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Overview

Introduction

In Source, the term constituents refers to materials that are generated, transported and transformed within a catchment and affect water quality. Common examples include sediments, nutrients, salts and other dissolved solids. 

Processes that act on these constituents to generate, transport and transform them can be modelled in Source. These models are broadly categorised as Catchment Water Quality models and Storage and Link Water Quality models.

Catchment water quality models are classified as:

• Constituent generation models (only for catchment scenarios)  - describe how constituents are generated in the within a functional unit and the resulting concentrations or loads delivered to the sub-catchment nodelink;
• Constituent filtering models (only for catchment scenarios) - represent any transformation of constituents between generation within the FU and arrival at the link upstream of the sub-catchment node.link.

Storage and link water quality models include:

• Constituent routing (conservative constituents) models - describes the movement of constituents along a river channel network, including exchange of constituent fluxes between floodplains, wetlands, irrigation areas and groundwater;. Constituent processing models

Constituent Generation Models

Constituent Filtering Models

Constituent Routing Models

Constituent Processing Models

Water quality processes are modelled in links and for node types which have storage associated with them (see Nodes - SRG). Most node types have no storage and at these the mass balance of each water quality constituent being modelled is maintained with no decay or deposition processes.

Two approaches are provided for routing constituent movement through a reach. The first approach assumes the constituent is fully mixed within a reach. This approach can be appropriate where the user is interested in monthly or annual loads. A particle tracking method that is also provided routes the constituent at the velocity of flow (Close, 1996). This method is more accurate and is suitable where the user is interested in concentrations or loads at smaller scales. More information on these methods is available via Routing Type SRG.

A number of options are available for representing water quality modification processes in links and storages:

• There are two general models that can act on each constituent, in a manner similar to the processing of each constituent by filter models. These comprise a decay model (see Decay SRG) and, in links only, modelling of surface water-groundwater exchange fluxes (see Flux SRG).
• Simple storage nutrient and sediment deposition models have also been provided.  More information on these models is available via Decay SRG.

The above material has been sourced from Welsh, et al (2013) with adaptations to suit the needs of the SRG.

References

Welsh, W.D., Vaze, J., Dutta, D., Rassam, D., Rahman, J.M., Jolly, I.D., Wallbrink, P., Podger, G.M., Bethune, M., Hardy, M.J., Teng, J., Lerat, J. (2013) An integrated modelling framework for regulated river systems.Environmental Modelling & Software, 39: 81-102.

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• routing models are conservative, meaning that they do not alter the total mass of constituent stored in the system; 
• Constituent processing models - describe processes that can alter the mass of a constituent in a storage or river reach (link), such as via a decay process.

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