Approaches to Developing Watershed Plans Using CAST

This framework is offered as one approach for developing a nitrogen, phosphorus and sediment reduction plan for any geographic area within the Chesapeake Bay region. This framework is directed toward developing the Phase 3 Watershed Implementation Plan (WIP), but is applicable to development of other water quality improvement plans. The primary audience is local government planners who will use CAST to estimate the nitrogen, phosphorus and sediment loads resulting from selecting one or more BMPs to achieve their goals.

Steps to Developing a Plan

The following diagram summarizes a sequence of steps for developing a water quality improvement plan.

Steps To Developing Plans

  1. The first step is to establish programmatic goals that include quantifiable loads for nitrogen, phosphorus and sediment; co-benefits from implementing BMPs, and the costs of implementation.
  2. The second step is to translate a combination of programmatic goals into specific BMPs, and then use those BMPs in a CAST scenario to estimate nitrogen, phosphorus and sediment loads.
  3. The third step is to determine if the loads are achieving the programmatic goals. If not, it is necessary to revise programmatic goals, perhaps changing co-benefits and/or costs, and then revising the scenario to estimate loads again.
  4. Once the estimated loads meet programmatic goals, the final step is to finish the plan and send it to the next level of the planning process.

Identifying Goals

Non-Numeric Goals

Reducing nitrogen, phosphorus and sediment while increasing the return on investment requires identifying programmatic goals. Programmatic goals can be identified and then modeled to determine a load reduction. This quantifiable information informs the refinement of the programmatic goals. Many goals may be non-numeric but may imply numeric outcomes. Some programmatic goals can be translated to BMPs.

Examples of non-numeric goals may be a growth policy that includes a regulation on large lot development or zoning requirements. Changes in development strategies are another example of programmatic goals that may be translated to a quantifiable load reduction. Changes in development strategies result in a change to land use. An example is establishing a policy on large lot development that requires a certain percentage to have tree cover. Increased trees or forest land can be modeled with Tree Planting or Buffer BMPs. Land conversion from developed to agriculture can be modeled by Impervious Surface Reduction. Land conversation from impervious developed to pervious developed can be modeled as Tree Planting – Canopy.

Another example is an agricultural regulation on using a Phosphorus Site Index to determine the amount of fertilizer applied. This regulation translates into the BMP Nutrient Management Phosphorus Application Rate Reduction which has a quantifiable effect on phosphorus loads. The programmatic goal in this example is establishing the Phosphorus Site Index as a requirement and the numeric goal is the acres of the Nutrient Management BMP and associated phosphorus load reduction.

Numeric Goals For Nitrogen, Phosphorus And Sediment

There may be programs, policies, or regulations that encourage or require nitrogen, phosphorus and sediment reduction. Planners should be provided with goals, or targets, in one of three forms: percent reduction, pounds of load, pounds reduced. A plan developed in CAST can show the loads in any of these three forms. 

Percent Reduction is available from the Results, Compare page on the third tab. This requires the user to identify an initial condition and compare that condition to one or more plans. CAST shows the percent reduction from the initial condition. 

Total Pounds delivered to the edge of a stream (EOS) or to the tidal waters of the Chesapeake Bay (EOT) on an annual basis is available from the Results, Compare page on the second tab. The total pounds per load source per year is shown for the scenarios selected by the user. An initial condition should be identified and selected along with one or more plans.

Pounds Reduced also is available from the Results, Compare page on the second tab. Just as with total pounds, the user selects the initial condition along with one or more plans. Then the user can simply subtract the difference in load for nitrogen, phosphorus and sediment. 

Numeric goals may be provided at any geographic scale. For example, they could be at the scale of county, state-river basin, or watershed of any size. Plans can be developed in CAST at any of these geographic scales for any area or combination of areas in the Chesapeake Bay Watershed. Results may be viewed and downloaded at any geographic scale as well. Whenever a user creates a scenario or views results, the user is asked to select the geographic scale and area.

The goals may be specified for a particular source sector. Examples of non-point source sectors are developed, agriculture, and natural. The developed sector includes pervious and impervious land in urban areas and rural areas. There is no distinction between new development and redevelopment. The agricultural sector includes pasture, cropped land, animal feeding space and riparian pasture manure deposition. The natural sector includes streams, Chesapeake Bay shoreline, forests and wetlands. CAST input and output is separated into these source sectors. Users add BMPs separately for each source sector and may view results by source sector and for all sectors summed together. 

Reducing Costs And Maximizing Co-Benefits

In addition to nitrogen, phosphorus and sediment goals, there may be additional, complementary objectives. Examples of additional objectives include:

  1. Reducing implementation costs
  2. Prioritizing co-benefits.

Examples of co-benefits include: improve stream health, increase fish habitat, and reduce toxic contaminants. Identifying these additional objectives early in the planning process allows for selection of BMPs that meet the load reduction goals as well as achieve these complementary objectives.

Costs Of BMPs

BMP costs included in CAST are developed by contractors to the EPA and are in 2010 dollars. The original costs were reviewed with the states who provided input. Additional BMPs were approved by the Chesapeake Bay Program Partnership since the original TMDL costs were determined. The details associated with costs of the original BMPs and the more recently added BMPs are provided in the downloadable files below. The costs are also available by logging in to CAST and going to the Cost Profiles. There is a profile for each state. CAST users may edit costs to their liking and determine the cost for any scenario.

