> For the complete documentation index, see [llms.txt](https://help.fielddoc.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.fielddoc.org/resources/models/adapted-chesapeake-nutrient-and-sediment-load-reduction-model.md). # Adapted Chesapeake Nutrient and Sediment Load Reduction Model The [Chesapeake Assessment Scenario Tool (CAST)](https://cast.chesapeakebay.net/) is a web-based nitrogen, phosphorus, and sediment load estimator tool that streamlines environmental planning. CAST is the official planning and tracking tool for the [Chesapeake Bay Total Maximum Daily Load (TMDL)](https://www.epa.gov/chesapeake-bay-tmdl), developed and maintained by the [Chesapeake Bay Program](https://www.chesapeakebay.net/), a unique regional partnership that has led and directed the restoration of the Chesapeake Bay since 1983. **Load reduction estimates** in FieldDoc are based on data from the Chesapeake Bay Program's Phase 6 model. These estimates are calculated by combining the type of [best management practice (BMP)](/essentials/activities/general-settings/activity-type.md), the **size** or area it treats, and its **location** within a specific land-river segment. The system then applies the corresponding nutrient or sediment reduction efficiency. FieldDoc uses the **Adapted Chesapeake Nutrient and Sediment Load Reduction Model**, which relies on standard efficiencies. This model is designed to make data entry easy while still maintaining scientific credibility. This model may have different outputs than scenarios calculated with the Phase 6 model. This tool is not intended to replace official Chesapeake Bay Program scenario tools or provide exact numbers for TMDL targets. Instead, it offers a general estimate of nutrient or sediment reductions based on the size, type, and location of a practice—helpful for understanding potential environmental benefits before implementation. ## Source Documentation
Chesapeake Assessment Scenario Toolhttps://cast.chesapeakebay.net/cbp-vertical-full-color.jpg
BMP documentationhttps://cast.chesapeakebay.net/Documentation/BMPscbp-vertical-full-color.jpg
## Calculation Methodology FieldDoc's adapted CAST model calculates pollutant load reductions by applying BMP-specific efficiency parameters to the implementation extent (typically acres). Each BMP has defined reduction rates for nitrogen (TN), phosphorus (TP), and sediment (TSS) that were established through Expert Panel reviews and approved by the Chesapeake Bay Program Partnership. ``` # Core reduction calculation # For each pollutant parameter: tn_lbs_reduced = tn_efficiency * acres_implemented tp_lbs_reduced = tp_efficiency * acres_implemented tss_lbs_reduced = tss_efficiency * acres_implemented ``` ### BMP Types and Categories BMPs in CAST are organized into three primary categories based on how they achieve pollutant reductions. The model applies these BMPs in a specific sequence to avoid double-counting reductions. * **Efficiency BMPs:** Apply a percentage reduction to pollutant loads passing through or generated by the practice. Examples include cover crops, nutrient management, and stormwater BMPs. * **Load Source Change BMPs:** Convert land from one use to another with different loading rates. Examples include forest and grass buffers, which convert agricultural land to lower-loading vegetation types. * **Animal BMPs:** Reduce nutrient loads from livestock operations through improved manure management, barnyard runoff controls, and mortality composting. ### Load Calculation Process The model calculates loads using a multi-step process that accounts for land use, nutrient sources, BMP applications, and delivery to receiving waters. ``` # Step 1: Calculate edge-of-stream (EOS) load eos_load = land_use_acres * loading_rate_per_acre # Step 2: Apply BMP reductions (in prescribed sequence) treated_load = eos_load * (1 - bmp_efficiency) # Step 3: Apply delivery factor for fate and transport delivered_load = treated_load * delivery_factor # Delivery factor accounts for: # - Distance to receiving water # - Intervening land uses and BMPs # - Stream and river attenuation ``` ### Land-River Segments CAST divides the [Chesapeake Bay watershed into approximately 2,058 land-river segments](https://www.arcgis.com/apps/mapviewer/index.html?layers=28bb80f62236422f896eed2f530cd1fc)—unique geographic units defined by the intersection of county boundaries and local watershed boundaries. Each segment has specific loading rates and delivery factors based on its soil types, hydrology, and proximity to tidal waters. **Delivery factors** represent the fraction of pollutant load that reaches the Chesapeake Bay from a given land-river segment. Segments closer to tidal waters or with more direct hydrologic connections have higher delivery factors, making BMPs in these areas more effective at reducing bay-wide loads. ### BMP Targeting and Effectiveness BMP effectiveness varies by location due to delivery factors. The Chesapeake Bay Program publishes [BMP targeting maps](https://cast.chesapeakebay.net/Documentation/BMPs) that identify land-river segments where practices would achieve the greatest reductions in delivered loads. These maps help prioritize conservation investments for maximum water quality benefit. ### Acronyms
BMPBest Management Practice
CASTChesapeake Assessment Scenario Tool
CBPChesapeake Bay Program
CBWMChesapeake Bay Watershed Model
EOSEdge of Stream
TMDLTotal Maximum Daily Load
TNTotal Nitrogen
TPTotal Phosphorus
TSSTotal Suspended Solids
WIPWatershed Implementation Plan
### Attribution The Adapted Chesapeake Nutrient and Sediment Load Reduction Model is based on Phase 6 best management practice isolation run outputs provided by the Chesapeake Bay Program. CAST is the Chesapeake Bay Program's official watershed model, enabling planners to develop strategies for meeting nitrogen, phosphorus, and sediment load allocations. For more information, visit the [Chesapeake Assessment Scenario Tool (CAST)](https://cast.chesapeakebay.net/) or view the [model documentation](https://cast.chesapeakebay.net/documentation/modeldocumentation). --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://help.fielddoc.org/resources/models/adapted-chesapeake-nutrient-and-sediment-load-reduction-model.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.