Subsurface Snowmelt Patterns Identified using Time-Lapse Electrical Resistivity Imaging.

In the American Intermountain West, snow provides the majority of the annually available water.  With recent droughts and increasing demands by a rapidly growing population, water is becoming a scarcer resource.  In addition, recent concerns about regional climate shift indicate more rapid and earlier snowmelt in the year that may increase the out-of-phase cyclical availability of water.  Understanding the dynamic fate of snow packs in the melting phase is therefore central for maximizing the harvestable water.  Some of the open questions regard the localized infiltration of water and the nature and patterns of subsurface flow in the soil overlying bedrock.  Electrical resistivity imaging non-invasively images processes with high spatial and temporal resolution if state variables (e.g. water content) give rise to contrast in electrical conductivity.  In the Vadose Zone, the interpretation of images is hampered by the equivalency of different model solutions.  However, in time-lapse measurements, changes in electrical resistivity can be more directly related to changes in water content.  This study was conducted within the well characterized and instrumented T.W. Daniels Experimental Forest located in the Bear River Range of northeastern Utah.  To monitor the spatio-temporal fate of snow melt, we installed an array of 72 Electrodes with 5 m spacing on a sloping profile in the fall of 2007.  Automated measurements at this remote site were collected daily at 5 p.m. over several months using a Wenner/Schlumberger acquisition array.  Measured data were inverted using simultaneous inversion of preceding data sets to produce apparent resistivity images of the subsurface.  Subsequent temperature correction and differential comparison of temporal changes in subsurface resistivity were accompanied by soil surveys and depth-to-bedrock soundings that guided the interpretation and calibration of measurements.  Results suggest localized infiltration of snowmelt and subsurface flow were impeded by areas of lower hydraulic conductivity.  Such behavior was consistent with visual observations of localized snowpack reduction and soil surface runoff patterns.