Controls on critical zone thickness and hydrologic dynamics at the hillslope scale

The critical zone is defined as the thin outer veneer of Earth’s terrestrial surface, extending from the top of the vegetation canopy to the base of weathered bedrock. Very little is known about how the critical zone is structured and how its structure controls the storage, transport, and chemical evolution of the biosphere’s most important resource- water. In hilly or mountainous landscapes, the critical zone often includes tens of meters of weathered rock beneath the surface and this weathered rock hosts a dynamic hydrologic system that is virtually unexplored. Below weathered bedrock, lies an unmapped three dimensional fresh bedrock surface, Zb, that defines the bottom boundary of the critical zone. This dissertation develops novel theory to predict how this fresh bedrock surface is structured across ridge and valley topography and illustrates, through a field study, how that structure influences the routing of water within the landscape. I report, for the first time, how the structure and hydrologic dynamics of the critical zone vary across an entire hillslope, from channel to topographic divide.Current models for development of the critical zone emphasize top-down processes associated with infiltrating waters and gases, as well as fracturing due to the differential stresses generated by topography. I propose a distinctly different theory, which enables a prediction of the thickness of weathered bedrock across a landscape. I hypothesize that as fresh bedrock, saturated with nearly stagnant fluid, is advected upward into the near-surface through uplift and erosion, channel incision produces a lateral head gradient within the fresh bedrock inducing drainage towards the channel. Drainage of the fresh bedrock causes weathering through drying (i.e. repeated cycles of wetting and drying) and permits the introduction of atmospheric and biotically controlled acids and oxidants such that the boundary between weathered and unweathered bedrock is set by the uppermost elevation of undrained fresh bedrock, Zb.At steady-state the rate at which fresh bedrock crosses the Zb boundary is equal to the channel incision rate (which commonly is less than 1 mm/yr). Hence, this slow drainage of fresh bedrock, progressively allowing weathering to proceed, exerts a “bottom up” control on the advance of the weathering front. The thickness of the weathered zone is calculated as the difference between the predicted topographic surface profile (driven by erosion) and the predicted groundwater profile (driven by drainage of fresh bedrock). For the steady state, soil-mantled case, a coupled analytical solution arises in which both profiles are driven by channel incision. Lithology of the fresh bedrock influences the thickness of the weathered zone through the saturated hydraulic conductivity of the bedrock. Measurements of rate processes and topography, as well as depth to fresh bedrock at the divide can be used to estimate the saturated hydraulic conductivity and porosity of the fresh bedrock. Two non-dimensional numbers corresponding to the mean hillslope gradient and mean groundwater table gradient emerge and their ratio defines the proportion of the hillslope relief that is unweathered. The model predicts a thickening of the weathered zone upslope and consequently, a progressive upslope increase in the residence time of bedrock in the weathered zone. Despite its simplicity, the model makes testable predictions and is consistent with field data from three sites.To investigate how the critical zone is structured across a hillslope and how water is routed throughout the critical zone, I conducted an intensive field investigation on a steep (average 30 degree), actively eroding (0.2-0.4 mm/yr), 135 m long soil-mantled hillslope within the Northern California Coast Ranges (referred to as Rivendell). The 4000 m2 hillslope is located within the 17 km2 Elder Creek watershed, in the Angelo Coast Range Reserve. The hillslope is forested with up to 60 m tall Douglas fir (Pseudotsuga menziesii) and mixed evergreen hardwoods including live oak (Quercus wislizeni), madrone (Arbutus menziesii), and California bay (Umbellularia californica), and is underlain by vertically dipping argillite with sandstone interbeds. The climate is seasonally dry, and characterized by warm, dry summers (May- Sept) and cool, wet winters within which all of the precipitation (1800 mm mean annual precipitation) falls.A network of 12 wells, as deep as 30 m, were drilled across the hillslope into fresh bedrock and an extensive sensor network of over 750 sensors records soil moisture and rock moisture, and meteorological and groundwater conditions across the site. Streamflow at the base of the hillslope is recorded at a United States Geologic Survey station a short distance upstream. To document the spatial and temporal dynamics of rock moisture, I performed periodic neutron probe surveys within deep wells.Drilling revealed a 4-25 m thick zone of variably weathered, fractured bedrock underlying, thin (<50 cm) soils. Intensely fractured argillite forms a saprolite in the upper 4 m, below which fracture density, porosity, and mechanical strength decreases with depth. Fresh bedrock at the base of the profile (revealed through large increases in standard penetration resistance and an absence of signs of oxidative weathering) bounds the weathered zone from below. The boundary between unweathered and weathered rock, Zb, is progressively deeper upslope, forming an upslope thickening wedge of fractured, weathered bedrock that is increasingly weathered upslope.The seasonal addition of rainfall to this structured weathering profile, leads to the development of three distinct hydrologic zones: a near surface 4-18 m thick zone that remains unsaturated year round, a 4-15 m thick seasonally saturated zone that fluctuates largely within the same elevations year after year, and a zone which remains chronically saturated below an annually repeatable minimum water table position.A significant consequence of the development of the weathering front into bedrock is that infiltrating rainfall travels through and is stored within weathered rock as rock moisture. Rock moisture is the exchangeable water within unsaturated weathered and fractured bedrock. It has been identified as an important source of moisture to vegetation, but is poorly documented due to its inaccessibility and