Cost Effectiveness of BMPs

Cost effectiveness analysis is a necessary component for implementing pollutant reduction strategies. The tables below show the cost of BMPs per pound of nitrogen, phosphorus, or sediment reduced. These are estimates that were developed with the 2013 progress scenario and the Phase 2 WIP on 2025 Base Conditions in CAST using the Draft Final 9/25/2017 model version. The data may be applied to similar scenarios. For example, if the objective is to determine the cost of a WIP, then use the Phase 2 WIP scenario as it will be most comparable. These values are not applicable to scenarios that differ substantially from these scenarios. That means that you should not use progress cost effectiveness to estimate cost effectiveness of BMPs in a WIP.

BMP Co-Benefits

BMPs are ranked to indicate their impact on the co-benefits evaluated. This information will be incorporated into CAST scenario results at a later date. In the meantime, the BMP co-benefit impacts are provided at the links below.

Translating Goals to BMPs

Create A Scenario

The starting place for developing a plan for reducing nitrogen, phosphorus, and sediment is to create a new scenario. In CAST, you will be asked to select a geographic scale and area, e.g.: state, county, small watershed, etc. You will be asked to select the initial conditions and wastewater scenario. The Phase 3 WIP is to be developed on 2025 initial conditions. Select the 2025 base conditions and then select the appropriate wastewater scenario. Wastewater is handled through permits so may not be edited. However, multiple wastewater scenarios are available to facilitate your planning if that is a load that you are considering.

Next, determine the appropriate scenario from which to copy BMPs. The BMPs copied in are your building block. From there, you will add BMPs to meet your planning goal. There are several options from which to select for copying in BMPs. You may choose the latest annual progress scenario. The annual progress scenario includes all the BMPs that are implemented and functioning in that year, regardless of the year in which they were constructed or performed. For example, a 2017 Progress scenario includes cover crops planted in 2017, street sweeping performed in 2017, and forest buffers implemented in any year that are still present and functioning. Selecting a progress as the initial building block allows maximum flexibility. The planner may then remove BMPs that are expected to lose functionality (i.e.: not maintained) and add any BMPs to meet the goals.

Another option is to begin with the last WIP that was developed. The last WIP was the Phase 2 WIP developed on the 2010 initial conditions. Using the previous WIP will get you closer to the goal than the progress. However, the BMPs selected for the previous WIP may no longer be the most appropriate choice. Goals, planning priorities, and the public will to implement certain BMPs may have changed. Better information about the effectiveness of BMPs may have been gained since the last WIP, resulting in a different level of effectiveness for some BMPs. New BMPs and technological innovation may have occurred allowing for better options than were in the previous WIP. Also, the goals may be allocated to sectors or geographic areas differently. If you have other goals, like co-benefits or reducing costs, these may not have been considered in the same way for the previous WIP.

The Phase 3 WIP is developed on 2025 initial conditions. This means that the number of animals, septics, and amount of each load source (cropland, pasture, developed, etc.) will be different. So not all the BMPs in the Phase 2 WIP may be credited if the BMPs are at the limit of the land, animals, or septics are available. These changes may make it necessary to change some of the BMP selections in the previous WIP.

Beginning with no BMPs is an option, but is not recommended. It is expected that planners have been reporting BMPs to the states, that in turn report the Chesapeake Bay Program, so that the annual progress scenario represents the current condition.

Once the initial BMPs are copied into a scenario and your scenario is created, then you may edit the BMPs to reflect current and planned policies. You may edit the BMPs in the scenario to increase implementation levels, or you may add new BMPs. 

Methods For Adding BMPs To A Scenario

There are three ways you can add BMPs:

  1. Copy from an existing scenario
  2. Upload from a file
  3. Add one by one from the input box on the screen for each sector

Whichever way or combination of ways you choose, you can add BMPs for any geographic scale. CAST allows BMPs to be uploaded from a file. Uploaded BMPs are tab-delimited text files. The downloadable ables below show the list of required columns and sample data for the files. More information is available in the document, Uploading BMPs to CAST.

Transitioning Scenarios From The Previous Version Of The Model To CAST- 2017

CAST users who want to use old versions of CAST scenarios in the new CAST-2017 will need to transition their scenarios. Differences in the model versions require users to modify BMP data. Modifications can vary depending on your intention. The downloadable document and spreadsheet provides suggestions on how to relate the old BMP data to valid CAST-2017 data. The suggestions are provided only as a starting point or example for CAST users.

Additional Considerations For Scenarios

When selecting BMPs, consider the ability to actually implement these BMPs. Is there the public will? Is there funding? Are the practices voluntary or mandated? A plan should be realistic and have the support of the land owner or responsible party. As such, it is critical that these land holders or responsible parties are included in the plan development process.

To get the largest reduction for the least amount of implementation, consider several factors. First, determine the highest loading load sources and geographical areas. This is where the BMPs can have the most effect. The load sources with the highest initial load is where the greatest opportunity exists for reductions. Likewise, those geographic areas with high loads also have high opportunity for reductions. This may be in a county with lots of agricultural animals, or it may be in an urban area with a particularly high load for various reasons related to how it was developed and the geographic characteristics.

Remember to bring in the information on non-numeric goals, like policies or regulations that can have an impact. Also consider the costs and co-benefits that are important to the community. The BMPs selected should be efficient, maximize return on investment, and improve quality of life. 


Sometimes users find the names that are used for the load sources and BMPs are unfamiliar. Definitions pop-up throughout CAST when hovering over the names. It may also be useful to download the table of source data that contains all of the load source and BMP definitions. (link to source data page)

In addition, a crosswalk of commonly-used BMP names to the CAST BMP name may be helpful in selecting the CAST BMP that best represents your management action.