Modeling dissolved organic carbon export from water supply catchments in the northeastern United States.

Natural organic matter (NOM) in rivers is an important energy source to sustain aquatic ecosystem health. However, in surface water supply systems where chlorination is often used for disinfection, NOM is also a precursor for the carcinogenic and mutagenic disinfection byproducts such as trihalomethanes and haloacetic acids. Effective management of NOM in rivers to maintain both aquatic ecosystem functions and high-quality water supply requires better understanding of the NOM transport patterns. NOM is often operationally measured by dissolved organic carbon (DOC). Challenges of using DOC data for analysis on a catchment scale largely relate to the spatial and temporal variations in DOC, and low sampling frequency which fails to capture the multi-scale transport patterns. To help improve the understanding of DOC sources and transport, we analyzed its long-term patterns in six water supply catchments in the New York City Water Supply System using monitoring data and models. We tested six empirical models for DOC prediction including linear, nonlinear and time-series based model formulations. We found that generalized additive models (GAMs) produced the most robust results across catchments. Then, we applied the calibrated GAM to predict daily DOC concentrations to estimate fluxes and analyze for trends. Finally, we compared the relationships between temporal patterns in DOC and catchment features to investigate the regional differences, focusing on the catchment mechanistic processes associated with DOC by parsing out the hydrological signals. The results showed that hydrology plays a larger role on DOC temporal patterns in three catchments where the top 5 % streamflow corresponded to nearly 50 % of the annual DOC export, whereas nutrient associated production processes were more important in others. The study presents a robust approach for DOC prediction in streams and can inform targeted monitoring strategies for DOC management in water supply source waters.