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Water Quality Model

Components and results

To support water quality trading in the Bear River, a water quality model was developed that determines the amount of tradeable pollutant for each individual stakeholder in the watershed. The model quantifies point and non-point sources of pollution as well as pollutant behavior within the watershed. Water quality trading can occur on a watershed scale, while loads must be determined at a farm/field scale, so the supporting modeling application links estimates of field scale loads to whole basin behavior. This is done by incorporating the simulation of hydrology, nutrient loading from fields and point sources, and instream water quality response in the components shown in the figure

  • Hydrology Component: TOPNET, a rainfall-runoff model, is used to simulate basin-wide hydrology. TOPNET accounts for subsurface storage, baseflow recession, potential evapotranspiration, interception, and soil zone components.
  • Variable Source Area Component: This component uses output of the hydrologic model along with variables such as porosity, slope, and contributing area to determine surface runoff generation.
  • Loading Model Component: Given surface runoff generation, this component provides estimates of total phosphorus (TP) loads from the diffuse sources in the watershed based on TP event mean concentrations for each land use.
  • Water Body Response Component: QUAL2E, a publicly available instream water quality model, is used to accumulate TP loads from the subbasins and route them downstream. The Loading Model, the TOPNET output, and additional external data drive the water body response component, which also accounts for point loads, diversions, and additional processes that may affect the fate of TP.

The ultimate outputs of these modeling components are TP loads determined for each farm field and TP delivery ratios for each subbasin, both of which are essential for establishing water quality trading. Because processes such as algal uptake, chemical binding, settling, and diversion can affect the amount of TP leaving a reach, delivery ratios are used to relate the amount of TP that enters a stream reach to the amount that leaves the reach downstream. Delivery ratios between locations within a watershed are important for trading so that the equivalent loading at one location can be determined at another location.

The model has been implemented for the Bear River between Oneida Narrows Reservoir and Cutler Reservoir including the Cub River and for the Little Bear River to Cutler Reservoir. Data is incorporated for 1989-2004. A daily time step is used for all calculations, and model output (loads and delivery ratios) are averaged by season. The figures below show an example of model output on Bear River and Cub River. Modeled loads are shown for each pixel averaged by season. The seasonal averages of loads and delivery ratios are used in the water quality trading applications. In addition to facilitating water quality trading, the water quality modeling framework can also be used to help estimate TP reductions from non-point sources, rank areas of importance for BMP implementation, and help to identify potential problem areas or land uses.

TP Load Map1
TP Load Map2

Model Input Data

The water quality model requires the following data for the watershed areas that are represented in the model:

  • Streamflow data
  • Water quality data in the river
  • Land use data
  • Topographic data (elevation, slope)
  • Soil type
  • Irrigation diversion locations and amounts
  • Precipitation/weather data

Model Outputs

The outputs from the water quality model include:

  • The quantity of phosphorus that comes from each land use (e.g., forest versus agricultural fields)
  • How much phosphorus comes from a farm field (pounds per season). This is the seasonal farm field load.
  • How much phosphorus is delivered to particular points in the river. From this the delivery ration can be derived.

Additional Resources

Neilson, B. T., C. Bandaragoda, M.E. Baker, J. S. Horsburgh, D. K. Stevens (2009), Watershed modeling for water quality trading, in Proceedings of the AWRA Summer Specialty Conference on Adaptive Management, Snowbird, UT, June 29 - July 1. [PDF 0.2 MB]