# Impervious Cover Analysis — Data Provenance

All pulls 2026-07-22. Working CRS: EPSG:5070 (CONUS Albers, NLCD native), 30 m grid.

| File | Source | Notes |
|---|---|---|
| `aoi_grid.json` | derived | Analysis grid: 1560 × 1453 cells, bbox 1170435,1352565 → 1217235,1396155 (EPSG:5070), snapped to the NLCD L48 grid (origin -2493045 / 177285). Covers the union of the 8 HUC12s. |
| `city_clemson_place_tigerweb.json` | US Census TIGERweb, Places_CouSub_ConCity_SubMCD MapServer layer 11 (Incorporated Places, ACS 2025 vintage) | City of Clemson, GEOID 4514950. Held fixed across all analysis years (no annexation artifact). |
| `clemson_university_tigerweb_slua.json` | US Census TIGERweb, Special_Land_Use_Areas MapServer layer 2 (Colleges and Universities) | 4 polygons, ~955 AC core campus. UNDERSTATES total university holdings — replace with parcel-based boundary (Pickens/Anderson owner = Clemson University) when available. |
| `huc12_wbd.json` | USGS hydro.nationalmap.gov WBD MapServer layer 6 | 8 HUC12 subwatersheds intersecting the padded city bbox. |
| `NLCD_epoch\NLCD_YYYY_Impervious.tif` | MRLC GeoServer WCS 1.0.0, coverage `mrlc_download:NLCD_YYYY_Impervious_L48` | 9 vintages: 2001, 2004, 2006, 2008, 2011, 2013, 2016, 2019, 2021. Clipped server-side to the AOI grid at native resolution. Values = percent impervious per cell (0–100). Big-endian tiled GeoTIFF. |
| `impervious_curve_epochs.csv` | output of `..\Scripts\build_impervious_curve.ps1` | Per boundary per year: mean impervious %, impervious AC (Σ fraction × 900 m²), boundary AC. |

Method: boundaries projected 4326→5070 with in-script Albers math (validated to <1 m against
hydro.nationalmap.gov geometry service); masks via even-odd scanline rasterization at cell centers;
city mask area matches TIGER ALAND+AWATER within 0.04%.

Exhibit: `..\Exhibits\impervious_curve.html` — published artifact
https://claude.ai/code/artifact/570c2368-97bb-4cdf-aa12-3fc01e80f8a4

| `NLCD_annual\FctImp_YYYY.tif` | USGS EROS GeoServer, workspace-scoped time-enabled WCS: `dmsdata.cr.usgs.gov/geoserver/mrlc_Fractional-Impervious-Surface-Native_conus_year_data/wcs`, coverage `...:Fractional-Impervious-Surface-Native_conus_year_data`, `TIME=YYYY-01-01T00:00:00.000Z` | Annual NLCD Collection 1 fractional impervious, all 39 years 1985–2023, clipped to the AOI grid. All 39 SHA1 hashes verified distinct (TIME honored per year). Pulled 2026-07-22. |
| `impervious_curve_annual.csv` | output of `..\Scripts\build_impervious_curve_annual.ps1` | Primary analysis series (39 years × 10 boundaries). Cross-check vs epoch product at 2021: city +1.22 pts, campus −0.98 pts (inter-collection difference; both products keep their own internal consistency — do not mix series on one curve). |

Endpoint discovery note: the annual products are NOT on www.mrlc.gov's GeoServer and the S3 bucket is
requester-pays; the open door is the workspace-scoped WCS on dmsdata.cr.usgs.gov (advertised as WMS-only
on MRLC's data-services page, but the workspace exposes WCS at the same path). Root-level GetCapabilities
on that server times out — always use workspace-scoped requests.

Thresholds referenced in exhibits: CWP Impervious Cover Model — <10% sensitive, 10–25% impacted,
>25% non-supporting.

## GIC / SC Forestry Commission figures (map gallery + canopy numbers)

The `maps/` images and all canopy, runoff-coefficient, heat, and ecosystem-service figures on the
page are reproduced from two public reports prepared for the City of Clemson by the Green
Infrastructure Center Inc. under a South Carolina Forestry Commission urban & community forestry
grant (figures pulled 2026-07-22):

- Urban Tree Canopy Assessment (June 2023) — https://online.fliphtml5.com/gavqz/rann/
- Tree Canopy Ecosystem Services Analysis — https://online.fliphtml5.com/gavqz/pctk/

Key values quoted: existing canopy 48% (goal ≥48%, aim 50%); potential planting area 189 AC (4.1%);
canopy interception 11.6M gal per 1-yr/24-hr (3.21") storm; +1% canopy ≈ +200,000 gal/event; runoff
coefficients 27,000 gal (paved AC) vs 750 gal (forested AC) per 1" storm; per-event filtering
18,360 lb N / 1,496 lb P / 955 tons sediment; extreme-heat days 4 → 44 → 78 (1971–2000 / 2036–65 /
2070–99); 58 AC priority planting (~4,980 trees).

Two surface-temperature maps, two dates, two legend ranges (each figure states its own scale on its
colorbar): **75.7–94.3 °F on 2022-06-05** (Tree Canopy Ecosystem Services Analysis, Fig-5) and
**87–127 °F on 2025-07-08** (Green Infrastructure Assessment, April 2026, Fig-6 — range read from the
map's own legend, which the text extraction of the PDF does not capture). The page uses the 2025 map
as the lead heat plate and the 2022 map as the corroborating earlier read; the two are never merged.

The "ten million gallons" figure is this page's screening-level arithmetic: measured +383 AC city
impervious growth (1985→2023, NLCD series above) × (27,000 − 750) gal/AC ≈ 10.05M gal per 1" storm.
GIC geographies are parcel/NAIP within city limits (their A–I watersheds ≠ our HUC12s; different
denominators — corroborate direction only, never merge on one axis).