Dynamic responses of DOC and DIC transport to different flow regimes in a subtropical small mountainous river

Transport of riverine dissolved carbon (including DOC and DIC) is a crucial process linking terrestrial and aquatic C reservoirs, but has rarely been examined in subtropical small mountainous rivers (SMRs). This study monitored DOC and DIC concentrations on a biweekly basis during non-event flow periods and at 3 h intervals during two typhoon events in three SMRs in southwestern Taiwan between January 2014 and August 2016. Two models, HBV (the Hydrologiska Byräns Vattenbalansavdelning model) and a three-endmember mixing model, were applied to determine the quantities of DOC and DIC transport from different flow paths. The results show that the annual DOC and DIC fluxes were 2.7-4.8 and 48.4-54.3 C km^-2 yr^-1, respectively, which were approx. 2 and 20 times higher than the global mean of 1.4 and 2.6C km^-2 yr^-1, respectively. The DIC/DOC ratio was 14.08, which is much higher than the mean of large rivers worldwide (1.86), and indicates the high rates of chemical weathering in this region. The two typhoons contributed 12 %-14 % of the annual streamflow in only 3 days (about 1.0 % of the annual time), whereas 15.0 %-23.5 % and 9.2 %-12.6 % of the annual DOC and DIC flux, respectively, suggested that typhoons play a more important role in DOC transport than DIC transport. The endmember mixing model suggested that DOC and DIC export was mainly from surface runoff and deep groundwater, respectively. The unique patterns seen in Taiwan SMRs characterized by high dissolved carbon flux, high DIC/DOC ratio, and large transport by intense storms should be taken into consideration when estimating global carbon budgets.