HEM Land Use Change Summary by Drainage Basin

Author

Tampa Bay Estuary Program

Published

August 12, 2026

Habitat Evolution Model (HEM) projections summarized by SWFWMD/FDEP drainage basin rather than by county. Each table gives mean acres across the accretion × sea level rise ensemble, ± one standard deviation, under protect existing development (PD) and allowing habitats to migrate (AM) land policy. Values in green are above the 2025 baseline; values in red are below it.

A county-based version of these results is available in hemchange.qmd.

The drainage basin layer covers 3,660,135 acres of the project area. The remaining 2,891,069 acres are open Gulf and nearshore waters with no basin assignment, carried here as a separate Coastal Waters domain derived by differencing the project extent against the dissolved basin layer.

Reporting only the basins would silently drop that area, and it is disproportionately Open Water and Subtidal - Seagrass — precisely where sea level rise drives change. Note that Tampa Bay itself is not part of the coastal residual; it is carried in the basin layer as a BAY feature, as are Sarasota Bay and Boca Ciega Bay.

Totals: 465 drainage basins and 8 coastal units, 6,551,204 acres combined.

Project area totals

By watershed (HUC8)

Drainage basins grouped by eight-digit hydrologic unit, ordered by area. Tab labels name the two largest basins in each unit.

Coastal waters

Residual project area outside the drainage basin layer, attributed to the county whose limits it falls within, with the remainder offshore.

By hydrologic feature type

Basins grouped by the FEATURE attribute of the source layer. BAY units carry the open estuarine water that would otherwise be mistaken for part of the coastal residual.

Largest individual basins

465 basins carry data, too many to present individually. The 12 largest by baseline acreage are shown; per-basin values for the full set are in data/output/basin/.

Notes on the numbers

Standard deviations are computed at the level they are reported. For any rollup above a single basin, acres are summed within each accretion × sea level rise combination first, and the standard deviation is taken across those ensemble totals. Pooling the individual basins’ standard deviations would assume the basins vary independently, which they do not — every basin in a watershed sees the same sea level curve — and would overstate the spread.

Zero-filling. Where a habitat category is absent from one ensemble member it is treated as zero acres rather than as a missing observation, so means are taken over the full set of members.

Baseline is the 2025 condition from data/tbcmp_hem_filled.tif, processed through the same zonal routine as the scenarios.

Boundary cells. Basin summaries count whole cells assigned to the basin containing the cell centre; the coastal summaries use partial-cell coverage fractions. At 2 m resolution the difference is immaterial at these areas, but it means basin and coastal acreages are not derived by identical methods.