Global Controls on DOC Reaction Versus Export in Watersheds: A Damköhler Number Analysis

Abstract

The relative capacity for watersheds to eliminate or export reactive constituents has important implications on aquatic ecosystem ecology and biogeochemistry. Removal efficiency depends on factors that affect either the reactivity or advection of a constituent within river networks. Here, we characterized Damköhler number (Da) for dissolved organic carbon (DOC) uptake in global river networks. Da equals the advection to reaction timescale ratio and thus provides a unitless indicator for DOC reaction intensity during transport within river networks. We aim to demonstrate the spatial and temporal patterns and interplays among factors that determine DOC uptake across global river networks. We show that watershed size imposes a primary control on river network DOC uptake due to a three orders of magnitude difference in water residence time (WRT) between the smallest and largest river networks. DOC uptake capacity in tropical river networks is 2–6 times that in temperate and the Arctic river networks, coinciding with larger DOC removals in warm than in cold watersheds. River damming has a profound impact on DOC uptake due to significantly extended WRTs, particularly in temperate watersheds where most constructed dams are situated. Global warming is projected to increase river network DOC uptake by ca. 19% until year 2100 under the RCP4.5 scenario.

Sensitivity of river catchments to discharge-controlled dissolved carbon export: a study of eight catchments in southern Patagonia

“…We found that in terms of unit area, small catchments (e.g., RB and RT) and especially mountainous catchments (RB) are very efficient DOC exporters (Mattsson et al 2005;Ågren et al 2007;Hilton et al 2011). This is in line with Liu et al (2022), who found that small river networks are DOC export-dominated systems, while very large river networks (stream order 7-9) are DOC reaction-oriented systems.…”

“…9). The effect of in-stream DOC degradation leading to an increase in the proportion of DIC (or CO 2 ) was suggested to be negligible for catchments in cold (boreal) areas with a low residence time of water in the system (Wallin et al 2010;Liu et al 2022). DOC in our rivers is predominantly coloured and refractory, typical for such boreal landscapes, and thus less prone for rapid photochemical or microbial degradation, so that within river losses of DOC had no major influence here.…”

Increasing riverine export of dissolved organic carbon from China

“…was not found to be as important in regulating the trend in C DOC (see Figure S14). This finding contradicts with what was previously hypothesized (Liu, Maavara, et al, 2022;Xu & Xu, 2022) but is in agreement with results by Gommet et al (2022) for the European river network. The regional analysis of the relationship between temperature and C DOC also indicates that temperature is only weakly and positively related to C DOC for most basins (Figure S23).…”

Modelling stream nutrient source and yield by coupling hydrological processes and nutrient budgets from an experimental headwater catchment with orchard expansion in the Changjiang River Delta area

“…Nutrient removal rate is depending on both nutrient uptake velocity and hydrological parameters, such as stream depth and retention time. Values of $$$ vf $$$ for nutrients are often considered constant in small headwater streams (Liu et al, 2022) and we assumed that the errors in stream nutrient removal caused by varied nutrient $$$ vf $$$ were ignorable in the YRC (Table 3) (Ensign & Doyle, 2006; Mulholland et al, 2008; Wollheim et al, 2006).…”