Isotope‐based Fluvial Organic Carbon (ISOFLOC) Model: Model formulation, sensitivity, and evaluation

Abstract

Watershed-scale carbon budgets remain poorly understood, in part due to inadequate simulation tools to assess in-stream carbon fate and transport. A new numerical model termed ISOtope-based FLuvial Organic Carbon (ISOFLOC) is formulated to simulate the fluvial organic carbon budget in watersheds where hydrologic, sediment transport, and biogeochemical processes are coupled to control benthic and transported carbon composition and flux. One ISOFLOC innovation is the formulation of new stable carbon isotope model subroutines that include isotope fractionation processes in order to estimate carbon isotope source, fate, and transport. A second innovation is the coupling of transfers between carbon pools, including algal particulate organic carbon, fine particulate and dissolved organic carbon, and particulate and dissolved inorganic carbon, to simulate the carbon cycle in a comprehensive manner beyond that of existing watershed water quality models. ISOFLOC was tested and verified in a low-gradient, agriculturally impacted stream. Results of a global sensitivity analysis suggested the isotope response variable had unique sensitivity to the coupled interaction between fluvial shear resistance of algal biomass and the concentration of dissolved inorganic carbon. Model calibration and validation suggested good agreement at event, seasonal, and annual timescales. Multiobjective uncertainty analysis suggested inclusion of the carbon stable isotope routine reduced uncertainty by 80% for algal particulate organic carbon flux estimates.

Nitrate Pathways, Processes, and Timing in an Agricultural Karst System: Development and Application of a Numerical Model

“…However, by including only hydrologic sets that also produce satisfactory N model results and represent behavior processes, the remaining sets ( n = 419) indicate that 42% of water is discharged by epikarst, 39% by phreatic zone, and 19% by quick flow. The utility of multiple response variables to reduce equifinality has been noted in other systems such as in surface streams using stable isotopes (Ford et al, ; Ford & Fox, ), in watershed‐scale models using remote‐sensing data (Silvestro et al, ), and in vegetation zones using carbon data representative of different time scales (Carvalhais et al, ). We add to this list with an application of equifinality reduction to water flow dynamics in an agricultural karst system using an N data set and numerical modeling.…”

Reducing equifinality using isotopes in a process‐based stream nitrogen model highlights the flux of algal nitrogen from agricultural streams

“…Fluvial nitrogen researchers now recognize the pairing of carbon and nitrogen cycles in stream channels. Organic matter breakdown, stabilization and mineralization impact particulate C and N storage and regeneration simultaneously, while organic carbon quality and quantity impacts rates of microbial N processing, namely nitrifying and denitrifying bacteria [ Arango and Tank , ; Martinelli et al ., 2011; Lane et al ., ; Ford and Fox , ; Hotchkiss and Hall , ]. Researchers desire continuous and integrated estimates of nitrogen fluxes for assessing the net function of the fluvial system, regardless of the apparent complexity of nitrogen dynamics [ Seitzinger , ].…”

“…Over‐parameterization of numerical models for stream nitrogen suggest the high potential for equifinality. Recent research suggests that these advanced model calibration and uncertainty subroutines might be coupled with ambient isotope tracers to reduce equifinality within water quality modeling [ Ford and Fox , ; Fox and Martin , ; Adiyanti et al ., ].…”

“…We propose several possible approaches that might address problems with equifinality within numerical modeling of stream nitrogen dynamics including: (1) the use of novel multiobjective calibration procedures with multiple response variables, (2) the use of nitrogen stable isotopes as a response variable to assist with calibration, and (3) application a robust uncertainty estimation procedure to quantify the extent of nutrient equifinality. Multiobjective calibration enables modelers to establish numerical‐based criteria that considers calibration statistics for multiple model response variables or evaluation of model performance at several different timescales [ van Griensven and Bauwens , ; Rode et al ., ; Ford and Fox , ]. Therefore, researchers can evaluate and reduce parameter ranges based on their sensitivity to unique or multiple responses, which in turn reduces the solution space of the results.…”

“…Agricultural streams often are rich in autotrophic production that can exert control on benthic and transported sediment organic matter composition at seasonal and longer‐term timescales via algal stabilization, i.e., breakdown and integration of algae into benthic storage zones [ Arango and Tank , ; Griffiths et al ., ; Ford and Fox , ], Further, nitrogen focused studies from agricultural streams suggest that organic carbon and nitrogen transformations are tightly coupled [ Butturini et al ., ; Arango et al ., ; Arango and Tank , ; Newcomer et al ., ]. Based on our previous work, the authors recognize the potential for benthic algae fate and transport to impact nutrient processes and net nitrogen fluxes through its significance to agroecosystem C budgets [ Ford and Fox ; Hotchkiss and Hall , ; Ford and Fox , ] prompting the motivation to quantify the fluxes in this paper. Further, the authors recognize the importance of algal sloughing in that under estimation of algal sloughing as a temporary sink of nitrogen could result in overestimation of denitrification, a permanent removal pathway, and therefore overestimate net nutrient attenuation by the stream.…”