Rain, rock moisture dynamics, and the rapid response of perched groundwater in weathered, fractured argillite underlying a steep hillslope

TLDR

Shallow groundwater in weathered bedrock beneath hillslopes dominates base and stormflow, yet recharge processes through the unsaturated zone are poorly understood; the bedrock zone varies from ~4 m to ~19 m deep and is overlain by a 0.5 m porous soil layer. We deployed five measurement systems to monitor soil and rock moisture dynamics across a 4000 m² zero‑order basin where all runoff passes through weathered argillite. During the first rains, water quickly infiltrates meters into the weathered bedrock, causing some groundwater points to respond before the wetting front exceeds ~1 m, while the lower unsaturated zone remains largely unchanged; these patterns indicate fracture‑dominated flow and suggest that seasonal moisture changes in the first 5–10 m of rock can exceed soil changes, representing a significant unmapped reservoir.

Abstract

Various field studies have concluded that shallow groundwater in weathered bedrock underlying hillslopes can contribute to both base and stormflow and thus dominate runoff. The processes associated with recharge from the ground surface, through this unsaturated zone, have received little study, yet they influence runoff dynamics, the chemical evolution of water, and moisture availability. Here we use five measurement systems to document soil and rock moisture dynamics within a 4000 m 2 zero‐order basin in which all runoff occurs through weathered argillite. At this site, the weathered bedrock zone (in which the groundwater fluctuates by 8 m seasonally) varies in depth from ∼4 m at the base of the hillslope to nearly 19 m near the hill top. An aggregate‐rich, porous, 0.5 m thick soil overlies the weathered bedrock. We find that during the first rains of the wet season, water rapidly travels meters into the weathered bedrock zone. Consistently, however, groundwater at some places responds quickly to the first major storm, well before the wetting front has been detected much beyond about 1 m. Furthermore, throughout the wet season, the lower portion of the unsaturated weathered bedrock shows little or no moisture change. These observations suggest a fracture‐dominated flow path, leading to a highly variably groundwater response across the hillslope for a given storm. Seasonal changes in rock moisture content are greatest in the first 5 to 10 m depth and may exceed the magnitude of moisture changes in the soil, suggesting that it could constitute a significant unmapped moisture reservoir.