Watershed Treatment Model

Download the WTM spreadsheet in Excel format here (420kb)
View WTM User's Notes

Note: The Watershed Treatment Model, Version 3.1, is available for free download on this website. The accompanying documentation, The Watershed Treatment Model, Version 3.1, is available for purchase on www.cwp.org and both documents the assumptions incorporated into the model and provides guidance for user input. The results and inputs from the WTM may be difficult to interpret without the accompanying documentation.

About the Watershed Treatment Model

The Watershed Treatment Model (WTM), a simple spreadsheet-based approach that evaluates loads from a wide range of pollutant sources, and incorporates the full suite of watershed treatment options. In addition, the model allows the watershed manager to adjust these loads based on the level of effort put forth for implementation. Although the simple algorithms in this model are no substitute for more detailed watershed information, and model assumptions may be modified as the watershed plan is implemented, the WTM acts as a starting point from which the watershed manager can evaluate multiple alternatives for watershed treatment.

The watershed manger has to meet multiple objectives of improving or maintaining water quality, enhancing habitat, meeting the requirements of environmental regulations, making the resource enjoyable to watershed residents, and staying within a limited budget. Either to justify existing programs, develop future ones, or evaluate their progress, watershed managers are frequently asked hard questions about the effectiveness of their programs. Thus, managers need to define the treatment of their programs, and practices put in place. Watershed treatment is the benefit of a practice or program to a water resource, in terms of pollutant removal or habitat improvement. Just a few where treatment needs to be defined include:

In each case, the watershed manager must accurately estimate pollutant loadings and/or habitat conditions both now and at some time in the future. In addition, he or she needs to determine how conditions will improve in response to various treatment options. While most models, simple or complex, can estimate the loads from various current and projected land uses, they often do not incorporate some sources, such as Sanitary Sewer Overflows (SSOs) and illicit connections, that may be critical in urban and urbanizing watershed. In addition, they are not particularly well suited to evaluating the effect of watershed treatment, which ultimately depends on effort, staffing, design, and the inherent treatability of the different sources.

Challenges of Estimating Watershed Treatment

A number of factors make it difficult to estimate how much treatment is likely to actually occur in a watershed. First, a great deal of uncertainty exists in estimating both pollutant sources and watershed treatment options. Second, both the magnitude of pollutant sources and the effectiveness of watershed treatments depend on factors that vary both in time and between subwatersheds. Finally, an accurate estimate of watershed treatment needs to incorporate factors that are difficult to predict, such as human behavior.

Pollutant Sources

All pollutant sources vary in time, either due to changes in weather pattern, population, or economic trends. On top of this variability, each pollutant source is unique. While some can be predicted using readily available land use and climate data, others need to be estimated from other parameters. For example, loads from sanitary sewer overflows need to be extrapolated from available data about the sanitary sewer system, such as the age and extent. Similarly, predictions of habitat degradation need to incorporate changes in the forested buffer over time. Even with the most sophisticated modeling, none of these sources can be forecasted perfectly.

In addition, it is not always easy to compare pollutant loads because the nature of each source is different. While some sources are episodic in nature, others are more continuous. While some sources are found primarily in storm flows, others occur mostly within baseflow. In addition, impacts to habitat can be both episodic and continuous, and are difficult to compare or integrate with other pollutant source estimates.

Treatment Options

The watershed manager has a wide range of watershed treatment options to choose from, but often no way of estimating the benefits of many of these choices, since little monitoring data are available to assess their effectiveness. Consequently, the watershed manager cannot easily demonstrate the benefit of many programs, such as watershed education. Without some tool for estimating these benefits, it becomes difficult to justify the funds necessary to support these programs.

Even when practices have monitoring data, the watershed manager needs to estimate both the extent of treatment in a watershed, the treatbility, and the level of implementation. Treatability reflects the fraction of a pollutant source that can realistically be served by a program or practice. For example, it is unrealistic to assume that the loads from every acre of managed turf can be reduced by a lawn care education program, or that every acre of land can be served by stormwater retrofits. This is because not every lawn owner over-fertilizes to begin with, and space and budget constraints make it unrealistic to design retrofits on every acre of land.

Another challenge is that most practice monitoring data reflect relatively well-designed and newly constructed practices. In reality, practice effectiveness will often be compromised due to the level of effort a community puts toward implementing and maintaining practices with budget and staffing limitations. Accounting for this imperfect practice application presents a challenge to the watershed manager.


At the same time, we have better data on watershed variables now than at any time in the past. With the advent of readily available GIS data, watershed managers can rapidly characterize watershed land use. This data availability allows the watershed manager to characterize loads that are based on land use very rapidly, allowing more room in the budget to a program, and more accurately characterize its true effectiveness.

In addition, regional and national monitoring averages are now available to characterize the flows and concentrations from many sources, and the effectiveness of some practices, giving the watershed manager greater confidence in initial estimates. Finally, simple models such as the Simple Method are available that allow the watershed manager to effectively characterize the load given various treatment options, at least at the site level.