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Water: Watershed Central

Set Goals and Identify Solutions - Determine Load Reductions Needed

This phase of the watershed management process should result in element b of the nine elements for awarding section 319 grants. Element b is "an estimate of the load reductions expected from management measures."

To estimate the load reductions expected from the management measures, you need to understand the cause-and-effect relationship between pollutant loads and the waterbody response. Establishing this link allows you to evaluate how much of a load reduction from watershed sources is needed to meet waterbody targets. There are several options for establishing such links, ranging from qualitative evaluations to more detailed receiving water computer modeling. The approach for quantifying pollutant loads will depend on several factors including data availability, pollutants, waterbody type, source types, time frame, and spatial scale. Most important, the approach must be compatible with the method used to quantify loads and must be able to predict the necessary load reductions to meet targets.

A number of techniques, some more rigorous and detailed than others, can be used. Sometimes models or analytic techniques that allow for careful calculation of appropriate loading are used, but at other times you might have only limited data to estimate loadings. Commonly used options that move toward increased complexity include:

  • Qualitative linkages
  • Mass balance approach
  • Empirical relationships
  • Statistical and mathematical relationships
  • Reference watershed approach
  • Receiving water models

Regardless of what approach you use to estimate your allowable loadings or necessary reductions, it's likely that several scenarios or combinations of source reductions will meet your targets. Depending on the magnitude of your load reductions, you might be able to distribute them among your sources or you might have to focus on one dominant source to meet your targets.

If you calculate the load reduction only at the mouth of the watershed, several scenarios will most likely meet the load reduction target at least on paper. Sometimes impacts from load reductions are not adequate to meet targets at downstream locations. Although the upstream reductions will likely improve downstream conditions, they might account for such a small portion of the overall load that they won't have a measurable effect on the overall watershed loading. In addition, the load reductions calculated at the bottom of the watershed might not capture the more significant reductions needed in smaller upstream subwatersheds. Be sure to estimate your load reductions at a few key locations in the watershed to capture the major problem areas and sources and to support efficient and targeted management. See table 9-3 below for examples of linkage approaches that should be taken for specific waterbody/pollutant combinations.

Table 9-3 . Example Approaches for Linking Indicators and Sources

Waterbody;Pollutant Combination

Example Linkage Approach

River Pathogens

Instream response using HSPF (data collection consideration)

Lake Nutrients

Lake response using BATHTUB

More detailed option using CEQUAL-W2 or EFDC

River Nutrients

Stream response using mass balance, QUAL2E low-flow model, or WASP

River Pesticides/Urban

Allowable loading determination based on calculation from identified target at design flow or a range of flows

River/Estuary Toxic Substances

Allowable loading determination based on calculation from identified target at design flow or a range of flows

River Sediment

Load target determined from comparison with desired reference watershed

Geomorphic/habitat targets derived from literature

River Temperature

SSTEMP or SNTEMP stream flow and temperature analysis

QUAL2E stream flow and temperature analysis

River Biological Impairment

Comparison of estimated watershed/source loads with loads in reference watershed

Estuary Nutrients

Estuary response using Tidal Prism, WASP, EFDC, or similar model

Coastal Pathogen

Response using WASP, EFDC, or similar model

Alternatively determine correlation of coastal impairment with tributary loading

 

Abstract for TMDL Model Evaluation and Research Needs - The full document is linked to the abstract page.

 


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