Developing an Online Tool for Facilitating Local Involvement in Watershed Implementation Plan

Olivia H. Devereux and Jessica R. Rigelman


The Chesapeake Assessment Scenario Tool (CAST) is developed as an online nutrient and sediment load estimator to streamline and facilitate Watershed Implementation Plan (WIP) and Milestone preparation consistent with the Chesapeake Bay Total Maximum Daily Load (TMDL). The purpose of the tool is to simplify the process for building scenarios and to provide initial estimates of nitrogen, phosphorus, and sediment load reductions using a variety of best management practices (BMPs). CAST builds on the science that went into development of the Watershed Model-HSPF by providing a broadly accessible online tool to facilitate efficient management decision making and greater environmental protection. CAST approximates loads using similar logic, rules and assumptions as the Watershed Model, which was used in determining the TMDL and allocations. This ensures consistency with the TMDL so that when progress is measured using the Watershed Model, the loads are comparable to CAST’s predicted loads. However, CAST does not actually “run” the Watershed Model, but rather approximates that model’s output for rapid online scenario development. The approximations are calculated using linear equations with coefficients generated through the Watershed Model using factorial inputs over combinations of BMPs and land use change. Loads are estimated for each land use and each modeling segment. Validation with the Watershed Model showed 99% agreement for TN with urban land uses.

CAST facilitates an iterative process to determine if TMDL allocations are met. Because CAST is online and easy to use, local planners may deeply engage, thereby improving local management decisions and securing a higher likelihood of actual implementation. CAST allows users to select the geographical area for their plan and identify the level of implementation of various BMPs. CAST outputs predicted pollutant loads and compares these against allocations. Local planners can compare among different scenarios that they create, so they can determine which scenario meets the allocation

and is politically feasible. In this way, planners can develop multiple options rapidly. Each local jurisdiction that uses CAST uses the same methodology, which ensures the ability to replicate results. CAST also automatically can create inputs to the model that pass the model’s validation, saving time and effort for Chesapeake Bay Program staff. This presentation will provide an overview of CAST, the data requirements necessary for its development, the method for approximating the Watershed Model, and its validation with the Watershed Model-HSPF.

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