Soil Temperature
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Unit: Degrees Celsius (°C)
Soil temperature is measured at varying depths, typically 0–10 cm for fire weather applications. It reflects both the surface energy balance (heating from solar radiation, cooling at night) and the slower thermal response of subsurface layers.
Why it matters for fire weather
Section titled “Why it matters for fire weather”Soil temperature affects fire conditions indirectly through its influence on fuel moisture and vegetation:
- Surface fuel drying — warm soils accelerate evaporation of water from surface fuels and organic layers. This is especially important for the deep fuel moisture tracked by the DC (Drought Code) in the FWI system, which represents moisture in organic layers at 10+ cm depth.
- Vegetation stress — warm soils combined with low soil moisture stress plant root systems. When roots are in warm, dry soil, plants lose the ability to replace moisture lost to the atmosphere through transpiration (driven by VPD), making live vegetation more flammable.
- Thermal lag — soil responds more slowly than air temperature. After several hot days, the soil retains heat even if air temperature drops temporarily. This stored warmth continues to drive evaporation from fuel layers and keeps the surface drier than air temperature alone would suggest.
How it works
Section titled “How it works”Soil temperature follows a damped version of the surface air temperature cycle:
- Near the surface (0–7 cm) — responds within hours to changes in solar radiation and air temperature. Shows a clear daily cycle.
- At depth (7–28 cm and below) — the daily cycle is barely detectable. Temperature reflects the average conditions over days to weeks.
The ECMWF IFS weather model (which provides Wildflyer’s soil data) uses the same four-layer structure as for soil moisture: 0–7 cm, 7–28 cm, 28–100 cm, and 100–289 cm. Deeper layers respond to seasonal changes rather than daily weather.
Key thresholds
Section titled “Key thresholds”Soil temperature thresholds are context-dependent — what matters is the combination with soil moisture and recent weather history.
| Soil temp (0–10 cm) | Physical significance |
|---|---|
| < 10°C | Cool soil. Biological activity and evaporation rates are low. Fuel drying is primarily atmospheric, not soil-driven. |
| 10–20°C | Moderate soil warmth. Normal growing-season conditions. Evaporation from the soil surface contributes to fuel drying. |
| 20–30°C | Warm soil. Evaporation from organic layers accelerates. Combined with low soil moisture, vegetation root stress increases. |
| > 30°C | Hot soil. Common in Mediterranean summers under direct sun. Organic layers at the surface are drying rapidly. In organic soil environments (peatlands), elevated soil temperatures indicate conditions where subsurface smouldering is possible. |
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”Soil temperature appears in the expert view alongside soil moisture. The two variables are most useful together: warm, dry soil indicates active fuel drying at depth, while warm, moist soil indicates that recent rain has been absorbed and vegetation roots still have access to water.
Sources
Section titled “Sources”- Balsamo, G. et al. (2009). A revised hydrology for the ECMWF model: Verification from field site to terrestrial water storage and impact in the Integrated Forecast System. Journal of Hydrometeorology, 10(3): 623–643.
- Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.