Direct measurements of seasonal groundwater and rock moisture storage in the deep Critical Zone reveal how lithology controls water availability and thus ecosystem characteristics in the Northern California Coast Ranges

Abstract:

Deciphering critical zone (CZ) mediation of water availability to ecosystems relies on an understanding of how rainfall is partitioned between runoff and storage. This hydrologic partitioning is controlled by the structure of the CZ, which is not readily observable. Thus, attempts at understanding hydrologic partitioning at the landscape scale, and its influence on the distribution of terrestrial and aquatic ecosystems, are typically accomplished inferentially via integrated streamflow measurements. Here, we present an alternative, direct approach. We investigate how the CZ influences hydrologic partitioning through an intensive subsurface investigation of CZ structure and the spatial and temporal pattern of water storage. We compare two sites with similar rainfall across a lithologic contact in the Franciscan Formation in the seasonally dry Northern California Coast Ranges. At each site, we pair groundwater, streamflow, transpiration, and water storage measurements (via successive neutron logs) with surface and borehole geophysical surveys and core logs. Despite intra-site heterogeneity, general patterns of subsurface CZ structure emerge that explain the strongly contrasting hydrologic dynamics of our sites. In the sheared turbidite sequences of the Coastal belt, a deep, porous CZ has developed. Rainfall transits vertically through a thick unsaturated zone in weathered bedrock before recharging groundwater perched on fresh bedrock that sustains perennial flow in salmonid-supporting streams. The weathered bedrock stores rock moisture that is used by a mixed coniferous-broadleaf forest year-round. In contrast, modest storms quickly fill a thin, low-porosity CZ in the Central belt argillite-matrix melange, resulting in widespread saturation overland flow and flashy, ephemeral stream runoff. The small dynamic water storage capacity is only sufficient to support grasses and a drought-tolerant oak (Q. garryana); and though groundwater remains near the surface at the end of summer, it is too tightly held in unweathered low-conductivity melange for plant extraction. Our findings highlight the links between CZ structure and ecologically-relevant water storage in seasonally dry climates, and explain unexpected ecosystem diversity in zones of similar climate in Northern California.