Riverine dissolved organic carbon (DOC) is an integral aspect of ecosystems, as a source of carbon and energy, and is an important component in regional and global carbon cycles. Additionally, DOC is a precursor to toxic disinfection byproducts formed during treatment of river water, an increasingly important source of drinking water. Research shows that in natural-temperate watersheds, DOC is primarily terrestrial in origin and DOC concentration is influenced by many factors, including terrestrial vegetation and stream discharge. In these watersheds, the riverine export of DOC is often highest during storm events. However, little is known about DOC dynamics in urban and arid/semi-arid watersheds where runoff is increased by impermeable surfaces and watersheds are largely unvegetated. To develop a better understanding of DOC dynamics during storm events this study explores changes in concentration and source of DOC during storms in a semi-arid-urbanized basin.

To examine DOC during storm events, we collected samples hourly during four rainstorms (July-September, 2014) from a concrete channel draining into the Rio Grande River in Albuquerque (New Mexico, USA). To characterize organic matter, we measured DOC concentration and fluorescence index (FI). Trends in concentration and source are nearly identical between storm events, despite large-scale differences in discharge. DOC concentration increases with discharge during the storm event, peaking shortly after peak discharge and the total carbon flux is positively correlated with total discharge. During storm events, trends in DOC concentrations are similar in shape and magnitude to those identified in temperate regions. FI indicates that DOC source transitions from microbially dominated to terrestrially dominated and back during storm events. Terrestrially-derived DOC scales with greater DOC concentration and discharge, suggesting that precipitation events connect previously unutilized terrestrial organic matter pools within the watershed to the river. Overall, the DOC contributed to during storm events is dominated by microbial sources. This stands in contrast to natural-temperate regions where DOC export is dominated by terrestrial sources (e.g. leaf fall) and concentration is dependent on contribution from terrestrial pools.