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Solano County, California
Shrink-swell clay limits houses without basements on 50% of Solano County's mapped land, a higher share than 91% of US counties and the 2nd highest of 54 California counties. A shallow water table limits basements on 21%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 67 flood losses here since 2016, 7% of them in buildings with basements. 45% of homes were built before 1980, against 50% nationally; the median home dates from 1982.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Solano County
| Expansive clay (limits slab homes) | 50.1% |
| High water table (limits basements) | 21.3% |
| Septic fields very limited | 76.4% |
| Clay swell (LEP, by area) | 6.7% |
| High-swell land (LEP 6%+) | 52.8% |
| Mapped soil | 582,142 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Capay | 10% | n/a |
| Yolo | 6% | n/a |
| San Ysidro | 6% | n/a |
| Dibble | 5% | n/a |
| Altamont | 4% | n/a |
| Diablo | 4% | n/a |
How we count · survey areas CA095 (2025-09-03)
What the numbers mean here
Clay swell averages a linear extensibility of 6.7% by land area here, and 53% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 50% of the mapped land.
On 21% of Solano County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.
Rain over the 12 months to August 2026: 17.85 in, 2.98 in below the 1991-2020 average year of 20.83 in.
USDA rates 76% of the land very limited for septic absorption fields.
Drought
0% of the county is in drought (D1 or worse) on the October 13, 2026 map. No part of the county was in drought in the last 52 weeks.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 19.9% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
67 flood losses since 2016, $1.7 million paid. The worst year was 2023, with 21. Foundations of the buildings, where recorded: 7% basements, 49% slabs, 35% crawl spaces, 9% elevated. Median water depth outside the building: 8.5 in.
| Year | Flood losses |
|---|---|
| 2016 | 5 |
| 2017 | 18 |
| 2018 | 2 |
| 2019 | 3 |
| 2020 | 1 |
| 2021 | 13 |
| 2022 | 0 |
| 2023 | 21 |
| 2024 | 3 |
| 2025 | 0 |
| 2026 to date | 1 |
Rain
| Average year, 1991-2020 | 20.83 in |
| 12 months to August 2026 | 17.85 in (-2.98) |
| 2025 | 16.61 in (-4.22) |
| Wettest year since 1895 | 1983: 41.24 in |
| Driest year since 1895 | 2013: 5.10 in |
| Days with 1 inch or more, normal at FAIRFIELD (7 mi away) | 6.2 |
Homes
| Housing units | 164,693 |
| Median year built | 1982 |
| Built before 1980 | 45% |
| Built 2000 or later | 19% |
| Owner-occupied | 63% |
| Median value, owner-occupied | $617,700 |
| Single-family detached | 72% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Contra Costa County, CA | 25 mi | 48% | 11% |
| Napa County, CA | 27 mi | 14% | 6% |
| Yolo County, CA | 29 mi | 44% | 23% |
| Sacramento County, CA | 35 mi | 16% | 26% |
| San Joaquin County, CA | 43 mi | 30% | 23% |
| Alameda County, CA | 43 mi | 38% | 8% |
| Marin County, CA | 46 mi | 20% | 6% |
| Sutter County, CA | 55 mi | 41% | 37% |
Sources: USDA SSURGO (September 3, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status