Quantifying dynamic water storage in unsaturated bedrock with borehole
nuclear magnetic resonance
Abstract
Quantifying the volume of water that is stored in the subsurface is
critical to studies of water availability to ecosystems, slope
stability, and water-rock interactions. In a variety of settings, water
is stored in fractured and weathered bedrock as rock moisture. However,
few techniques are available to measure rock moisture in unsaturated
rock, making direct estimates of water storage dynamics difficult to
obtain. Here, we use borehole nuclear magnetic resonance (NMR) at two
sites in seasonally dry California to quantify dynamic rock moisture
storage. We show strong agreement between NMR estimates of dynamic
storage and estimates derived from neutron logging and mass balance
techniques. The depths of dynamic storage are up to 9 m and likely
reflect the depth extent of root water uptake. To our knowledge, these
data are the first to quantify the volume and depths of dynamic water
storage in the bedrock vadose zone via NMR.