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CAST Data Input Disaggregation from County and Land-River Segment Scale to National Hydrography Dataset Plus, Version 1.1

April 29, 2022

The detrimental effects of excess nutrients and sediment entering the Chesapeake Bay estuary from its watersheds have necessitated regulatory actions. Federally-mandated reductions are apportioned to bay jurisdictions based on the U.S. Environmental Protection Agency's Chesapeake Bay Time-Variable Watershed Model (CBPM). The Chesapeake Assessment Scenario Tool (CAST version CAST-19; cast.chesapeakebay.net; Chesapeake Bay Program, 2020) is a simplified, on-line version of the Phase 6 CBPM that simulates watershed nutrients delivery to the estuary using the original model's annual land-surface nutrient source and removal inputs and time-averaged climatological forecasting. Because it runs much faster than the CBPM, CAST facilitates rapid generation and comparison of alternate input reduction scenarios. The purpose of this data release is to make the baseline annual nitrogen, phosphorus, and sediment input data used by CAST available to the scientific community in a standardized, public-domain format, such that CBPM baseline predictions can be corroborated, or the model can be refined through independent scientific investigations. Because it constitutes the best available estimate, as of 2019, of past and projected future land-surface nitrogen, phosphorus, and sediment inputs over the entire extent of the Chesapeake watershed, this data set also supports broader USGS Chesapeake Bay Studies through fiscal year 2025. Source-specific annual nutrient source and removal inputs for years 1985 through 2025 were downscaled from the CBPM land-river segment scale (2,049 segments; mean area 118 square kilometers) to the National Hydrography Dataset Plus version 2.0 (NHDPlus) 1:100,000 catchment scale (83,331 segments, mean area 2.1 square kilometers). Eleven source or removal categories are represented for all counties that intersect the Chesapeake Bay watershed. These categories are listed below and further defined in the Purpose section. 1. Atmospheric deposition (atm. dep.) 2. Biosolids 3. Combined sewer overflow (CSO) 4. Direct deposit (manure directly excreted on pasture and in streams) 5. Fertilizer 6. Manure applied as fertilizer 7. Nitrogen fixation by agricultural crops (Nfix) 8. Rapid infiltration basins (RIB) 9. Septic systems 10. Nutrient uptake by agricultural crops that is removed from the field 11. Wastewater For most of these categories, nutrient source and removal inputs are tabulated for five species: ammonia, nitrate, organic nitrogen, phosphate, and organic phosphorus; sediment inputs are provided as total suspended sediment. Consistent with CBPM, plant uptake is specified only as total nitrogen and total phosphorus, and wastewater inputs are specified as biological oxygen demand and dissolved oxygen (Chesapeake Bay Program, 2020). In addition to these sources, annual proportional land-use layers used in the downscaling process are provided, also at NHDPlus 1:100,000 scale. Layers for each year represent proportional coverage of 14 Chesapeake Bay 2013 1-meter Land Use Data classes, interpolated (1985-2013) based on evolution of land-cover derived from NLCD 1992, 2001, 2006, and 2011 layers, and projected (2014-2025) using land use estimated for 2025 using the USGS Chesapeake Bay Land Change model (USGS, 2020). Best management practices (BMPs) are not included in this data release. BMPs have varying effects on nutrient inputs and runoff. These effects are best represented in CAST. Moreover, the BMP history is regularly revised by the states and the most current history is available as a downloadable file from CAST. Chesapeake Bay Program, 2020. Chesapeake Assessment and Scenario Tool (CAST) Version 2019. Chesapeake Bay Program Office, Last accessed November 2021.