Water quality remains a challenging issue for the Chesapeake Bay despite decades of efforts to protect water resources. The export of nitrate, the dominant nitrogen species, exhibit diverse behavior at the watershed scale: the concentration-discharge relationship of nitrate has exhibited dilution (decreasing concentrations with increasing discharge), chemostatic (relatively constant), and flushing behavior (increasing concentrations with increasing discharge). The underlying mechanism of the differing behaviors remains elusive. Here we develop a physically-based biogeochemical module, bioRT-Flux-PIHM, to integrate hydrological processes, land-surface interactions, and multicomponent biogeochemical reactions at the watershed scale. The model enables simulation of coupled water and aqueous biogeochemical processes to understand controls of nutrient export under dynamic hydrologic conditions, including the shifts between different nitrate sources and flow pathways. Here we demonstrate the model application in two sub-watersheds in the Chesapeake Bay, the Chesterville Branch and the Conewago Creek Watershed. The Chesterville Branch Watershed exhibits a dilution concentration-discharge (CQ) while the Conewago Creek Watershed shows a contrasting enrichment behavior. Model results show that hydrologic conditions determine the dominant nitrate source and flow pathway connected to the stream. Under base flow conditions, stream nitrate imitates the groundwater nitrate concentration because groundwater is the dominant source. Under wet conditions, however, the soil lateral flow becomes the dominant contributing flow and largely determines the stream nitrate concentration. Groundwater at the Chesterville Branch Watershed is enriched with nitrate concentration and is diluted by soil interflow during big precipitation events, leading to a dilution CQ behavior. In contrast, groundwater at the Conewago Creek has lower nitrate concentration and the stream is enriched by interflow with high nitrate concentration during large events, leading to flushing behavior. These insights potentially explain different nutrient observations in different watersheds.
Li Li, Wei Zhi*, Christopher Duffy, Gopal Bhatt (2018): Understanding Hydrobiogeochemical Controls of Nutrient Export Using Process-based Watershed Modeling. Abstract H12E-03 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.
This Paper/Book acknowledges NSF CZO grant support.