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.

地下储水通过影响植被生长、地表径流与地下水补给，在陆地水循环（terrestrial water cycle）中发挥着关键作用。过往针对坡向驱动的入射太阳辐射差异所产生影响的研究，大多聚焦于由此引发的植被与浅层土壤水（shallow soil moisture）分布格局。然而，鲜有研究对土壤下方风化基岩（weathered bedrock）中的水分进行量化，这限制了我们对坡面尺度水循环格局的认知，也制约了合理水资源管理工作的开展。
本研究以加利福尼亚海岸山脉地中海气候汇水区（Mediterranean California coast range catchment）为研究对象，该区域具有旱季生长期，且不同坡向的植被类型存在显著差异。研究对"朝向赤道坡（equator-facing slopes）始终比朝向极地方向坡（pole-facing slopes）更为干燥"这一主流观点提出了挑战。在2023水文年中，我们采用地表与井下地球物理测量（downhole geophysical measurements）手段，对地下水分（subsurface moisture）进行了量化。结果显示，尽管朝向赤道坡接收的入射太阳辐射更强，但其以草本植物为优势的植被覆盖的水分含量，反而高于以橡树为优势的朝向极地方向坡。
上述研究结果凸显了植被类型与地下水分之间的复杂关联，这对于理解存在显著坡向驱动生态差异的流域的水资源状况至关重要。