therefore remains an unaccounted for, but important, component of the hydrologic cycle. Here, for the first time, I directly document the spatial and temporal dynamics of rock moisture throughout the critical zone.Periodic surveys in deep wells reveal a seasonal cycle of rock moisture addition and depletion across the hillslope. This cycle begins with the first rains that mark the end of the dry season, which advance moisture into the soil and often up to 1 m into the weathered bedrock. Subsequent rains advance a wetting front through the upper 5-12 m of the profile, where increases in rock moisture storage are proportional to the addition of rainfall. In some instances, these early wet season storms generate a small, rapid but short-lived response of the water table.Once cumulative rainfall has caused the local rock moisture storage to reach a capacity beyond which rock moisture no longer increases, groundwater responds to rainfall. Further incoming water is passed rapidly, via fracture flow, to the groundwater table. This rock moisture storage capacity, which is observed to be approximately the same each year, increases upslope from 85 to 615 mm, corresponding to the upslope increase in weathering of the bedrock. The average rock moisture storage in the chronically unsaturated zone across the hillslope is about 280 mm. The upslope increase in rock moisture storage needed to initiate the seasonal groundwater response leads to the condition where, early in the wet season, runoff is generated from the lower part of the hillslope while the upper part of the hillslope is still gaining moisture.Once rock moisture is seasonally elevated, all infiltrating precipitation travels vertically through soil, saprolite, and weathered rock (we observe no overland flow or saturated flow within the soil). The timing of the rapid response of the groundwater system (~ hours) is highly variable for a given depth and does not appear to depend on travel distance to the water table. Additional storms throughout the wet season do not alter the structure or magnitude of rock moisture storage within the hillslope. Rock moisture storage is most significant in the upper 5-12 m and diminishes with depth to a zone where no detectable changes in rock moisture are observed despite the rise and fall of the water table within this zone. Rock moisture may occur as water along fracture surfaces or as water that penetrates the matrix blocks bounding fractures. At depth, the constant saturation of matrix blocks leads to the dominance of fracture flow, which drives the rapid (10-5 to 10-3 m/s) and significant (up to 11 m in a single storm) rise of the water table. On average, rock moisture changes of only 5% are needed to achieve saturation in the seasonally saturated zone. The dynamic and responsive, fracture dominated groundwater system leads to 97-99% of runoff in Elder Creek occurring during the wet season.The final storm of the wet season marks the initiation of the slow decline of rock moisture and groundwater within the hillslope throughout the long (>120 days) dry season.The 30-130 mm of seasonal soil moisture storage is rapidly depleted within the first sev- eral weeks following the final storm. Approximately 12 weeks into a typically 18-week dry season, less than 15 mm of soil moisture remains, while up to 120 mm (53 mm average across the hillslope) is stored in the upper 10 m as rock moisture. An annual cycle is repeated each year: drying to a characteristic value and then wetting to a similar rock moisture storage capacity. Because the precipitation exceeds the rock moisture storage capacity even in dry years, the spatial pattern and magnitude of dry season rock moisture are insensitive to the precipitation magnitude and temporal pattern of the immediately preceding wet season, even in a significant drought year (approximately half mean an- nual precipitation). The drop in groundwater level over the last 2 months of the dry season (equivalent to approximately 5-50 mm) is also similar in different years. Runoff of just 5 mm per month is sufficient to sustain flow in Elder Creek and support salmonid populations.Rock moisture, a direct consequence of the alteration of bedrock in the near surface, is a virtually unknown and unmapped component of the hydrologic balance. Here, I show that year after year, 30-60% of the incoming precipitation is stored seasonally as rock moisture. Nearly all of that water must be used in transpiration.Hence, rock moisture is a major source of water for vegetation. Because incoming rainfall first restores this moisture content before generating runoff, even in strong drought years, the rock moisture is available, and provides drought resilience. The ecohydrologic function of the critical zone at this site must therefore be divided between 1) near surface rock moisture storage that controls the exchange of gases and solutes and supports dry season transpiration and 2) the fracture dominated seasonally perched groundwater system that routes most precipitation as runoff and controls both peak and low streamflow and aquatic habitat. The dual function of seasonal storage and rapid transmission of water is the defining feature of the rock moisture system.The rock moisture dynamics documented here explain several previously observed processes at Rivendell. The seasonal build up of rock moisture leads to mixing of waters and damping of the stable isotope signature of storm events. Within the dynamic rock moisture zone, seasonal stimulation of subsurface microbial communities (as expressed in gas composition) occurs and cation exchange processes likely drive the solute chemistry of water recharged to the groundwater and drained as runoff. Rock moisture is likely important in a wide range of settings. I propose here the possibility that there may be a co-evolution of vegetation and critical zone structure wherein the water extraction by trees accelerates weathering of the bedrock, which in- creases rock moisture retention. Further field characterization is needed in a range of climates lithologies, and tectonic settings to document the critical zone structure, its rock moisture characteristics, and ecosystem dynamics. Modeling has begun to account for rock moisture dynamics and runoff through fractured bedrock, however, further field investigation will help guide models that predict critical zone development over sufficiently large areas to inform regional hydrologic, climate, and ecologic models