Johnson County, Iowa
A shallow water table limits basements on 56% of Johnson County's mapped land, a higher share than 72% of US counties. Shrink-swell clay limits houses without basements on 40%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 8 flood losses here since 2016. 37% of homes were built before 1980, against 50% nationally; the median home dates from 1992.
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 Johnson County
| Expansive clay (limits slab homes) | 40.3% |
| High water table (limits basements) | 56.5% |
| Septic fields very limited | 96.6% |
| Clay swell (LEP, by area) | 4.7% |
| High-swell land (LEP 6%+) | 20.3% |
| Mapped soil | 398,900 acres |
How we count · survey areas IA103 (2025-09-08)
What the numbers mean here
Clay swell averages a linear extensibility of 4.7% by land area here, and 20% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 40% of the mapped land.
On 56% of Johnson 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: 34.69 in, 2.66 in below the 1991-2020 average year of 37.35 in.
USDA rates 97% 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. Part of the county was in drought in 19 of the last 52 weeks; the most was 100% of its area, in 14 weeks, most recently on the March 31, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
8 flood losses since 2016, $228,504 paid. The worst year was 2016, with 4.
| Year | Flood losses |
|---|---|
| 2016 | 4 |
| 2017 | 0 |
| 2018 | 3 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 0 |
| 2022 | 0 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 1 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 37.35 in |
| 12 months to August 2026 | 34.69 in (-2.66) |
| 2025 | 28.46 in (-8.89) |
| Wettest year since 1895 | 1993: 58.31 in |
| Driest year since 1895 | 1988: 20.73 in |
| Days with 1 inch or more, normal at IOWA CITY (6 mi away) | 9.5 |
Homes
| Housing units | 68,217 |
| Median year built | 1992 |
| Built before 1980 | 37% |
| Built 2000 or later | 37% |
| Owner-occupied | 59% |
| Median value, owner-occupied | $308,600 |
| Single-family detached | 48% |
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 |
|---|---|---|---|
| Iowa County, IA | 24 mi | 57% | 69% |
| Washington County, IA | 25 mi | 65% | 88% |
| Cedar County, IA | 25 mi | 39% | 43% |
| Muscatine County, IA | 28 mi | 20% | 50% |
| Linn County, IA | 28 mi | 9% | 57% |
| Louisa County, IA | 36 mi | 38% | 70% |
| Benton County, IA | 38 mi | 33% | 73% |
| Keokuk County, IA | 38 mi | 71% | 87% |
Sources: USDA SSURGO (September 8, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status