Looking forward to seeing you at our 2019 Symposium.

Symposium Tuesday February 5th – Thursday February 7th

Short Courses Monday February 4th

Field Trip Friday February 8th

Interested in what we have done in the past? Check out the past symposia

More information is coming soon!

Bill Dietrich


The Critical Zone:  Where Trees get their Rock Moisture and Summer Streams get their Base Flow

In 2001, the US National Research Council report on Basic Research Opportunities in the Earth Sciences proposed the term “critical zone” to describe a region of the earth that extends from the vegetation canopy to the “base of the groundwater zone”.  The term “critical” was assigned to this zone to emphasize that it “sustains nearly all terrestrial life”.   The base of the critical zone then was conceptual, as it is difficult to assign an absolute boundary across the terrestrial earth below which no meteoric water travels.  As the critical zone concept became translated into US National Science Foundation-supported Critical Zone Observatories a narrower, simpler definition emerged that is applicable to many landscapes:  the thin veneer of Earth that extends from the top of the vegetation to the base of weathered bedrock.    With this definition we can envision the critical zone as a distinct entity with a well-defined top and a fairly well-defined bottom.  It is a zone of co-evolving processes and, importantly, much of this zone is well below the soil mantle.    Weathering advance into bedrock creates a hydrologically-conductive skin that mediates runoff and solute chemistry, stores water used by vegetation, influences soil production and hillslope evolution, and feeds gasses to the atmosphere.  In essence, we can now see hillslopes as reactive filters through which watershed currencies (water, gasses, sediment, biota, solutes, energy and momentum) are mediated. The critical zone is commonly 10 times thicker than the soil mantle.   But the spatial structure of the critical zone is still poorly known.

In seasonally dry environments, we have found that rock moisture, i.e. moisture that is exchangeable and potentially mobile water in the matrix and fractures of the weathered bedrock of the critical zone, can be a significant source of moisture to plants and a large component of the water budget. This moisture is not included in regional or global climate models.  We are now debating whether rock moisture use in the summer by trees influences baseflow.  This is key forest management question.

Summer baseflow is derived from groundwater drainage from the critical zone.  In the northern California Coast Ranges critical zone structure strongly depends on characteristics of the underlying bedrock.  In the Coastal Belt of the Franciscan formation (mostly shale and sandstone) which underlies the Angelo Coast Range Reserve, the critical zone thickness exceeds 25 m at hilltops and ~300 mm of annual precipitation (about 2000 mm) is seasonally stored as rock moisture.   A seasonal groundwater table develops on the underlying fresh bedrock and slowly drains via fractures to springs that mostly remain flowing throughout the summer, sustaining salmon populations.  A dense, mixed evergreen forest dominates the vegetation.  Just 20 km away the landscape is underlain by the Central Belt of the Franciscan formation, which is mélange (intensely deformed shale with blocks of sandstone and many other rock types mixed into it).  Here the critical zone is less than 3 m thick, winter runoff occurs as saturation overland flow, rock moisture storage is small, and the dominant vegetation is grass.  The only springs come from the sandstone blocks and in smaller watersheds there is no summer baseflow.  The springs from the sandstone blocks are now being exploited for marijuana production, threatening local ecosystems and base flows of larger watersheds.

Critical zone structure strongly influences terrestrial and river ecosystems.  Research is advancing theory and observation for explaining and predicting the structure and how it mediates watershed currencies.


William E. Dietrich became a professor in the Department of Earth and Planetary Science at UC Berkeley in 1982 after completing his PhD dissertation work on river meander mechanics at the University of Washington.  He is a geomorphologist who has worked on hillslope and fluvial processes and contributed to some 250 papers on such topics as sediment budgets, soil production and transport, weathering, landsliding, debris flows, runoff processes, weathering, river incision into bedrock, gravel transport, river meandering, floodplain deposition, channel networks, landscape evolution, and surface processes on Mars.  He is co-founder of the NSF supported National Center for Airborne Laser Mapping and Director of the Eel River Critical Zone Observatory.