The Earth's critical zone extends from the top of the vegetation canopy through the soil and down to fresh bedrock and the base of active groundwater. It is a co-evolving system that mediates the movement of water, biota, sediment, dissolved elements, gases, energy, and momentum (the "watershed currencies"). To better understand this system, nine Berkeley faculty formed the Eel River Critical Zone Observatory, which established three field monitoring sites in the Northern California Coast Range. The team was interdisciplinary, with expertise in the fields of climate science, geomorphology, hydrology, ecology, biogeochemistry, and microbiology.

Two sites on the western wet side of the Coast Range share the same climate but differ in bedrock properties, resulting in a deep critical zone (up to 25 m deep) and a shallow critical zone (< 3m). The deeply weathered site supports old growth evergreen forest, while the shallow site is mostly grass-covered with patches of deciduous trees.  The third site on the eastern edge of the range had a similar deep critical zone and geology to the first site but just one quarter the annual rainfall, and supports just grass and sparse deciduous trees. All three sites consist of a hilly landscape dissected by gravel-bedded river channels.

On the hillslopes at each site, we drilled many deep wells through the critical zone and into the underlying fresh bedrock.  In these wells we lowered detectors to document the moisture held in the weathered bedrock above the water table ("rock moisture"). Weather stations were set up to measure rainfall, humidity, temperature, and other climate attributes.  Probes monitored continuously groundwater levels in wells and soil moisture at many locations.  A unique pair of slanted boreholes enabled the sampling of water and gas at 10 intervals to depths of 16 m below the surface.  The collected samples were used to track the geochemical evolution of the downward-penetrating waters. At frequent intervals, we cored trees, and sampled soils, groundwater, and nearby streams for isotopic analysis. An overarching goal of the project was to connect fluxes leaving the hillslope critical zone to the atmosphere (and thus regional climate) as well as to rivers, estuaries, and eventually, the ocean. We studied seasonal changes in the algae, invertebrates, and fish to investigate the influence of river discharge and temperature on salmon-bearing food webs at the western sites, which drain via the Eel River to the Pacific Ocean.  The influence of river flow and temperature on rearing, feeding, and movement strategies of juvenile salmonids (for which the Eel is a critical habitat), as well as on the seasonal migrations of an invasive non-native predatory fish, which feeds voraciously on small salmonids, was also documented.   

Collectively, many discoveries were made, some of which are mentioned here.  We proposed a mathematical theory that predicts the depth to fresh bedrock underlying hillslope, matching our field observations. We found that below the soil, weathering makes the bedrock porous, and where this weathering has progressed deep below the surface there is much more plant-available moisture ("rock moisture") storage capacity in the weathered bedrock than in the soil. In the dry summers, trees live on rock moisture, and differences in the amount of rock moisture can explain the presence of forest versus grassland plant communities in areas experiencing similar climate. Motivated by insights from the field sites, a major mapping project across the continental United States indicated widespread use of bedrock water by vegetation, and revealed that, in California, trees use more water from bedrock each year than is typically stored behind all the Californian dams combined. However, in drought years, rainfall may be insufficient to replenish soil and rock moisture in some landscapes, leading to forest water stress and mortality. Hence, rock moisture water storage needs to be considered in analyses of forest resilience to drought. As much as 8 m deep in the critical zone, CO2 gas is produced by microbes.  Some gas travels upward and into the atmosphere and some combines with water to form an acid contributing to bedrock weathering. Our model predicts this weathering process.  Stream flows (including winter floods and summer low flows) originate from the hillslope critical zone, and the depth and thus water storage capacity of the critical zone can strongly influence these flows.  The timing of runoff in streams affects when salmon runs begin, and low flows and elevated temperatures in summer can lead to toxic algae blooms, salmon disease, and the upstream advance of invasive non-native fish and bullfrogs, threatening native vertebrates. 

Over 55 undergraduates, 26 graduate students, and 12 postdocs have been associated with the ERCZO. The team has hosted workshops and worked with government agencies, non-governmental organizations, and Native American tribes.  Our findings are guiding ongoing studies by the US Forest Service and stimulating investigations to include rock moisture storage in global climate models.

Last Modified: 03/31/2023
Modified by: William E Dietrich