ABSTRACT:

This data repository houses neutron probe data and processing information for projects at UC Berkeley's Blue Oak Ranch Reserve (BORR) investigating vadose zone moisture dynamics across hillslopes with opposing aspects.

Throughout our field studies, we encountered several challenges with interpreting the neutron probe data. First, there were many dates where the neutron probe did not pass an in-field chi-squared test indicating that the probe was not producing accurate estimates of neutron counts. Second, in several instances, we encountered anomalous increases or decreases in neutron counts across surveys between time periods where there were no precipitation inputs or evapotranspiration, thus, no significant change in moisture content and neutron count was expected. This indicates that the instrument was likely experiencing drift which has been observed in several previous studies (e.g., Hoedlmoser et al. 2012; Vachaud, Royer, and Cooper 1977; Yao et al. 2004; Zhang 2017). To account for these issues, we eliminated survey dates that failed the chi-squared test or produced anomalous count values. We then quantified the amount of instrument drift by averaging measurements across multiple survey periods at depths in the borehole where no change in neutron count was expected to apply a drift correction. Data, scripts, and further explanation for neutron probe data processing is available as part of this Hydroshare resource.

This research was supported by the National Science Foundation CAREER Grant (Award 2046957), Mildred E. Mathias Graduate Student Research Grant by the University of California Natural Reserve System, and the Betty and Gordon Moore Foundation.

ABSTRACT:

Water stored in the subsurface plays a crucial role in the terrestrial water cycle by influencing vegetation growth, streamflow, and groundwater recharge. Past studies on the impact of aspect-driven differences in incoming solar radiation have largely focused on resulting vegetation and shallow soil moisture patterns. However, few studies have quantified moisture below soil, in weathered bedrock, limiting our understanding of hillslope-scale water cycling patterns and proper water resource management. In a Mediterranean California coast range catchment with a dry growing season and vegetation type differences between aspects, our study challenges the notion that equator-facing slopes are consistently drier than pole-facing slopes. In the 2023 water year, we quantified subsurface moisture using surface and downhole geophysical measurements. Despite greater incoming solar radiation, grass-dominated equator-facing slopes showed greater moisture content than pole-facing slopes with oak trees. These findings underscore the intricate link between vegetation type and subsurface moisture, which is crucial for understanding water resources in watersheds with clear aspect-driven ecosystem differences.