Complex patterns of catchment solute–discharge relationships for coastal plain rivers

Abstract

Riverine solute versus discharge (C–Q) relationships provide information about the magnitude and dynamics of material fluxes from landscapes. We analysed long‐term patterns of C–Q relationships for 44 rivers in Florida across a suite of geogenic, nutrient, and organic solutes and investigated land cover, watershed size, and surficial geology as controls on these patterns. Solute concentrations generally exhibited far less variability than did discharge, with coherent solute‐specific behaviours repeated across watersheds. Geogenic solutes generally diluted with increasing discharge, whereas organic solutes generally enriched; patterns for nutrients were highly variable across watersheds, but on average exhibited chemostasis. Despite strong evidence of both geologic and land cover controls on solute flow‐weighted concentrations, these variables were poor predictors of C–Q slopes (β) or relative coefficients of variation (CVC:CVQ). CVC:CVQ generally increased with watershed size, and wetland area appeared to influence C–Q patterns for base cations and organic solutes. Perhaps most importantly, we observed significant slope breaks in C–Q association in approximately half of our observations, challenging the generality of using single power functions to describe catchment solute export patterns. For all solutes except phosphorus (P), C–Q slopes decreased above statistically identified breaks (slopes for P increased), with breaks consistently at or near median flow (i.e., 50% flow exceedance probability). This common pattern significantly impacts solute load estimates; failing to account for slope breaks overestimates nitrate and total organic carbon loads as much as 125% and underestimates P loads as much as 35%. In addition to challenging generic power‐law characterization of C–Q relationships for these coastal plain rivers, and exploring the load estimate consequences thereof, our study supports emerging insights about watershed hydrochemical behaviours across a wide array of solutes.

Flow Extremes as Spatiotemporal Control Points on River Solute Fluxes and Metabolism

“…Despite the unique hydrogeology, our results generally comport with findings from other catchments that solute concentrations exhibit far less variability than Q (Godsey et al, ; Musolff et al, ; Thompson et al, ), and C ‐ Q associations often exhibit significant slope breaks (Diamond & Cohen, ; Meybeck & Moatar, 2012; Moatar et al, ). Consistent with previous findings, our slope breaks occured at or near‐median Q conditions and across all solutes (Diamond & Cohen, ).…”

Concentration versus streamflow trends of major ions and tritium in headwater streams as indicators of changing water stores

“…Variations in solute concentrations with streamflow (discharge) in rivers and streams contain vital information on residence times, processes of solute generation, biogeochemical processes, and sources of stream water; thus, documenting the controls on solute geochemistry is vital to understanding catchment functioning (Ameli et al, ; Bieroza, Heathwaite, Bechmann, Kyllmar, & Jordan, ; Bouchez et al, ; Clow & Mast, ; Creed et al, ; Diamond & Cohen, ; Duncan, Band, & Groffman, ; Godsey, Kirchner, & Clow, ; Guan et al, ; Hunsaker & Johnson, ; Ibarra, Moon, Caves, Chamberlain, & Maher, ; Kim, Dietrich, Thurnhoffer, Bishop, & Fung, ; Koger, Newman, & Goering, ; Uhlenbrook & Hoeg, ; Wlostowski, Gooseff, McKnight, & Lyons, ; Zhang, Harman, & Ball, ). There are several potential sources of the water in streams and rivers that may have significantly different geochemistry (Cartwright, Gilfedder, & Hofmann, ; Cook, ; Gonzales, Nonner, Heijkers, & Uhlenbrook, ; Nathan & McMahon, ; Yu & Schwartz, ).…”

“…(Bieroza et al, ; Clow & Mast, ; Diamond & Cohen, ; Godsey et al, ; Koger et al, ; Musolff, Schmidt, Selle, & Fleckenstein, ; Wlostowski et al, ). Equation is a power law relationship where the exponent b is the slope of the log C versus log Q trend and a is a constant.…”

“…This overcomes the problem that solute concentrations in rainfall can be spatially and temporally variable (Hrachowitz, Fovet, Ruiz, & Savenije, ; Kirchner, Tetzlaff, & Soulsby, ) and are thus difficult to account for in detail. The concentrations of major ions such as Cl, Na, Ca, and K do decrease with increasing streamflow in many catchments; however, values of b are commonly significantly greater than −1 (Clow & Mast, ; Diamond & Cohen, ; Godsey et al, ; Musolff et al, ; Wlostowski et al, ).…”

Seasonal dynamics and exports of elements from a first‐order stream to a large inland lake in Michigan

“…This inference is congruent with the saturated variable source area model (Andrews, Lin, Zhu, Jin, & Brantley, ; Boyer, Hornberger, Bencala, & McKnight, ; Inamdar et al, ; Lambert et al, ). The variable source area model holds that rising water tables, and more extensive soil saturation allow for flushing of soil C to the stream from distal wetland areas that are disconnected from the stream at lower flows (Brown, McDonnell, Burns, & Kendall, ; Diamond & Cohen, ; Gannon, Bailey, McGuire, & Shanley, ; Grabs, Bishop, Laudon, Lyon, & Seibert, ; Hornberger, Bencala, & McKnight, ; Inamdar et al, ; Lottig et al, ). Importantly, because TSS also showed a positive (flushing) C‐Q relationship, it is likely that suspended solids account for an additional flux of C during high flows and peak events when saturated areas are most extensive and high‐energy stream flows can transport particulate organic matter down watershed.…”