Dissolved organic matter dynamics in storm water runoff in a dryland urban region

The temporal and spatial dynamics of dissolved organic carbon (DOC) during storm events are well characterized in mesic urban regions, but equivalent studies in arid regions are less frequent, and our understanding of the impacts of arid-land urban storm events on downstream ecosystems remains poorly constrained. We sampled storm runoff in an urban dryland storm drainage system during four summer monsoon storms to determine DOC concentration and composition and to develop a conceptual carbon transport model during storm events. DOC patterns were consistent between storm events despite large differences in discharge and DOC concentration, with the lowest DOC concentrations (2.1-11.7 mg L-1) during first flush, highest concentrations shortly after peak discharge (18.3-46.0 mg L-1), and decreasing concentrations on the descending limb of the hydrograph. Fluorescence Index (FI) values suggested the DOC source shifted throughout events in three distinct phases: Phase 1- autochthonous DOC, Phase 2 - fresh allochthonous DOC, and Phase 3 - processed allochthonous DOC, representing the sequential input of previously disconnected sources during each storm event. Peak DOC concentrations generally lagged peak discharge, supporting distal allochthonous inputs of DOC to the drainage system. A single 11-h storm event delivered more than half the average daily Rio Grande discharge and approximately four times the average daily DOC load. Our findings indicate the significant impact monsoon storm events exert on DOC budgets in dryland rivers, particularly in watersheds drained by urban storm-water conveyances.