Concentration‐Discharge Relations in the Critical Zone: Implications for Resolving Critical Zone Structure, Function, and Evolution

Abstract

Critical zone science seeks to develop mechanistic theories that describe critical zone structure, function, and long‐term evolution. One postulate is that hydrogeochemical controls on critical zone evolution can be inferred from solute discharges measured down‐gradient of reactive flow paths. These flow paths have variable lengths, interfacial compositions, and residence times, and their mixing is reflected in concentration‐discharge (C‐Q) relations. Motivation for this special section originates from a U.S. Critical Zone Observatories workshop that was held at the University of New Hampshire, 20–22 July 2015. The workshop focused on resolving mechanistic CZ controls over surface water chemical dynamics across the full range of lithogenic (e.g., nonhydrolyzing and hydrolyzing cations and oxyanions) and bioactive solutes (e.g., organic and inorganic forms of C, N, P, and S), including dissolved and colloidal species that may cooccur for a given element. Papers submitted to this special section on “concentration‐discharge relations in the critical zone” include those from authors who attended the workshop, as well as others who responded to the open solicitation. Submissions were invited that utilized information pertaining to internal, integrated catchment function (relations between hydrology, biogeochemistry, and landscape structure) to help illuminate controls on observed C‐Q relations.

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

“…In another example, a coordinated effort emerged to measure and understand the relationships among solute concentrations and water discharge in streams (e.g., Kirchner, 2003;Godsey et al, 2009). A special issue on the topic (Chorover et al, 2017) points the way toward the use of knowledge of subsurface structure to explain concentration-discharge behavior a priori.…”

Fire, Flood, and Drought: Extreme Climate Events Alter Flow Paths and Stream Chemistry

“…Critical questions concerning watershed function include the following: Where does water go after it falls as rain or snow in a watershed, and what happens to its chemistry as it travels to the stream? Evaluating relationships between stream discharge and concentrations has yielded insight into the role of climate, lithology, geomorphology, and flow paths on solute export from rivers (e.g., Anderson et al, ; Chorover et al, , and references cited therein; Dessert et al, ; Evans & Davies, ; Ibarra et al, ; Maher, ; Maher & Chamberlain, ; Moquet et al, ; Stallard & Murphy, ; Torres et al, ). Such work typically evaluates concentration‐discharge (C‐Q) relationships with a power law equation, log ( C ) = a + b*log ( Q ), where C is the concentration, a is a constant, b is a slope, and Q is the stream discharge.…”

“…The slope is largely determined by how long water takes to reach the stream compared to reaction rates of constituents (Hrachowitz et al, 2016;Maher, 2011;Oldham et al, 2013). When reaction rates are rapid compared to water transit times, chemostasis is the dominant C-Q relationship; when water transit times are fast relative to reaction rates, dilution occurs (Chorover et al, 2017;Maher, 2011;Oldham et al, 2013). Bedrock-derived constituents (e.g., SiO 2 , Na, Ca, Mg, and alkalinity), which tend to be in greater concentrations in subsurface flow paths, often show chemostasis or slight dilution (Chorover et al, 2017, and references cited therein; Godsey et al, 2009).…”

“…When reaction rates are rapid compared to water transit times, chemostasis is the dominant C-Q relationship; when water transit times are fast relative to reaction rates, dilution occurs (Chorover et al, 2017;Maher, 2011;Oldham et al, 2013). Bedrock-derived constituents (e.g., SiO 2 , Na, Ca, Mg, and alkalinity), which tend to be in greater concentrations in subsurface flow paths, often show chemostasis or slight dilution (Chorover et al, 2017, and references cited therein; Godsey et al, 2009). Constituents associated with surface or near-surface flow paths (e.g., dissolved organic carbon [DOC] or total suspended solids [TSS]), often show enrichment (e.g., Asselman, 2000;Chorover et al, 2017, and references cited therein; Godsey et al, 2009;Herndon et al, 2015;Hinton et al, 1997;Raymond & Saiers, 2010;Stallard & Murphy, 2014).…”

“…Bedrock-derived constituents (e.g., SiO 2 , Na, Ca, Mg, and alkalinity), which tend to be in greater concentrations in subsurface flow paths, often show chemostasis or slight dilution (Chorover et al, 2017, and references cited therein; Godsey et al, 2009). Constituents associated with surface or near-surface flow paths (e.g., dissolved organic carbon [DOC] or total suspended solids [TSS]), often show enrichment (e.g., Asselman, 2000;Chorover et al, 2017, and references cited therein; Godsey et al, 2009;Herndon et al, 2015;Hinton et al, 1997;Raymond & Saiers, 2010;Stallard & Murphy, 2014). Thus, understanding hydrologic flow paths and hydrologic connectivity, or how water moves from one landscape compartment to another (Bracken & Croke, 2007), is critical to understanding stream concentrations.…”