Development of an advanced eco‐hydrologic and biogeochemical coupling model aimed at clarifying the missing role of inland water in the global biogeochemical cycle

Abstract

Recent research showed that inland water including rivers, lakes, and groundwater may play some role in carbon cycling, although its contribution has remained uncertain due to limited amount of reliable data available. In this study, the author developed an advanced model coupling eco‐hydrology and biogeochemical cycle (National Integrated Catchment‐based Eco‐hydrology (NICE)‐BGC). This new model incorporates complex coupling of hydrologic‐carbon cycle in terrestrial‐aquatic linkages and interplay between inorganic and organic carbon during the whole process of carbon cycling. The model could simulate both horizontal transports (export from land to inland water 2.01 ± 1.98 Pg C/yr and transported to ocean 1.13 ± 0.50 Pg C/yr) and vertical fluxes (degassing 0.79 ± 0.38 Pg C/yr, and sediment storage 0.20 ± 0.09 Pg C/yr) in major rivers in good agreement with previous researches, which was an improved estimate of carbon flux from previous studies. The model results also showed global net land flux simulated by NICE‐BGC (−1.05 ± 0.62 Pg C/yr) decreased carbon sink a little in comparison with revised Lund‐Potsdam‐Jena Wetland Hydrology and Methane (−1.79 ± 0.64 Pg C/yr) and previous materials (−2.8 to −1.4 Pg C/yr). This is attributable to CO2 evasion and lateral carbon transport explicitly included in the model, and the result suggests that most previous researches have generally overestimated the accumulation of terrestrial carbon and underestimated the potential for lateral transport. The results further implied difference between inverse techniques and budget estimates suggested can be explained to some extent by a net source from inland water. NICE‐BGC would play an important role in reevaluation of greenhouse gas budget of the biosphere, quantification of hot spots, and bridging the gap between top‐down and bottom‐up approaches to global carbon budget.

Biogeochemical contrast between different latitudes and the effect of human activity on spatio-temporal carbon cycle change in Asian river systems

“…The continental values are estimated by a compilation of the major river basins in the world (153 basins/watersheds, 325 river channels) as in Nakayama (2017a). Though the model simulated only the large rivers, the hydrologic and carbon cycles were simulated within the range of previous researches in 27 different rivers (Fig.…”

“…The details are described in the author's recent paper (Nakayama, 2017a(Nakayama, , 2017b. The above modifications helped to simulate a huge number of global rivers automatically in one simulation and enabled the NICE-BGC to be a process-oriented biogeochemical cycle model of human-nature-coupled phenomena; mechanisms of transport, mineralization, and sequestration of carbon in terrestrial-aquatic linkages, interplay between inorganic and organic carbon (DOC, POC, DIC, pCO2, etc.)…”

“…In order to fill the gap in the current eco-hydrology, Nakayama (2016Nakayama ( , 2017a has recently coupled the original NICE 25 framework with various biogeochemical cycle models, including those for terrestrial ecosystems such as LPJWHyMe (LundPotsdam-Jena Wetland Hydrology and Methane) (Wania et al, 2010), those for water quality in aquatic ecosystems such as QUAL2Kw (Pelletier et al, 2006) and SWAT (Soil and Water Assessment Tool) (Neitsch et al, 2011), those for carbon weathering such as RokGeM (Rock Geochemical Model) (Colbourn et al, 2013), and CO2SYS (CO2 System Calculations) (Lewis and Wallance, 1998) In the modified NICE model (NICE-BGC), all submodels were coupled together with the original NICE to conserve the carbon budget (Nakayama, 2016(Nakayama, , 2017a. Each submodel offers iterative simulation in the most efficient way by combining on-line and off-line modes (on-line: data input/output through I/O memory, off-line: data input/output through file).…”

“…Finally, the model was extended to continental scales to evaluate eco-hydrological cycle and the role of Asian river systems in the seasonal and diurnal biogeochemical cycle change. The new model could show that there is a great variability of DOC, POC, and DIC horizontal transport to the ocean reflecting biologic and hydrologic processes, and vertical flux of CO2 degassing affected by both 15 terrestrially derived CO2 and CO2 production through aquatic metabolism, which were usually evaluated separately in the previous researches in regional and continental scales, and so there is a great improvement from them (Nakayama, 2016(Nakayama, , 2017a). …”

“…The revised NICE model (NICE-BGC) incorporates the connectivity of carbon, nitrogen, and phosphorus cycles accompanied by the hydrologic cycle between surface water and groundwater, hillslopes and river networks, and other intermediate regions (Nakayama, 2016(Nakayama, , 2017a. The model simulates both horizontal transport to the ocean and vertical fluxes and includes aquatic metabolism and terrestrially derived Biogeosciences Discuss., https://doi.org /10.5194/bg-2017-447 Manuscript under review for journal Biogeosciences Discussion started: 7 November 2017 c Author(s) 2017.…”