Core Ideas Studying the critical zone requires targeted research on water, energy, gas, solutes, and sediments. The SSHCZO targets a 165‐km2 watershed on sedimentary rocks in the northeastern United States. One SSHCZO subcatchment, Shale Hills, provides extraordinary data describing a shale CZ. The Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) was established to investigate the form, function, and dynamics of the critical zone developed on sedimentary rocks in the Appalachian Mountains in central Pennsylvania. When first established, the SSHCZO encompassed only the Shale Hills catchment, a 0.08‐km2 subcatchment within Shaver's Creek watershed. The SSHCZO has now grown to include 120 km2 of the Shaver's Creek watershed. With that growth, the science team designed a strategy to measure a parsimonious set of data to characterize the critical zone in such a large watershed. This parsimonious design includes three targeted subcatchments (including the original Shale Hills), observations along the main stem of Shaver's Creek, and broad topographic and geophysical observations. Here we describe the goals, the implementation of measurements, and the major findings of the SSHCZO by emphasizing measurements of the main stem of Shaver's Creek as well as the original Shale Hills subcatchment.

Complex subsurface flow dynamics impact the storage, routing, and transport of water and solutes to streams in headwater catchments. Many of these hydrogeologic processes are indirectly reflected in observations of stream chemistry responses to rain events, also known as concentration-discharge (CQ) relations. Identifying the relative importance of subsurface flows to stream CQ relationships is often challenging in headwater environments due to spatial and temporal variability. Therefore, this study combines a diverse set of methods, including tracer injection tests, cation exchange experiments, geochemical analyses, and numerical modeling, to map groundwater-surface water interactions along a first-order, sandstone stream (Garner Run) in the Appalachian Mountains of central Pennsylvania. The primary flow paths to the stream include preferential flow through the unsaturated zone (''interflow''), flow discharging from a spring, and groundwater discharge. Garner Run stream inherits geochemical signatures from geochemical reactions occurring along each of these flow paths. In addition to end-member mixing effects on CQ, we find that the exchange of solutes, nutrients, and water between the hyporheic zone and the main stream channel is a relevant control on the chemistry of Garner Run. CQ relationships for Garner Run were compared to prior results from a nearby headwater catchment overlying shale bedrock (Shale Hills). At the sandstone site, solutes associated with organo-mineral associations in the hyporheic zone influence CQ, while CQ trends in the shale catchment are affected by preferential flow through hillslope swales. The difference in CQ trends document how the lithology and catchment hydrology control CQ relationships. Plain Language Summary Stream chemistry serves as a fingerprint for the processes that occur in the critical zone, which extends from unweathered bedrock to the top of the tree canopy. The critical zone thus includes all resources critical to life. This paper evaluates chemical and physical processes in the critical zone, specifically in soils, streams, and groundwater. Our work suggests the important influence of groundwater-surface water interactions on mountain streams with an emphasis on the transport of water and solutes through the streambed. By comparing these transport processes on sandstone and shale, we hypothesize that the type of rock underlying a watershed can influence the relative importance of groundwater-surface water interactions on stream chemistry.

Core Ideas Two new subcatchments are used to test the importance of lithology and land use. Differences in lithology and land use result in differences in soils and waters. Despite differences, all catchments have a shallow and a deep water table. The relative importance of flow paths controls distinct chemistry response to discharge. Cross‐site comparison will ultimately enable upscaling from the catchment to large scale. The footprint of the Susquehanna Shale Hills Critical Zone Observatory was expanded in 2013 from the forested Shale Hills subcatchment (0.08 km2) to most of Shavers Creek watershed (163 km2) in an effort to understand the interactions among water, energy, gas, solute, and sediment. The main stem of Shavers Creek is now monitored, and instrumentation has been installed in two new subcatchments: Garner Run and Cole Farm. Garner Run is a pristine forested site underlain by sandstone, whereas Cole Farm is a cultivated site on calcareous shale. We describe preliminary data and insights about how the critical zone has evolved on sites of different lithology, vegetation, and land use. A notable conceptual model that has emerged is the “two water table” concept. Despite differences in critical zone architecture, we found evidence in each catchment of a shallow and a deep water table, with the former defined by shallow interflow and the latter defined by deeper groundwater flow through weathered and fractured bedrock. We show that the shallow and deep waters have distinct chemical signatures. The proportion of contribution from each water type to stream discharge plays a key role in determining how concentrations, including nutrients, vary as a function of stream discharge. This illustrates the benefits of the critical zone observatory approach: having common sites to grapple with cross‐disciplinary research questions, to integrate diverse datasets, and to support model development that ultimately enables the development of powerful conceptual and numerical frameworks for large‐scale hindcasting and forecasting capabilities.

Understanding streamflow generation and its dependence on catchment characteristics requires large spatial data sets and is often limited by convoluted effects of multiple variables. Here we address this knowledge gap using data‐informed, physics‐based hydrologic modeling in two catchments with similar vegetation and climate but different lithology (Shale Hills [SH], shale, 0.08 km2, and Garner Run [GR], sandstone, 1.34 km2), which influences catchment topography and soil properties. The sandstone catchment, GR, is characterized by lower drainage density, extensive valley fill, and bouldery soils. We tested the hypothesis that the influence of topographic characteristics is more significant than that of soil properties and catchment size. Transferring calibration coefficients from the previously calibrated SH model to GR cannot reproduce monthly discharge until after incorporating measured boulder distribution at GR. Model calibration underscored the importance of soil properties (porosity, van Genuchten parameters, and boulder characteristics) in reproducing daily discharge. Virtual experiments were used to swap topography, soil properties, and catchment size one at a time to disentangle their influence. They showed that clayey SH soils led to high nonlinearity and threshold behavior. With the same soil and topography, changing from SH to GR size consistently increased dynamic water storage (Sd) from ~0.12 to ~0.17 m. All analyses accentuated the predominant control of soil properties, therefore rejecting the hypothesis. The results illustrate the use of physics‐based modeling for illuminating mechanisms and underscore the importance of subsurface characterization as we move toward hydrological prediction in ungauged basins.

Recently, geophysicists discovered that seismic images in crystalline bedrock under crustal compression show a "bowtie" structure. Such a structure shows a seismic velocity pattern that mirrors the topography, with shallow depths (relative to an elevation datum) to unweathered bedrock under the valleys and deep depths under the ridgelines (St. Clair et al., 2015). For example, St. Clair et al. (2015) reported a bowtie pattern for almost the entire range of imaged velocities (i.e., <1,000->4,000 m/s) for two landscapes developed on crystalline rock (Figures S1a and S1b). This pattern is of interest because it matches theoretical calculations of the potential for fracturing. St. Clair et al. (2015) attributed the bowtie structure at two locations in the Piedmont Province of eastern North America (Pond Branch, MD; Calhoun, SC) to fracturing driven by coupling between tectonic stress and topography. They argued that topographic perturbation to a strong horizontal tectonic compression created the potential for opening fractures to greater depths under ridges than valleys when compared to an