How does low subsoil moisture levels affect wildfire behaviour?

Low subsoil moisture levels have a significant impact on wildfire behavior by influencing both the flammability and availability of fuels, as well as the potential intensity and spread of fires.

• Fuel Dryness and Curing: When subsoil moisture is low, plants experience water stress, leading to drier live and dead fuels. This accelerates the curing process, making vegetation more flammable and increasing the likelihood of ignition and rapid fire spread56. In grasslands and forests, large wildfires during the growing season are strongly associated with low subsoil moisture, as measured by the fraction of available water (FAW). For example, 91% of large wildfires in the southern Great Plains occurred when FAW was below 0.5, indicating significant water stress in the root zone6.

• Fire Probability and Size: Low subsoil moisture increases the probability of wildfire occurrence and the potential size of fires during the growing season. As soil moisture drops below certain thresholds (e.g., FAW < 0.3), the probability of rapid fuel curing and large wildfires rises sharply56. This relationship is especially pronounced in ecosystems where vegetation is actively growing and dependent on subsoil moisture.

• Seasonal and Biome Differences: The effect of subsoil moisture on wildfire behavior varies by season and vegetation type. During the growing season, low subsoil moisture is a strong predictor of large fires, whereas in the dormant season, its influence is less pronounced, particularly in grasslands where dead fuel moisture is less dependent on soil water content56.

• Root Depth Sensitivity: The impact of subsoil moisture is also related to the rooting depth of dominant vegetation. Shallow-rooted ecosystems (such as grasslands and some boreal forests) are more sensitive to changes in subsoil moisture, making them particularly vulnerable to intense wildfires when subsoil moisture is low5.

• Predictive Value: Subsoil moisture is often a better predictor of wildfire risk and size than traditional indices based on surface conditions or precipitation alone. Soil moisture data can provide earlier and more accurate warnings of high fire danger, allowing for improved fire management and preparedness56.

In summary, low subsoil moisture levels increase wildfire intensity and spread by promoting drier, more flammable fuels and accelerating fuel curing, particularly during the growing season and in shallow-rooted ecosystems. This makes subsoil moisture a critical variable in predicting and understanding wildfire behavior56.

1. https://www.sciencedirect.com/science/article/abs/pii/S0378112721004679
2. https://www.nature.com/articles/s41598-020-67530-4
3. https://lfs-mlws-2020.sites.olt.ubc.ca/files/2021/10/Cheng-2021-A-Review-of-Wildfire-Effects-on-Soils-Hydrologic-Processes-and-Water.pdf
4. https://www.fs.usda.gov/rm/boise/AWAE/labs/awae_flagstaff/publications/neary_fireeffectsonsoils.pdf
5. https://ntrs.nasa.gov/api/citations/20230014040/downloads/IJWF%20Accepted_Manuscript.pdf
6. https://www.climatehubs.usda.gov/sites/default/files/Ochsner_Soil_Moisture.pdf
7. https://criticalzone.org/uploads/bigdata/Boisrame_2018_AWR_Published.pdf
8. https://www.publish.csiro.au/wf/wf21064
9. https://cwfis.cfs.nrcan.gc.ca/background/summary/fwi
10. https://www.nature.com/articles/s41561-024-01563-6
11. https://news.okstate.edu/magazines/engineering-architecture-technology/impact/articles/2024/the_science_behind_post-wildfire_soild_behavior.html
12. https://www.mdpi.com/2571-6255/4/3/45
13. https://pubmed.ncbi.nlm.nih.gov/32620812/
14. https://nwfirescience.org/sites/default/files/publications/WF22056.pdf
15. https://www.publish.csiro.au/wf/fulltext/WF22056
16. https://assessments.epa.gov/risk/document/&deid=357096