Watershed Treatment Model

The Watershed Treatment Model (WTM) is a spreadsheet-based tool designed for rapid watershed planning. It computes annual pollutant loads from a wide range of sources within a watershed and estimates the load reductions achieved by a comprehensive suite of treatment options and management programs. The model is intended for watershed managers, planners, and engineers to evaluate multiple treatment alternatives, justify existing programs, develop future strategies, and prioritize restoration efforts.

While the simple algorithms in the WTM are not a substitute for detailed, dynamic watershed modeling, the model serves as an effective starting point for analysis. It allows users to compare different scenarios quickly, understand the relative importance of various pollutant sources, and assess the potential effectiveness of management interventions before committing resources to more complex studies or on-the-ground implementation.

What the Model Calculates

The primary output of the WTM is the estimated average annual pollutant load, calculated for the entire watershed or for individual subwatersheds. The model provides a detailed accounting of these loads, breaking them down by individual source. This source-based approach helps managers identify the most significant contributors to water quality degradation.

The model calculates annual loads for the following key parameters:

• Nutrients: Total Nitrogen (TN) and Total Phosphorus (TP).
• Sediment: Total Suspended Solids (TSS).
• Bacteria: Fecal Coliform or E. coli.
• Runoff Volume: The total annual volume of surface runoff.

By quantifying loads from each specific source and the potential reductions from each treatment practice, the WTM provides a clear framework for developing targeted and cost-effective watershed management plans.

The Two Assessment Components

The WTM’s methodology is built on two core components: a comprehensive inventory of pollutant sources and a full suite of potential treatment options. This structure ensures that the model reflects the complex reality of watershed management, where pollution originates from many places and can be addressed through diverse strategies.

1. Pollutant Sources

Unlike models that focus solely on land use, the WTM incorporates a wider variety of pollutant sources that are often critical in urban and developing watersheds. These are categorized as primary and secondary sources.

Primary Sources are directly related to land use and land cover. The model uses standard export coefficients, similar to the Simple Method, to calculate baseline loads from different land use types, such as residential, commercial, industrial, forest, and agricultural areas. This calculation is often informed by the principles of the Impervious Cover Model.

Secondary Sources are discrete or diffuse sources not directly tied to a general land use category. The inclusion of these sources provides a more complete and accurate pollutant inventory. Key secondary sources addressed in the WTM include:

• On-site septic systems, particularly their contribution to nutrient loads in baseflow.
• Combined Sewer Overflows (CSOs) and Sanitary Sewer Overflows (SSOs), which can be major episodic sources of bacteria and nutrients.
• Illicit connections and failing sewer lines.
• Stream channel erosion, a significant source of sediment.
• Application of road sand and deicing agents.
• Atmospheric deposition.
• Loads from construction sites.

2. Treatment Options

The model allows users to evaluate the effectiveness of a broad spectrum of watershed treatment options. This goes beyond traditional structural stormwater controls to include programmatic and non-structural approaches that are often difficult to quantify.

Structural Stormwater Practices: These are engineered systems designed to capture and treat stormwater runoff. The WTM can model the pollutant removal benefits of practices such as ponds, wetlands, infiltration trenches, bioretention areas, and proprietary systems. Performance data for these practices can be derived from resources like the national pollutant removal database or a local Design Manual.

Better Site Design and Non-Structural Programs: These options focus on preventing runoff generation and pollutant loading at the source. Examples include conservation design, impervious cover reduction, rooftop disconnection, and stream buffer ordinances.

Education and Pollution Prevention Programs: The WTM provides a framework for estimating the benefits of programs aimed at changing human behavior. This includes pollution prevention practices such as lawn care education, pet waste management campaigns, septic system maintenance programs, and municipal good housekeeping practices (e.g., street sweeping).

How Discounts Work

A key feature of the Watershed Treatment Model is its system for “discounting” the ideal or theoretical pollutant removal efficiency of a practice to reflect real-world conditions. Most monitoring data for treatment practices reflect newly constructed, properly designed, and well-maintained systems. In reality, performance is often compromised by budget limitations, staffing constraints, and imperfect application. The WTM accounts for this by applying three adjustment factors to derive a more realistic estimate of actual pollutant reduction.

The three discount factors are:

1. Coverage (or Treatability): This factor represents the fraction of a pollutant source that can realistically be addressed by a given practice or program. For example, it is not feasible to install stormwater retrofits on every acre of existing impervious cover due to space, cost, and ownership constraints. Likewise, a lawn care education program will not reach every household, and not all households that are reached will change their behavior. This factor adjusts the potential load reduction to only the portion of the source that is actually treated.
2. Performance: This is the underlying pollutant removal efficiency of a practice when it is properly designed, installed, and maintained. This “book value” is often based on national or regional monitoring data. For example, a bioretention facility might have a theoretical performance of 80% removal for total phosphorus.
3. Implementation: This factor accounts for how well a program or practice is actually executed and maintained over time. It reflects the level of effort a community puts toward its programs. A well-funded inspection and maintenance program for stormwater ponds would have a high implementation score, while a program with infrequent or inadequate maintenance would have a lower score, reducing the overall pollutant removal credit.

By multiplying these three factors (Coverage × Performance × Implementation), the model calculates an adjusted, real-world pollutant removal rate that is more credible and defensible than relying on idealized performance data alone.

Inputs Required

To run the WTM, a user must gather data to characterize the watershed and the management programs being evaluated. While the model includes many national and regional default values, its accuracy is significantly improved when local data is used. Key inputs include:

Strengths and Limitations

Strengths

• Comprehensive Scope: The model evaluates a uniquely broad range of pollutant sources and treatment options, including secondary sources and non-structural programs often ignored by other models.
• Realistic Estimates: The discounting system for coverage, performance, and implementation provides more defensible estimates of actual pollutant reduction achieved on the ground.
• User-Accessible: As a spreadsheet-based tool, it does not require specialized modeling software or extensive training, making it accessible to a wide range of municipal staff.
• Scenario Planning: It is highly effective for comparing the cost-effectiveness and potential outcomes of different management strategies, helping to prioritize actions and justify budgets.
• Answers Key Management Questions: The WTM is designed to help answer practical questions, such as identifying which subwatersheds have the greatest restoration potential, evaluating strategies to meet a TMDL, or assessing the effectiveness of a municipal stormwater program.

Limitations

• Planning-Level Accuracy: The WTM provides estimates of average annual loads and is not a substitute for more detailed, dynamic simulation models (e.g., SWMM, HSPF). It cannot predict the effects of individual storm events or generate hydrographs.
• Reliance on Assumptions: The model’s outputs are sensitive to the quality of its inputs and the many underlying assumptions and default values. Results should be interpreted as planning-level estimates, not precise predictions.
• Not a Design Tool: The model is intended for strategic planning at the watershed or subwatershed scale. It cannot be used for the detailed engineering design of specific stormwater practices.

Frequently Asked Questions