Geophysical Investigations of Shallow and Deep Critical Zone Processes at the Reynolds Creek Critical Zone Observatory

Abstract:

At the Reynolds Creek Critical Zone Observatory (RCCZO), located in southwest Idaho, we conducted a series of geophysical experiments designed to investigate shallow and deep critical zone (CZ) hydrologic processes. To investigate water infiltration in the shallow (<1m) CZ, we installed 7x8m² electrical resistivity tomography arrays at low elevation (1406m) and mid-elevation (1653 m) sites. To capture acute rainfall response and seasonal variability, we monitored the sites weekly during the spring, summer, and fall of 2015. Comparing pre and post rainfall surveys, the resistivity volumes show substantial spatial heterogeneity in rainfall response both within a plot and between sites. Such heterogeneity is difficult to characterize with laterally sparse moisture probe measurements. Previous studies of weathering at RCCZO and similar watersheds have revealed that northerly aspects are more deeply weathered than southerly aspects. The cause of this asymmetry is thought to be a function of greater snowpack on north facing slopes combined with the water storage and insulating properties of snow which act to increase the amount of water propagating into the deeper CZ. To explore how changes in snowpack correlate to weathering asymmetry, we acquired four seismic refraction profiles within an east-west striking watershed. The profiles were roughly equally distributed throughout the elevation range of the drainage. Based on seismic velocity, we estimated mean depths to fractured and un-weathered bedrock. We observed that the maximum N-S difference in weathering occurs ¾ of the way up the drainage, coincident with the maximum difference in snow accumulation. Below this point differences in weathering depth and snow accumulation decrease and are nearly equal at the outlet. Our results support the hypothesis that deeper snow accumulation leads to deeper weathering when all other variables are held equal. Global land models do not explicitly capture the role of correlated structures such as topography, vegetation, and soil properties in modulating water, energy, and nutrient cycling. Our work illustrates understanding of these correlations may be enhanced through hydrogeophysical investigation providing potential to improve parameterizations of CZ processes in large scale modeling.