Nutrients ModuleNutrient enrichment is arguably the most important environmental degradation factor within the Patuxent watershed, especially in the estuary. The PLM's process based algorithms are designed to track the quantities of labile forms of phosphorous and nitrogen through the landscape during a simulation. Only phosphorous and nitrogen are tracked as they are the most likely nutrients to limit plant growth rates. Nutrient inputs of anthropogenic origin respond to socioeconomic forces and the effects of these inputs can can be observed in the plant production within the various habitats. 
Nutrients sector of modified GEM. n_SF and n_SD represent total available phosphorus (P) or nitrogen (N) on the surface and in the sediment, respectively. P_SS is the sorbed phosphorus of the sediment that is immobilized and deposited. Various nitrogen forms, NO2-, NO3- and NH4+ are aggregated into one variable representing all forms of nitrogen that are directly available for plant uptake. Available inorganic phosphorus is simulated as orthophosphate. In the model the nutrients on the surface are not associated with surface water and therefore need not be in the dissolved form only. On the contrary, since most of the time most of the cells have no surface water, n_SF represents the dry deposition of nitrogen and phosphorus on the surface. Over dry periods n_SF continues to accumulate with incoming fluxes from air deposition or mineralization of organic material. When rainfall occurs, a certain proportion of the accumulated n_SF becomes dissolved and therefore is made available for horizontal fluxing and infiltration. Further modification of the nutrient dynamics was required to accommodate the aggregation of surface and shallow subsurface flows in the hydrologic sector. In the PLM a proportion of nitrogen and phosphorus stored in the upper soil layer is made available for fast horizontal fluxing along with nutrients on the land surface. We have assumed this layer to be 10 cm thick. At present, nutrient storage in the biomass is estimated from empirical data on 12 US east coast mixed forest plots (Johnson and Lindberg 1992). Dissolved inorganic nutrients are removed from the system through the growth of biomass and released through mineralization of soil or suspended organics. Nutrient uptake and release are modeled as proportional to total biomass growth and decay at rates estimated from elemental carbon ratios measured in similar ecosystems. 
Dynamics of N and P in the model for various habitat types Nutrients is the other model component that is significantly altered by spatial dynamics. Phosphorus did not seem to be readily available for horizontal transport in terrestrial ecosystems, whereas nitrogen in its dissolved forms is closely related to hydrologic fluxes. Therefore we isolated a submodel that in addition to hydrology represented the nitrogen dynamics, referred to here as the Water Quality submodel. Calibration tests were carried out, similar to the ones performed for the hydrologic model alone. Unfortunately the data available for dissolved nitrogen is mostly limited to observations of NO3 content in the estuarine part of the Patuxent river. The northern-most point in that data set is provided by the USGS National Stream WQN at Bowie. The calibration was performed for this station and then for the full watershed. 
Calibration results for the spatial water quality model. Data measured at Bowie Station within the framework of the USGS National Stream WQN After the initial adjustment period, the model results were in quite
good agreement with the data.
For full list of model parameters and some sensitivity analysis results
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