The Development Of TRIPLEX-HYDRA Model And Its Spatio-Temporal Simulations Of Organic Carbon Flux Exported By Global Rivers

As an important unidirectional pipeline,global rivers contact the two carbon pools:terrestrial ecosystems and marine ecosystems,and play a key role in global carbon cycles.According to biodegradation and solubility,riverine carbon can be divided into four forms:particulate organic carbon?POC?,dissolved organic carbon?DOC?,particulate inorganic carbon?PIC?,and dissolved inorganic carbon?DIC?.As important organic nutrients for microbes in rivers and coastal waters,the organic carbon?DOC and POC?deeply affect the biogeochemical cycles and ecosystem environment of rivers and coastal waters.Estimation of organic carbon flux by global rivers always represents a hot spot of global carbon budget.However,previous riverine carbon flux estimations were usually based on the field surveys and simple empirical model,which could still have large uncertainties.In fact,the riverine organic carbon fluxes involve many eco-hydrologic processes and are affected by a lot of environment factors,which consequently showed high spatio-temporal variability.The ecological process based models could be powerful tools to explore the spatial and temporal dynamics of some key ecological variables,such as river carbon flux.Here we developed an eco-hydrologic processes based model,TRIPLEX-HYDRA,which integrated important carbon and water cycles of soil and river systems,to simulate the dynamics of DOC and POC,including production,transportation and consumption processes.Due to the different production process and pathways?e.g.,DOC originating from soil carbon leaching;but POC originating from the soil erosion process?,we developed two modules to simulate DOC and POC flux dynamics,respectively.In summary,TRIPLEX-HYDRA model comprise four parts:vegetation dynamics and soil biogeochemistry module;hydrological module;DOC module and POC module.After model calibration and validation,we applied the TRIPLEX-HYDRA model for estimating the global riverine DOC,POC and TOC?total organic carbon?fluxes and simulating its spatio-temporal dynamics.The main results of this work as follows:?1?As a crucial driver and input variable for DOC and POC flux simulation,simulated discharge by the hydrological module should be tested firstly.We examined the simulated river network and river pathways by comparing them with the measured river network maps,and validated simulated river discharge with long-term discharge measurements.The results showed that our hydrological module can capture both the spatial distribution of global river networks and the temporal dynamics of river discharge.Besides,our model estimated that global rivers export 38540 km³ water into oceans every year.The high river discharge were found in the regions of tropical and northern high-latitudes,or some large rivers in temporal regions,such as Mississippi and Yangtze rivers.Meanwhile,we also found that the discharge of some rivers,such as Congo,Niger,Zambezi and Huanghe,were significantly decreased during 1951-2015;but of some rivers were significantly increased,such as Mississippi and Volga rivers.?2?On the basis of soil biogeochemistry module and hydrological module,we have simulated the production,adsorption,degradation and transportation processes of DOC from terrestrial ecosystems into global rivers.After model development,we performed sensitivity analysis of parameters,and found that R₁₀?soil respiration rate at 10??and Ks?solubility coefficient of SOC?are the most sensitive parameters related to riverine DOC flux.By adjusting R₁₀ and Ks,we calibrated the simulated DOC flux with measured DOC flux.Finally,we validated the simulations with global river annual DOC data,and found that simulated DOC flux is well correlated with the observed values?R²=0.96;n=71;P<0.01?.And then,we applied the TRIPLEX-HYDRA model for simulating the DOC flux of global rivers.The results showed that global rivers export 0.24 Pg DOC every year into the global oceans,and most of DOC flux were come from large rivers in tropical and boreal regions.Besides,the total DOC flux of global rivers have been significantly decreased by 0.38 Tg C every year during 1951-2015.For the regional analysis,DOC flux of most tropical rivers were significantly increased,which could be related to the increase of terrestrial organic carbon input.However,the DOC flux of most boreal rivers were significantly declined during 1951-2015,which could due to the increased flow path and DOC degradation due to climate warming.?3?We have integrated the revised universal soil loss equation?RUSLE?,soil biogeochemistry module,hydrological module and the processes of POC dynamics?including enrichment,degradation,transportation and trapping by global dams?,into the developed the POC module.Similar to DOC module,we also conducted sensitivity analysis of parameters,and then found that ER?POC enrichment during soil erosion?and K_poc?respiration rate of POC in river?are the most sensitive parameters in relation to riverine POC flux.After model calibration,we also validated the simulated POC flux,and found that simulated the POC fluxes were in good agreement with river POC observations?R²=0.96;n=71;P<0.01?.The results showed that global rivers delivered 0.29 Pg POC into oceans every year,and about 60%of global POC are exported from the rivers in low latitude regions?30°S-30°N?.Meanwhile,we found that global riverine POC flux have been increased by 0.13 Tg C every year during 1982-2015,but the increase trend is not significant.For the regional analysis,total POC flux of boreal rivers?60°N-90°N?were significantly increased,which could be related to the increase of discharge and soil carbon releasing from frozen soil or glaciers.However,the POC flux of some tropical or temporal rivers?e.g.,Murray and Yangtze?were significantly decreased,which could be as a result of discharge decreasing and many dam construction.?4?We also analyzed the spatial and temporal variations of riverine total organic carbon flux?TOC,TOC=DOC+POC?.In total,our results revealed that global rivers exported 0.53Pg TOC every year into global oceans,and greater than 60%of POC flux are from the rivers in tropical and boreal regions.During 1982 to 2015,the TOC flux of global rivers have been declined by 0.38 Pg C every year.For the regional rivers,TOC flux of boreal rivers?60°N-90°N?were significantly increased,but the riverine TOC flux of 30°S-60°S latitude bands were significantly decreased.At the same time,we also calculated the ratio of DOC flux and POC flux of global rivers?i.e.,DOC/POC?.We found that DOC/POC of most large rivers were greater than 1,which mean large rivers could export more DOC than POC flux.Compare to small rivers,most of POC flux in large rivers could be decomposed into DOC even CO₂ at the middle and lower regions of these rivers.