Greater rock moisture content on an equator-facing hillslope due to aspect-related variations in vegetation water use

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.