Recent research shows inland water may play some role in continental carbon cycling though its contribution has remained uncertain due to a limited data (Battin et al., 2009). This is also related to stronger heterogeneity in aquatic ecosystem, and therefore, general assumption that its role is almost negligible or within error range. While wetlands provide important role on hydrologic and biogeochemical cycle and preserving valuable species, boreal and subarctic ones might store relatively much soil carbon as peat and affect dynamics of greenhouse gases such as methane (Limpens et al., 2008). Rivers may contribute on emitting CO2 up to 10 % of net ecosystem exchange, and might alter carbon balance of terrestrial systems (Butman and Raymond, 2011). About scale similarity and discontinuity of eco-hydrological process, it is heuristically important to identify spatial coupling of local ecosystems including energy, materials, and organisms across ecosystem boundaries. So, it is powerful to re-evaluate the ecosystem as an extension of the metabolic theory of ecology (Brown et al., 2004) from the perspective of a meta-ecosystem analysis by considering multi-scaled aspects between global--regional--micro levels in the same way as the river continuum concept (Vannote et al., 1980).
The author has developed process-based National Integrated Catchment-based Eco-hydrology (NICE) model (Nakayama, 2008a-b, 2010, 2011a-b, 2012a-c, 2013; Nakayama and Fujita, 2010; Nakayama and Hashimoto, 2011; Nakayama and Shankman, 2013a-b; Nakayama and Watanabe, 2004, 2006, 2008a-b; Nakayama et al., 2006, 2007, 2010, 2012). NICE is consisted by complex sub-compartments of surface hydrology model such as hillslope and stream flows, land-surface model including urban and crop process, groundwater model, regional atmospheric model, mass transport model in sediment and nutrient, and vegetation succession model, etc. The model also incorporates surface-groundwater interactions, includes up- and down-scaling processes between local-regional-global scales, and can simulate iteratively nonlinear feedback between hydrologic-geomorphic-ecological processes.
Results and Discussion
In this study, NICE was applied to various catchments/basins including wetland (Changjiang, Yellow, Mekong, and Ob Rivers, etc.) and expanded to global scale in order to evaluate eco-hydrological process and its impact on biogeochemical cycle in inland water. River discharge and groundwater level simulated by NICE agreed reasonably with those in previous researches (Niu et al., 2007; Fan et al., 2013), and we extended to clarify lateral subsurface also has important role on global hydrologic cycle (Nakayama, 2011b; Nakayama and Shankman, 2013b) though the resolution was coarser. The model also shows there is a great difference in hydrologic cycle including surface-groundwater interaction between each basin. In particular, as example in China, NICE clarified impact of irrigation and urban water use on eco-hydrological processes, and predicted hydrologic change in the entire Changjiang and Yellow River basins after completion of Three Gorges Dam and South-to-North Water Transfer Project to estimate whether dilemmas between water stress, crop productivity, and ecosystem degradation would diminish. Further, NICE was developed to incorporate biogeochemical cycle including reaction between inorganic and organic carbons in terrestrial and aquatic ecosystems accompanied by hydrologic cycle there. The missing role of carbon cycle simulated by NICE, in particular, CO2 evasion from inland water in global scale (total flux was estimated as about 1.0 PgC/yr), was relatively in good agreement in that estimated by empirical relation using previous pCO2 data (Aufdenkampe et al., 2011; Global River Chemistry Database, 2013).
This advanced process-based model would help to clarify how substantial pressure of complicated problems can be overcome by effective trans-boundary solutions. The model would also play important role in identification of full greenhouse gas balance of the biosphere and spatio-temporal hot spots for boundless biogeochemical cycle (Cole et al., 2007; Battin et al., 2009). The coupling between eco-hydrology and biogeochemical cycle model is a new way forward to evaluate the close linkage between water and biogeochemical cycle in terrestrial-aquatic continuum and to bridge gap between top-down and bottom-up approaches (Frei et al., 2012).
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