# Wildfire Burn Severity & Vegetation Impact Brief
## Sierra Ridge Mutual Water Company — Service Area Assessment

**Prepared:** 2026-04-12
**Meeting:** Dana Morales, Operations Manager — 4:30 PM Pacific

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## Key Findings

| Metric | Value |
|--------|-------|
| Total service area analyzed | 318.3 km2 |
| Area with active burn signatures | 17.58 km2 (5.5%) |
| Area showing regrowth | 28.91 km2 (9.1%) |
| Mean NDVI (vegetation health) | 0.298 |
| Vegetation under stress (NDVI < 0.3) | 45.5% of area |
| Healthy vegetation (NDVI > 0.5) | 2.4% of area |

## Burn Severity Breakdown

Severity Class                   Pixels   Area (km2)   % of AOI
----------------------------------------------------------------
Enhanced Regrowth (High)           4465        1.786       0.6%
Enhanced Regrowth (Low)           67799       27.120       9.5%
Unburned                         599478      239.791      83.8%
Low Severity                      43484       17.394       6.1%
Moderate-Low Severity               432        0.173       0.1%
Moderate-High Severity               19        0.008       0.0%
High Severity                         5        0.002       0.0%
----------------------------------------------------------------
TOTAL VALID PIXELS               715682      286.273     100.0%

## Imagery Details

| Parameter | Pre-fire Scene | Post-fire Scene |
|-----------|---------------|-----------------|
| Scene ID | `S2B_MSIL2A_20241127T190639_R013_T10TEL_20241127T212155` | `S2C_MSIL2A_20251028T190521_R013_T10TEL_20251028T212222` |
| Date | 2024-11-27 | 2025-10-28 |
| Source | Sentinel-2 L2A (Copernicus) | Sentinel-2 L2A (Copernicus) |
| Resolution | 20m | 20m |

## Methods Summary

This analysis uses **Sentinel-2 Level-2A** surface reflectance imagery accessed from the
Microsoft Planetary Computer STAC catalog. The workflow:

1. **Scene selection** — Cloud-free (<15%) Sentinel-2 scenes were selected for
   a post-fire period (summer/fall 2025) and a pre-fire baseline (same season, prior year).

2. **Cloud masking** — The Scene Classification Layer (SCL) was used to mask clouds, cloud shadows,
   and cirrus pixels.

3. **Burn severity (dNBR)** — The Normalized Burn Ratio (NBR) was calculated from NIR (B08) and
   SWIR (B12) bands for both dates. The differenced NBR (dNBR = NBR_pre - NBR_post) was classified
   into USGS burn severity categories (Key & Benson, 2006).

4. **Vegetation health (NDVI)** — The Normalized Difference Vegetation Index was calculated from
   NIR (B08) and Red (B04) bands of the post-fire scene to assess current vegetation condition.

5. **Spatial clipping** — All rasters were clipped to the Sierra Ridge service area polygon and
   reprojected to UTM Zone 10N (EPSG:32610) for accurate area calculations.

6. **Area statistics** — Pixel counts per severity class were converted to km2 using the
   20m pixel resolution.

## Attachments

- `wildfire_assessment_maps.png` — Side-by-side burn severity and NDVI maps
- `ndvi_histogram.png` — Distribution of vegetation health values across the service area
- `service-area.geojson` — Prospect service area boundary

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*Analysis performed using Sentinel-2 L2A imagery via Microsoft Planetary Computer.
Classification follows USGS dNBR severity thresholds (Key & Benson, 2006).*