An integrated modelling system for management of the Patuxent River

The Patuxent River watershed is a heavily impacted basin (2290 km2) and estuarine tributary (120 km2) of the Chesapeake Bay, USA. To assist manage-ment of the basin, we are testing a coupled modelling system composed of a watershed model (HSPF), an estuarine circulation model (CH3D), and an estuarine water-quality model (CE-QUAL-ICM). The modelling system is being tested to guide the development of Total Maximum Daily Loads (TMDLs), and therefore errors in the models must be carefully evaluated. A comparison of daily total nitrogen (TN) concentrations simulated in HSPF with observations indicated that there was no significant bias, with an rms error of 37%. In contrast, modelled total phosphorus (TP) and total suspended solids (TSS) had significant bias with larger rms errors (65 % and 259%, respectively). In the estuary, CH3D accurately simulated tides, temperature, and salinity. CE-QUAL-ICM overestimated nitrogen (N) and phosphorus (P) in the upper estuary and underestimated in the lower estuary, primarily because intertidal marshes are not currently a model component. Model errors declined from short ((1 day) to long (multi-year) timescales as under- and overestimations cumulatively cancelled. Watershed model errors propagate into the estuarine models, interacting with each subsequent model’s errors, which limits the effectiveness of this TMDL management tool at short timescales. 1