Wildfire is an increasingly common occurrence in the Sonoran desert due to both encroachment of non-native grasses (e.g., buffelgrass, brome) and longer, more frequent droughts. We studied water chemistry in the intermittent desert stream, Sycamore Creek, Arizona (USA), whose watershed partially burned in 2020. The study focused on dissolved organic carbon (DOC) concentrations as they varied in time (pre- and postfire) and space (postfire).
When compared to historical DOC concentrations, there was an almost 3-fold increase in DOC concentration during storms immediately following the fire. There was also a significantly higher median DOC concentration during floods compared to base flow conditions. However, there was no evidence to support differences in median concentrations of DOC between partially burned sites versus unburned sites. Lack of data from unburned sites may have confounded this finding. Variability in DOC concentrations may be partially explained by the passage of time, with DOC concentrations declining roughly 2 mg/L every 30 days. We hypothesize that precipitation depletes pyrogenic organic matter by transport through runoff, and that the number of storms will better explain temporal variation in DOC concentrations.
These findings contribute to our growing knowledge of the effects of wildfires on the water quality of streams in arid regions. The increase in DOC concentration after the fire when compared to historical data suggests that fire increased availability of organic carbon for transport. Runoff generated by precipitation transported pyrogenic organic matter to the stream from burned portions of the watershed. This was evident in the higher DOC concentrations during floods in Sycamore Creek and in the decrease in DOC concentration over time. Taken together, these findings show that fire affects DOC dynamics of desert streams, and suggests that observed DOC flushed from the watershed can partially be explained by the transport of pyrogenic organic matter by runoff during storms.