The water quality module for CCHE1D has been developed to simulate the transport and fate of pollutants through drainage networks including the modeling of transformation and decay processes that result from biochemical activity that takes place in stream waters.

The model can be used to evaluate the response of stream water quality to man-induced changes in watersheds, such as deforestation, urbanization, and the implementation of new agricultural practices. The approach is based on the integration of watershed and channel network models for the continuous simulation of transport and fate of nutrients and other pollutants.

Pollutant Transport Model

A new model has been developed to simulate transport and fate of pollutants in channel networks under unsteady flow conditions. Implemented as an extension to the flow model CCHE1D, it is designed to compute time-dependent concentrations of a series of constituents, which are governed by the processes of advection, dispersion, and chemical reactions. The advection-dispersion equation is discretized using the Control Volume Method and the Exponential Scheme. Combined with a fully implicit solution method, the model is well suited for application in complex channel networks.

Nutrients and Eutrophication

Emphasis is given to the simulation of the biogeochemical transformation processes that determine the fate of nutrients, in particular the simulation of the aquatic cycles of nitrogen and phosphorus compounds. The model also considers the growth of phytoplankton in response to high concentrations of nutrients and other environmental conditions.
Focus on the determination of concentrations of nutrients in streams, primarily nitrogen and phosphorus, caused by nonpoint source loadings associated with upland hydrological and erosion processes.

Integration with AGNPS

The new water quality model also includes a series of routines that convert daily mass loads computed by AGNPS into spatially and temporally distributed lateral inflows, based on existing hydrological conditions. The method facilitates the combined use AGNPS and CCHE1D, provided the CCHE1D graphical interface is used to generate the watershed subdivision based on digital elevation models.

Future Enhancements

The current pollutant transport model will be extended to include a dynamic water temperature sub-model. The computation of heat exchange with the atmosphere would improve modeling of temperature dependent reaction kinetics, as the water temperature would be computed based on climate data, rather than specified by the modeler. The new temperature sub-model would also allow the application of CCHE1D in situations where high stream temperatures are identified as a cause of impairment of or detrimental to the environmental quality.

The current model does not simulate interactions between nutrients and sediment. Although a fraction of nutrients is attached to sediment particles, there is no consideration of interactions between nutrients in the sediment bed and dissolved in the water column. Exchanges of nutrients and pollutants with benthos will also be modeled in future versions.