegetation distribution at the land surface, variable soil depth and locations of thick sequences of weathered bedrock.

This research is part of the NSF-supported Southern Sierra Critical Zone Observatory, which is collocated within the U.S. Forest Service, Kings River Experimental Watershed.