CWP WTM

The WTM already has estimations of pollutant removal efficiencies in Table 5.1 of their model documentation categorized by 17 specific structural BMPs and 6 different pollutant conditions. Furthermore, performance factors of “treatability and discount” can be calculated for each specific treatment practice to get a more realistic removal efficiency of a user’s chosen BMPs, by taking into account that maintenance and lifetime of every BMP. The user takes these efficiency values and performance factor values into existing equations in the WMT depending on if the BMP is implemented during development or is a retrofit.

There are seven different worksheets for the user to directly input data into. This includes primary sources, secondary sources, existing management practices, future management practices, retrofit, future land use, and new development. The primary sources and secondary sources sheets are where the user would input information regarding the land use, rainfall, and soil, as well as sewage data and illicit discharges. The existing and future management practices sheets require information on the BMPs in place or planned in the future. This includes, for example, turf conditions, structural BMP information, and street sweeping.Each worksheet also has many default values that the user has the option to change, such as BMP efficiencies, pollutant concentrations, pollutant load rates/runoff rates for each land use, etc.

The final output is in the form of three summary sheets: loads with existing practices, loads with future practices, and loads with new growth. Each sheet has a table for the loads to surface waters from each source, and the loads to groundwater from urban land and septic systems. Each of the input sheets alsohas a data summary to allow the user to view, for example, the load reduction from each type of BMP.

• Tool may be used during the planning phase of stormwater program.
• No part of the spreadsheet is write protected, allowing the user to make custom edits to the algorithms as needed
• The tool is intended to be more for screening different options for watershed management, not for obtaining precise watershed information as it intentionally uses more simplified models
• Default values, such as pollutant concentrations, are based on data taken from studies conducted across the U.S., so the user may want to consider using data specific to the watershed being analyzed
• CWP WTM is free and available to the public

• The tool allows the user to input future management practices and new development in order to also analyze loads based on future practices and new growth
• Two versions of the tool exist: “off-the-shelf”, which is intended to be used as-is, for users who just want to input data, and a “custom” version, which is intended for users who want to be able to change aspects of the model and its algorithms